Making Time Lapse Motion Capture of the Night Sky

In previous blog posts, I’ve written about taking still pictures of the night sky, and how to take time exposures in daylight. Now, let’s look at the ten basic steps for capturing the motion of the stars:

1.       Leave town. I typically drive 1 to 2 hours north of the city to get away from its ‘light dome’.

2.       Make sure you have a sturdy tripod for your DSLR.

3.       Use as wide-angle a lens as you can get. For a full frame sensor, consider using anywhere from 14mm to 18mm focal length. For APS-C sensors (ie. Canon Rebel or equivalent), a 10 to 12 mm focal length will work. Camera brand lenses can be outrageously expensive, so consider much cheaper but still good third party brands like Rokinon or Tokina.

4.       Consider getting an intervalometer, or at least a wired shutter release for your camera. Either one will cost you less than $20 on eBay.

Intervalometer

5.       If the air temperature is expected to drop below the dew point, be ready to clear condensation off your lens. You could use a microfibre cloth between shots, buy a battery operated fan at the dollar store to keep the air moving across the front of the lens, or look into some form of lens warmer.

6.       Set your ISO to about 3200 and your shooting mode to Manual. Since you want to keep exposure to less than 30 seconds to prevent the stars from creating noticeable trails on each frame you shoot, select a value between 20 and 30 seconds and set aperture to maximum (ie. f/2.8). I use f/4 with success, but it might force the choice of 30 seconds over 20 seconds. You may also want to use a fixed White Balance mode instead of Auto, but I generally have not found Auto to be a problem.

Normal Infinity Focus Setting

7.       If your lens has a focus scale, set it to the vertical bar near the infinity mark, but not at the infinity mark. Take a test shot and zoom in on the playback to see if you have optimal focus. If not, tweak it very slightly and re-check. The actual infinity focus point varies slightly with focal length, so leave the lens at your intended focal length when setting the focus point, and only zoom in on the playback to verify it.

8.       If using an intervalometer, set your camera to Bulb (may be found in Manual mode settings or may be a separate mode depending on the camera). Set the intervalometer shutter  ‘on’ time to 20 to 30 seconds (depending on your test shots), set its interval between shutter actuations to something like 5 seconds (to allow you to wipe condensation off the lens between shots), and set the number of frames to about 150. I normally shoot with RAW+JPEG, but you may want to consider doing such a long sequence in JPEG only to conserve memory card space. Elapsed time will be close to an hour and a half, so bring a coffee flask!

9.       If using a simple wired shutter release, set the camera to burst or continuous mode instead of single shot mode. You will not use Bulb mode in this case. Instead, select either 20 or 30 seconds for your shutter speed in Manual mode. When ready to start, use the lock on the wired release to hold the shutter button on. The camera will take one shot immediately after the other with no breather in between. This will get the sequence over with more quickly than when using an intervalometer with a 5 second breather, but you won’t have an opportunity to clean off condensation between shots. Depending on the camera, you may also have to manually keep track of the number of shots taken.

10.   Find an interesting object on earth that you can anchor your shots to. Put it in the lowe part of your frame, about a third of the way in from the edge. You can have it silhouette against the sky, or use a flashlight to do a little light painting. In a sequence, however, you may want to forget the light painting because it is difficult to make the object look uniformly lit in all 150 or so frames. Now, consider putting Polaris somewhere in your shots, so you can see the stars rotate around it in your final sequence.

That takes care of the capture portion. Now for the post-production steps:

1.       I import all 150 frames (images) into Lightroom. Then, if corrections for exposure, white balance, etc. are needed, you can make the changes to one frame and sync the changes to all 150. Similarly, I have set up a custom crop mode in Lightroom for 16x9, so that the resultant video will frame properly on television. Again, I can crop one and sync-crop all the others.

2.       A sidebar on ‘stuck’ pixels: You are going to notice that there will be bright spots on all frames, which don’t move with the stars. These are generally due to the long exposures and the number of such exposures, which result in heating up of the sensor. If these are left alone, it will look weird once you put all the stars in motion, since stuck pixels don’t move. You could use the camera’s long exposure noise reduction feature to mitigate this, but you will be out there for three hours instead of one and a half hours capturing the 150 frames. My approach is to use the clone brush in Lightroom on one frame and sync to all other frames.This is the most time-intensive and frustrating part of the process due to thenumber of stuck pixels my camera produces, and the way you clone them out may adversely affect other frames once you sync. There is a bit of an art to it, and I could write a whole treatise on it. But not now.

