Catching a Falling Star

There I was at Point Clark, Ontario on the edge of Lake Huron with a friend, innocently taking a time lapse sequence of the gorgeous night sky. Just a few frames from the end of my 150-frame sequence, probably the brightest meteor I have seen in recent years streaked across the sky.

Since I was shooting 20-second exposures at ISO 3200, there was plenty of opportunity for the meteor’s short life span to be captured. I was just lucky, however, that it didn’t perform its burnout while my camera was taking a 5-second breather between shots.

My intent that night was to capture the elusive Milky Way. Not only did I get the best shots ever of the MW and its bright core, but to capture it and a meteor in one shot?….Serendipity.

Tick that box on my bucket list.

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.

Guide Numbers and Watt-Seconds

Whenever I think about hauling lighting equipment to a location, the question comes up: Should I take a.c. strobes, extension cords and power bars, or will a couple of hot-shoe flashes, umbrellas and stands do it? Hmmmm...I know what I'd prefer.

The trouble is, hot-shoe flash output is (usually) expressed in Guide Numbers (GN) while strobes are rated in Watt-seconds. Sadly, there is no easy way to relate the two, so I made a quick setup to measure the output of each. I've always assumed that a 150 Watt-second strobe would blow away a flash in terms of light output, but I was about to be surprised.

Now, we should start with definitions. GN is the product of the distance of the flash to the subject and the f stop being used at a particular ISO setting (or film speed). Example: I set my flash meter at a sensitivity of 100 ISO and placed it at 10 feet from the flash. I fired the flash and read the f stop it was recommending. For a flash rated at GN 80 (ft - ISO 100), you would read f/8 on the meter (f/8 x 10 ft). Indeed, my Sigma EF 500 came in close at f/7.6 (hence a GN of 76).

Next, I set up the 150 Watt-second strobe at the same location as the flash, set it for full power, and blasted away. Here's the surprise: it read f/7.9 on the light meter, meaning its GN equivalent is 79, which is barely better than the Sigma. Admittedly, there are a couple of variables here. One is that the light meter is older and uncalibrated, and the Sigma's batteries may not have been at peak voltage. The point, however, is that on a relative basis you can determine how your strobe/flash sources compare. This is particularly helpful when the manufacturer of the flash doesn't provide the GN.

So GN is a measure of the amount of light available for proper exposure of a subject at a particular combination of distance from the flash and camera aperture setting, on a camera set at a particular ISO sensitivity. On the other hand, Watt-seconds are a measure of the electrical energy expended in the strobe tube (Watts are the rate at which energy is used, so multiplying by time gives energy used over that time period). This measure doesn't take into account the conversion efficiency of the tube from electrical energy to light energy, or the effect of the reflector behind it. Hence, GN and Watt-seconds can't be related mathematically.

Bottom line: measure the output of each with a flash meter so you know how they relate. Similarly, if you double the Watt-seconds you can expect the guide number to double, since it should add another stop to the flash meter reading.

Then, you write your results down, as I did, so that confusion doesn't ensue.