3.       Once I’ve cleaned up all the frames, I export them from Lightroom as TIFF files. At this point, you may want to decide if you simply want to make a video clip of the stars rotating around Polaris, or create progressively longer and longer star trails as they rotate. For the latter, you could use StarStax. This software creates another set of files in which the star trails get progressively longer. This is done by ticking a checkbox in the settings menu to force it to save a file every time it adds another file to the sequence. If you don’t check the box, it will only create one file that consists of all 150 frames forming a single long trail for each star.

4.       Now to put things in motion. There are a number of software packages that can accomplish this, but I use my Corel VideoStudio video editor. In that software, you simply right-click on an empty timeline and select the time lapse option(‘Insert Photo for Time-lapse/Strobe’). You then tell it where your frames are and how long you want to make the clip (I generally start with about 10 seconds), and the frames load onto the timeline. If you want to have the stars rotate without making trails, use the files you exported out of Lightroom. On the other hand, if you want the stars to create progressively longer star trails, use the files created by StarStax.

5.       Now you can create a finished video (Share tab in Corel). I prefer to save mine in .mp4 format. Have a look at my two samples on YouTube. One is without StarStax (Milky Way video) and the other is with StarStax (observatory video).  I hope these inspire you to try out this fun technique.

TRY TIME EXPOSURES FOR ETHEREAL EFFECTS

When we think about taking long exposures with a camera, night time images usually come to mind. Only dark conditions generally call for slow shutter speeds, right?

True-but there is a way to take time exposures in broad daylight...with surprising results.

So the first question that comes to mind is: how do you take a long exposure in bright light? Won’t the camera try to set a fast shutter speed to ensure that the exposure is correct, regardless of how small the aperture is set?

Then there’s the other question: why would you want to take long exposures in daytime, anyway?

OK - a long time exposure (keeping the camera’s shutter open for a long time – from seconds to minutes) will occur when:

• Light level is very low
• A very small aperture is chosen, and
• ISO is set to the lowest value that the camera will allow.

So, to answer the first question, you need to simulate low light conditions by giving your camera dark sunglasses. The device that will do this is an inexpensive thread-on piece of glass called a neutral density (ND) filter. These filters come in a wide variety of grades and are either specified in ‘stops’ or ‘times’.  They are also available as variable grade versions, often referred to as Fader NDs. Be sure to buy one that has the same filter thread diameter as your lens.

A one-stop (or two times) ND filter, with aperture and ISO remaining the same, will require that you force the shutter to stay open twice as long to get the same exposure as you would without the filter. Similarly, a two-stop filter will require slowing the shutter by four times, since a stop means a factor of two in exposure. For the kind of images I’m proposing we take, you will need a ten-stop filter in order to get exposures that are minutes long. This is equivalent to increasing exposure by a factor of 1024 times, but is usually referred to as a 1000 times filter.

Why minutes? Well, in answer to the second question above, anything moving (water, clouds) will take on a foggy (in the case of waves) or streaky (in the case of clouds) look. As long as you have stationary elements in the scene, such as rocks, ground and a tree trunk in the sample shots shown in this post, the viewer is confronted with a paradox that doesn’t appear ‘natural’. This makes for a more interesting image than if you had captured the same scene with a shutter speed of, say, 1/250^th of a second. While these images were shot in colour, I converted them to black and white, since they tend to look more dramatic that way.

And now for the practical considerations. You will need:

• A stable tripod
• A remote shutter release that plugs into your camera, since you can’t hold down the shutter button on your camera for minutes without jarring it. You can also get an intervalometer, which allows you to set a specific time interval without having to manually time the shutter.
• A camera with a Manual mode that will allow you to set the shutter to ‘Bulb’, or one that has a dedicated Bulb mode.
• A day with calm winds, so that your tripod doesn’t get buffeted during the exposure.

Look for a scene that has a combination of moving and stationary components. Getting the right exposure is definitely going to take some experimentation. As a starting point, set your camera’s ISO to the lowest value it provides. This is typically 100. Do NOT use Auto ISO. In Manual / Bulb mode, set the aperture to a value anywhere in the range of f/16 to f/22. This will depend on whether it’s an overcast or sunny day. You should get exposures in the range of 2 to 4 minutes. Use the histogram to confirm good exposure and tweak the time (or aperture) as needed to get perfect exposure or desired motion effect.

With digital SLRs, a practical consideration is the heating of the sensor during long exposures. While it won’t damage the camera, this heating tends to amplify what are often referred to as ‘stuck pixels’. Every camera has them, and they appear as unexpected bright spots on the final image.

To combat this, DSLRs generally have a long exposure noise reduction setting which can mitigate this annoyance. Unfortunately, it requires that the camera will make you wait twice as long to see your image, since it takes as long again as the exposure time to process out the stuck pixels. This could result in an 8 minute wait until you take the next shot. While this technique is effective, I prefer to take my chances and fix the spots after in either Lightroom or Photoshop.

By way of a summary, here are the steps I take:

1. Set up the camera on the tripod and compose the scene with the chosen lens.
2. Ensure ISO is at its lowest setting, and choose a small aperture to help force a long exposure and to give me the depth of field I want.
3. Plug in the remote shutter release or intervalometer (and set it to the exposure time I want).
4. Thread on the ten-stop ND filter, being careful not to disturb focal length or focus settings.
5. Cover up the eyepiece if the sun is behind me, to prevent light leak onto the image (it has happened to me!)
6. Check my image and histogram, rinse and repeat until I get the right exposure and effect.

It’s a slow process that requires patience. At the end of it though, you will get images that will be set apart from the ordinary.

Sharpness and Contrast: Lenses Compared

There is a vast range of pricing in lenses, so I decided to find out for myself if the most expensive lenses are really any better.

High up in this rank is the name Zeiss, and yes, the prices are out of reach for most of us. So, rather than take a chance on potential high-priced disappointment through a purchase, I decided to rent a Zeiss ZE 85mm f/1.4 Planar T for the weekend.

NOTE: Not all Zeiss lenses are more expensive than the camera manufacturers' own lenses. In fact, the Zeiss Planar lens tested here is actually cheaper by far than Canon's 85mm f/1.2 L series. The Zeiss OTUS 85mm f/1.4 lens,  however, is more than twice the price of the Canon.

My plan was to test it on my Canon 6D against the Canon 24-105mm L series zoom set to about 85mm focal length and a Mamiya M645 55mm f/2.8 medium format lens through an adaptor. Right off the bat, I must point out that it is a bit unfair to compare the 24-105 zoom against prime lenses, but after all, it is an L series lens that Canon touts as very high quality.

Also, since the Mamiya has a larger image circle than the other lenses, which were designed for a full frame sensor, only the centre portion of the Mamiya’s projected image was being used. As we know, aberrations are more pronounced at the edges, so the Mamiya has the advantage of having only its sharpest central area used in the comparison.

The test was not a rigorous scientific one. I used a test chart downloaded from the Internet for my basic comparison of resolution and contrast, then took a few outdoor shots for a more subjective head-to-head. I tried to frame up the shots the same in each case, since I was using different focal lengths (85mm and 55mm). That way, the necessity of unfairly having to enlarge the 55mm shots (and enlarging the apparent pixel size) would create a disadvantage for the Mamiya lens. Similarly, I tried matching the exposures in the camera as much as possible, but inevitably, a bit of exposure tweaking in post was needed to align the white end of the histograms. I felt this was necessary to get a fair comparison of contrast.

Test Chart Results Compared
I must preface this comparison by saying that focus is critical for a fair comparison, and I can’t guarantee that in each case I was perfectly focused. I did, however, choose the shots out of many iterations that I think represented the best focus for each lens.

The images compared here were all cropped from 100% views in Photoshop. At 100% , you are very close to seeing the limitation of the sensor in resolving detail, so even a premium lens may not seem razor sharp at such magnification.

Having said that, and with focus differences aside, the comparison is fair. Looking at the chart centre comparisons below, the Zeiss and the Mamiya seem to be comparable on a sharpness basis, while the Canon is slightly softer. This is most apparent in the fine rings in the ‘bullseye’, and in the vertical tapered stripes (next to the 16, 18, 20 marks on the right) where the Canon looks a little mushy. In fact, the Mamiya seems to have the best contrast in that zone, but critical focusing may have given it a slight advantage over the Zeiss.

Zeiss - test chart centre
 

Mamiya -test chart centre

Canon - test chart centre

Strangely, the Canon seems to give the best contrast with blacker blacks, which I would have expected the Zeiss to excel at. This, however, may be due to the fact that the Canon was not resolving the faint dot pattern (from printing the chart) that made up the black areas, and was blocking them in as solid black. Keep in mind also that the exposure was tweaked on each to ensure the white area in the centre of the ‘bullseye’ was matched across all three in terms of intensity to allow a comparison of blacks, and thus contrast.

The corner comparisons are where the differences became apparent. While both the Zeiss and the Mamiya (ignoring the chart tilt) maintained nice right angled corners, the Canon showed flawed geometry. In addition, the Canon also went very soft. Again, perhaps it’s unfair to compare a zoom's performance versus a prime's, but it’s convincing enough to make you think twice about using a zoom in a critical shoot. Also, we don’t know how the Mamiya would stack up against the Zeiss in the corners, because we’re really only using the centre portion of the Mamiya’s image circle. While very subtle, I would say the Zeiss was slightly sharper than the Mamiya in this area.

Zeiss - test chart corner

Mamiya - test chart corner

Canon - test chart corner

Subjective Outdoor Results Compared
Ok – this is where it gets even less scientific, but after all, the subjective result is probably what will convince you whether or not a particular lens will work for your type of photography.

For this comparison I used a population of one (scene, that is). Not totally representative of what a lens can capture, of course, but time eludes me on this little project.

A tripod was used to centre on the same flower bud, but slight tweaks on tripod height and position, to frame all three shots equally to compensate for focal length differences, resulted in slight differences in perspective. In Photoshop I also tried to equalize white balance and exposure.

 

 

 

To me, the Zeiss is the winner in this scenario. While the Mamiya appears to be comparable in sharpness (using the little hairs on the leaves as a reference), the contrast overall is stronger on the Zeiss. While the Canon’s contrast seems comparable to the Zeiss’, its sharpness didn’t stack up to either the Mamiya's or the Zeiss'.
Conclusions: Are the elite lenses  worth the coin?
From the moment that you first look through the Zeiss and examine the first shot, the reaction is “Wow – this is a nice lens”. After all, the heft of the all-metal barrel and the size of the glass definitely give you the impression of quality.

Before succumbing to the desire to just buy it no matter the cost, soberly think about your budget and the type of shooting you do. If you do high end portraiture or regularly sell landscapes through galleries, you can probably afford to cut to the chase and buy the best. On the other hand, if photography is a part-time money-maker, or even just your obsession, the budget-conscious do have more affordable alternatives.

I should make one other point. While the maximum aperture of the Zeiss was a lovely f/1.4, every lens performs at its worst, aberration-wise, at maximum aperture. While bokeh is gorgeous at f/1.4, there was a definite soft-focus look at that aperture. Subjectively, I don’t think the lens cleaned up until about f/2.8. That’s ok, because the brighter the maximum aperture, the sooner you achieve optimum performance when you stop down.  Bokeh still looks great at f/2.8, whereas a lens that has a maximum  aperture of f/2.8 may not perform will until you stop down to f/5.6 or smaller.

For my budget, the Zeiss are tempting, but a little too rich. I would prefer to  find used medium format lenses and adapt them to the 6D, because both the test chart and subjective comparisons show that the sharpness (while perhaps not the contrast) compare favourably to the Zeiss. One caveat: medium format lenses are somewhat vintage. Be sure to confirm that there are no signs of yellowing or coating separation on the glass before making your purchase from afar on eBay or other sites.

As a final note, don't discount vintage 35mm film lenses (with appropriate adaptors). I was going to include one in this review, but felt I needed to pull in the reins. Subjectively, I can tell you the Asahi Pentax 55mm f/2 I have was as sharp as the Mamiya and Zeiss at the centre. I suspect corner performance may not be as good, though. But for the price......

USING SPEEDLITES FOR BETTER RESULTS

Speedlites , simply stated, are accessory flashes that attach to the hot shoe of your DSLR. By raising the light source above the camera, the speedlite produces a more pleasing effect than the harsh head-on shadow cast by the camera’s built-in popup flash.

But there’s more to speedlites than that. Most have swivel heads that allow you to “bounce” the flash off a ceiling or wall to provide a more diffuse light source. This has the effect of softening the shadows behind the subject and more evenly filling in shadow areas on the face. Because the flash head is now well separated from the camera body, you can also add light modifiers such as a collapsible softbox. As the name implies, it softens or diffuses the light, allowing you to aim the speedlite directly at the subject rather than depending on a bounce surface. Outdoors in daylight, these are often used as a “fill” light to overcome the harsh shadows cast on the face by mid-day sunlight.

Better yet, speedlites have their own internal battery supply, so they can be separated from the camera and used in the same way as studio strobes. By mounting them on light stands and firing them with inexpensive radio triggers, you can have studio lighting anywhere, without the need for AC power. Add lightweight bounce umbrellas to diffuse the light evenly, and you can have as many light sources as you like filling an area with even, soft light.

So how do you configure this portable studio? Most of the small stuff can be bought on eBay much cheaper than at local retailers, but bulky items like light stands and umbrellas, in my humble opinion, can be sourced locally for not much more than overseas online pricing. And you have the benefit of not having to wait 2-3 weeks for product.

When it comes to speedlites and radio triggers, however, I would like to make a recommendation. While I want to stress that I have no affiliation with the company, I have found products made by Yongnuo of China to be well made and very cost effective. Just be sure that when you make a purchase that the models you choose are designed for your specific DSLR model. In some cases, their triggers are also compatible with camera manufacturers’ speedlites, but why not buy a speedlite that works as well at a third of the price?

So what is a radio trigger? If you decide to use a speedlite off-camera, you will need a means of making the flash fire when you press the camera’s shutter button. The transmitter mounts on your camera’s hot shoe and sends a signal to the receiver, which you mount on the light stand. The receiver has a hot shoe built onto it, so you simply attach your speedlite to it. Some triggers, like the YN-603, are actually transceivers, so you can use them as either a transmitter (when mounted on the camera’s hot shoe), or a receiver (when the speedlite is attached to it).

Now, there are two types of speedlites that you should be aware of: manual (such as the YN-560 III) or full ETTL (such as the YN-565 EXII). ETTL stands for Evaluative Through The Lens, meaning the camera and speedlite work together through connections on the hot shoe to ensure that the light output from the speedlite gives optimal exposure.

YN-560 III in off-camera configuration

A manual speedlite, however, requires that you set the light output on the speedlite itself by either trial-and-error or by using a flash meter. Studio setups are typically all manual. The YN-560 III has a very nice feature: built in radio receivers that are compatible with the YN-603 trigger. This means that you only need a single YN-603 on the camera’s hot shoe, rather than also attaching one to each of the speedlites to act as receivers. One feature of a manual flash is that its hot shoe connection is comprised of only a single contact point. This makes it compatible with a wide range of cameras, as opposed to an ETTL speedlite that has an arrangement of multiple contacts that are specific to a camera brand. This is why you must buy an ETTL speedlite specifically for your camera.

 

 

Yongnuo also makes an ETTL radio trigger called the YN-622. Two of these transceivers will allow you to trigger a compatible off-camera ETTL speedlite like the YN-565 EXII in full ETTL mode, meaning you don’t have to manually set the speedlite output. The camera reads the light from the speedlite through its lens and sends commands to the speedlite over the YN-622 radio link to adjust its output. You can also control groups of speedlites with these so that each has a different output level, but still controlled by the camera.

YN-565 EXII operating ETTL through a YN-622

Another benefit of using the YN-622 and a compatible speedlite on the camera is that you can use that speedlite in ETTL mode while simultaneously triggering a manual flash with a second YN-622 off-camera. That way, you can quickly move from a studio setup with two lights to a roaming subject using the single ETTL speedlite on your camera (such as in a wedding reception). The YN-622 simply attaches to the camera’s hot shoe and the speedlite attaches to the YN-622’s hot shoe.



Stacked YN-603 on a YN-622

While the Yongnuo product line is great value for money, however, I get the sense that the company doesn’t have a uniform design vision. For example, the built-in trigger receiver in the YN-560 III doesn’t exist in the YN-565 EXII. Where this becomes an issue is if you want to use the 560 and 565 together in a setup (with the 565 set to manual mode) and trigger them both from a single YN-603 trigger on the camera. Also, you can’t mix and match YN-603 and YN-622’s in a setup because they’re not compatible. You can, however, stack one on top of the other on your camera’s hot shoe and have each trigger their respective receivers.

By doing a little research, you can configure a professional lighting setup for well under $500. Just be sure that you are buying product that is 100% compatible with your camera make and model

New Digital SLR Photography Book for 2014

"Digital SLR Photography Demystified" is my new venture in the world of photography instruction.

Originally conceived as a reference for students taking my Digital Photography for Beginners course, it also serves as a standalone guide for those who prefer self-instruction. Written in the same way as my easy-to-understand teaching style, it includes ten practice exercises to reinforce the concepts presented.

The content goes beyond the "Beginners" course by covering panoramic, high dynamic range (HDR) and night photography. Also included is a comprehensive Glossary of Terms and a Quick Solution Guide.

While no one book can explain the functions of all camera makes, a generic model is used to help the reader locate a button or menu item on their own camera, using industry-standard icons where applicable.

Currently, the book is available as a 96 page paperback from Lulu.com for $14.95 CDN + shipping (slightly less in $US). In the coming weeks, it will also become available on Amazon and Barnes & Noble websites under the ISBN 978-1-312-03258-3.

Use the QR code below to review the book on Lulu.com.

WINTER EXPOSURE

Have we had enough of cold and snow yet? Unfortunately, it doesn’t look like it’s going away any time soon.

So – if you have the itch to get out and take some photos anyway but are disappointed with the results, the problem may be with exposure. The photographic kind, that is. The camera meters (measures) the light coming through the lens and tries to ‘average’ the exposure to a mid-grey (referred to as 18% grey).

With Evaluative (Canon-speak) or Matrix (Nikon-speak) metering, the camera reads the light over several zones in the viewfinder. If a bright scene covers most of these zones, the computer in the camera will calculate the exposure based mostly on these bright zones. In winter scenes – guess what – snow tends to dominate the scene and the camera tries to expose it to a mid-grey. So, if your winter shots are a little dark, that’s the reason.

There are two fixes for this: spot/partial metering and exposure compensation. I find the latter is the easiest solution so I’ll address it first.

 
Exposure compensation is a means of overriding the camera’s metering, by allowing you to over- or underexpose by an amount chosen by you. It’s normally controlled by pressing the +/- button and turning the command wheel on the camera. As you do, a bar graph (depends on the camera) will indicate by how many stops you are changing the exposure.

Since the snow caused your camera to underexpose, you need to dial in some overexposure or + exposure compensation. I typically use about one and two-thirds stops, but you may need to use something different depending upon how much of the scene is occupied by snow. Just don’t forget to dial it back to zero when finished, because it won’t go away when the camera is turned off.

 

Partial or Spot metering relies on something that has a mid-grey tone (although it can be a colour like facial skin) centred in the viewfinder. It effectively ignores all the white snow around it and exposes based on the face or mid-grey object.

I hope this helps, because there’s nothing I can do about the weather. If you need a little more help,  check out my photography courses at www.filmscapes.ca

PERFECTING THE PANORAMIC

It’s getting easier to make panoramics with today’s cameras. Some will actually do the stitching together for you without the need for software. But unless you take control of the process, you may be selling yourself short on results.

Here is a checklist you can follow in preparation for shooting your panoramic. It may look a little daunting, but it will become old hat with a little practice:



• Arrive Early   In order to get through this checklist, you’ll need the extra setup time before the light changes!
• Use a Tripod   A panoramic is comprised of a series of adjacent photographs stitched together in software to form a single wide-angle image. In the stitching process, the software does its best to match adjacent images seamlessly, but there is usually a bit of twisting required to match them perfectly. This requires cropping the final panoramic. Unless the individual images are shot with the camera absolutely level, the twisting and subsequent cropping may get to the point where much of the scene is lost.
• Level the Tripod and Camera  I have a bubble level on the base of my tripod which I use to get the legs levelled up first. I also use a bubble level that slips onto the camera’s hot shoe, which I use to do fine levelling adjustments. The reason for using both is that uneven legs may give me the impression that I have a level camera at one position, using the hot shoe level. But as I pan, I would likely see the level drift off centre. Once you have tweaked both the legs and the pan/tilt head, try panning through the intended range of your panoramic to ensure you have optimized both.
• Set the Camera to Manual (M) mode   Imagine you’re shooting a panoramic made up of photos that have dramatically different light content. In any mode but Manual, each photo would be exposed differently because the light metering system built into your camera tries to set the exposure automatically to an average 18% grey level. In this scenario, the panoramic wouldn’t look right because the inherently darker scenes would be forced to look lighter compared to the others, resulting in a patchy-looking result. Sure, you could try matching the exposures in software before stitching, but it’s easier to get it right in the camera. I pan the entire scene first, using the digital meter to set a shutter speed/aperture combination that averages a normal exposure. As a result, some shots will be over-exposed slightly, others under. As long as the histograms for all photos don’t show any clipping at the black or white points, you’re OK.

  

  The camera's digital light meter. Zero is optimal exposure.

  

 

 

 

 

 

 

•  Set the Camera ISO setting to a fixed value  Some cameras have Auto ISO setting. Per the last point about exposure, ensure you choose a fixed value (eg. 200) rather than Auto.
• Set the Camera to manual focus  Most SLR lenses have a switch allowing you to disengage the motor that automatically focuses your lens before you take a shot. For panoramics, set it to manual focus (MF). The reason for this is that as you pan to take each of the shots, the camera may refocus on something close in or far away, especially if there’s an object in one of the shots that is prominent. That could result in uneven focus across the panoramic. Pick one focus setting that works for the whole panoramic and leave it there.
• Take the Camera out of Auto White Balance  Similar to the argument in the last two points above, you don’t want the camera to change the colour balance as you pan because of changing content. Choose one of the presets instead of Auto. For instance, if shooting on an overcast day, use the Cloudy Bright setting. If shooting an indoor scene under fluorescent lights, use the Fluorescent setting. If you have access to a photo editing program that allows white balance adjustment, don’t worry about what preset you choose, because you can correct all the shots by the same amount if the preset you chose results in wonky colour.
• Overlap the Photos  Panoramic software uses artificial intelligence to look for similarities between adjacent shots then join those features seamlessly. In order to provide those reference points, you need to provide overlap in content between adjacent shots. I typically overlap by 25%. If I start my panoramic on the left end then take subsequent shots by panning left to right, this means that my second photo contains the rightmost 25% of the first photo. I’ve made marks on my tripod head that tell me how far to pan before taking the next shot while allowing 25% overlap. I have marks for both landscape format and portrait formats. But, of course, those marks only work for one focal length, which brings me to the next point….
• Use a “Normal” focal length  For a consumer digital SLR with an APS-C sensor, normal focal length is about 33mm. Anything shorter is considered wide angle. The wider you go, the more barrel or pincushion distortion will be introduced. This may make it harder for the stitching software to match the photos without twisting the photos to counter this effect. The more twisting, the more cropping of the final panoramic you’re likely to have to do. Of course, you could use a longer focal length without this concern, but remember that depth of field reduces with focal length.  I use a 35mm prime lens for panoramics, adapted from an old film camera.
• Choose the Right Content  I like shooting sky panoramics. I found out the hard way that sometimes the stitching software will not work properly with some content – particularly soft images with few hard edges. Some sunset clouds fit that description. Sometimes, it helps to include some hard edge detail (like treetops) to assist in the stitching. You can always crop it out later.

• Choose the Right Software  I find that the ‘premier’ photo editing package gives too many choices when creating panoramics. You have to try them all to find out which one works best, but my experience is that I generally don’t like any of the results. On the other hand, a cheap and cheerful package sometimes works better and with less fuss. I particularly like ArcSoft’s Panorama Maker, which came bundled free with my point-and-shoot camera.

 

 

3 overlapping images stitched together, before cropping. Note how the software has to distort the images to make them match up.



As you can see, the process of shooting panoramics like a pro is very manual. If you’re shooting a scene with moving clouds, set up marks on your tripod so you can rapidly make all your shots before the clouds change position and shape. And finally, resist the temptation to use more than 3 or 4 shots in a panoramic, because you’ll end up with a long, skinny picture sitting in the middle of your print that will look silly.

Lenses for the DSLR: Crop Factor and Image Circles

(Not to be confused with crop circles)

 

A few months ago, I wrote about how sensor size affects the way different cameras capture an image when using the same lens. To refine the topic a bit further, I’m going to introduce two terms: crop factor and image circles.

Why bother with this techno-babble? Well, as stated in the earlier article, if you now own a consumer DSLR with an APS-C sized sensor, chances are that when you want to replace it down the road, your next camera will have a full frame sensor. This means that the lenses you have now are going to behave differently on your future camera, or, worst case, will force you to buy at least one new lens.

Now for my definitions:

Crop Factor – The ratio of apparent magnification provided by the same lens when switching between two different-sized camera sensors.

For example, a lens on an APS-C sized sensor will produce an image that appears magnified by a factor of 1.6 times over the image captured by that same lens on a full frame sensor. The actual image projected by the lens is the same in both cases (note that the focal length of the lens is assumed to be fixed), but the smaller APS-C sensor sees fractionally less of the projected image than the full frame sensor. In effect, the APS-C sensor is ‘cropping’ the image, thereby giving the impression that it is magnifying part of the image.

Image Circle – The diameter of a circular image projected by a lens when focused on a surface. It is usually expressed in millimetres.

In this context, the lens is projecting the image from a scene in front of the camera onto the back of the camera. At the back of the camera is the sensor. In order for the image to fully cover the sensor, the image circle must be at least as large as the diagonal measurement of the sensor. Otherwise, we would see a darkening in the corners of the resulting image where the projected image didn’t fully cover the sensor. This is called vignetting.

One important fact to note is that lens manufacturers make two different types of lenses for DSLRs: full frame and digital-only. For example, Canon full frame lenses are designated EF and digital-only are designated EF-S. They may have exactly the same focal length, but the difference is that the full frame version projects a bigger image circle, more than sufficient to cover the full frame sensor. The full frame version works fine on both full frame and APS-C cameras, whereas the digital-only version would cause vignetting on the full frame camera. Note: while the mounts look the same on the EF and EF-S lenses, there is a protruding 'shoulder' on the EF-S lenses that prevents you from mounting them on full frame cameras.

Now, you would be tempted to think that using a full frame lens on an APS-C sensor when compared with the digital-only lens would result in apparent magnification, just like the crop factor, because the smaller sensor is only capturing part of the full frame lens image circle. An important difference, however, is that if you looked at the projected images from the full frame and digital-only lenses (with the same focal length) under the exact same conditions, an object in the scene would be projected at exactly the same size. Why? - because the full frame lens actually captures a wider field of view. In other words, putting the full frame lens on an APS-C camera will project peripheral information outside of the sensor, while the digital-only lens will not project that peripheral information but will adequately cover the sensor.

If that didn’t do it for you, here’s a scenario that might explain the difference between crop factor and image circle. Hopefully, the accompanying graphics will help:

1.  I have an APS-C sensor DSLR in front of me. I also have two lenses – both are 50 mm focal length, but one is designed for full frame and one is designed for digital-only. I have the camera set up on a tripod aimed at an object (X) which sits in the middle of the viewfinder. I try each of the lenses in turn and focus on the X. I notice that the X appears to be the same size in each case, and fills the frame from top to bottom. As long as the focal length on both lenses is the same, there is no change to the size of the image or the apparent angle of view in the captured image. The only difference is that the full frame lens is projecting a bigger image circle with more peripheral information (the dots) that the sensor doesn’t see anyway.

 

2.  I now take the full frame 50 mm lens and put it on a full frame sensor DSLR. The angle of view now appears wider than it did on the APS-C camera and the X is smaller (doesn’t fill the frame from top to bottom) by a factor of 1/1.6 (62.5%) because of crop factor in reverse. Saying it another way, the full frame lens now just covers the full frame sensor as opposed to the APS-C sensor which cropped the full frame lens image circle and caused apparent magnification. Again, focal length remained the same, but sensor size changed. Note that the full frame sensor captures some of the peripheral image (dots) that the APS-C sensor didn't.

 

3.  If I put the digital-only 50mm lens on the full frame camera (not generally possible as explained above), the object size is the same as in 2., but I see severe vignetting in the corners. This is because the image circle of the digital-only lens is too small for the full frame sensor.

To tie this discussion up with a nice ribbon, we can summarize where crop factor and image circle come into play as follows:

 

• Crop factor is a consideration when using the same lens on two different cameras with different sensor sizes.
• Image circle is a consideration when using two different lenses with the same focal length but different image circle sizes on the same camera.

Taking this one step further, what if you had a full frame DSLR, but you wanted to use a lens of the same focal length (ie. 50 mm) but with a bigger image circle than the full frame lens provided? Those of us geezers who used to use medium format film know that the lenses for those old cameras projected an even bigger image circle than full frame DSLR lenses because medium format film sizes were larger than a full frame sensor. So, as long as you were comfortable shooting in manual mode, you can adapt your medium format film lenses to your DSLR for reasonable cost. Why would you need a bigger image circle if the full frame lens covered your sensor? Well, with a bigger image circle, you can (with the right adapter) use tilt and shift functions over a wider range than you could with a full frame lens.

But tilting and shifting is another subject for another blog post….