G Dan Mitchell Photography

Ivan just wrote and asked how I determine exposure times for night photography, so I thought I’d share my answer here in case the information is useful to others. This won’t be a comprehensive coverage of the subject by any means, but I’ll touch on a few key ideas.

First, for information about the techniques of nocturnal photography (and for some great examples of the photographic results and for workshop opportunities) I recommend a visit to the web site of The Nocturnes, the San Francisco Bay Area night photography group more or less run by Tim Baskerville. I’ve joined them on several night photography shoots, and the resources at their web site provide a ton of basic information that will save you many frustrating nights of trial and error.

Since so much of the basic information is available there, I’ll just add a few techniques and ideas that I find useful, especially for shooting with DSLRs. Here they are, in no particular order.

Use your camera’s long exposure noise reduction system – On my Canon DSLRs this setting causes the camera to make a second “dark frame” exposure following your “real” exposure. You might be tempted to forego this option since it doubles the time required for each exposure – your 60 second exposure will be followed by a 60 second dark frame exposure, while you stand there and twiddle your thumbs. (Or, do what I do – start composing your next shot.) The issue is that DSLRs are subject to increased noise and “hot pixels” with very long exposures. The dark frame provides the camera’s software with a reference that contains only the noise and hot pixels, and it can use this to map out (or subtract) the noise, etc from the actual exposure.

Rely on your histogram to check exposure – not on how the shot looks in the display. If the shot looks like what you see at night, in most cases it will be way underexposed – and, as a result, you’ll have a very noisy image and you may end up with artifacts such as banding. Instead, use an exposure that produces a balanced histogram curve – or, “expose to the right” as many of us like to say.

Shoot wide open at high ISO to calculate basic exposure – If you have a f/1.4 lens and can set your camera to ISO 3200, do so. Let your camera make an automatic exposure and use that as a starting point. For example, if you get a decent (though noisy and otherwise awful looking) exposure at ISO3200, f/1.4, and 1 second you can work backwards from there to get a better exposure. Switching back to ISO 100 means that you need 5 stops more light, so your exposure time will go from 1 second to 32 seconds. (Count 1, 2, 4, 8, 16, 32.) Changing your aperture back to a more reasonable f/8 means another 5 stops or about a 17 minute (!) exposure – if my quick calculations are correct. (Update: I just read today that if you can do this at ISO 6400 you can do a straight conversion from seconds to minutes – e.g. if the exposure at some aperture is 1 second at ISO 6400 the claim is that it will be 1 minute at ISO 100.)

Apertures and ISOs that work in the daytime are good starting points for night photography – If f/8 at ISO 100 is your preference for daytime landscapes, then there is a good chance that these will be about right for night time landscapes as well. It just takes longer. A lot longer.

Find something to focus on – Getting good focus stymies many night photographers. There are several approaches that you can try, and the right choice will vary depending on the shot.

• If there is a relatively bright spot in your composition – perhaps a light bulb, a reflection, the moon… – place an autofocus point right on top of it and see if it will acquire focus. Then switch AF off and recompose the shot.
• Sometimes there is nothing in the scene that will provide enough light/contrast for focus. When this happens, look around for something outside of your composition that is about the same distance away – focus on that, switch AF off, and recompose.
• Temporarily place a light in the scene – a bright flashlight will work  – and focus on it.
• You may be able to create a usable focus point in some cases by shining a laser pointer beam into the scene and focusing on it.
• If you have one “must work” shot to make and plenty of time, you could set up the shot and determine focus during the daylight and leave the camera in position.
• Manual focus may be a possibility – either by trying to get a point light source such as a bright star to focus manually through the viewfinder or by relying on the distance scale on the lens barrel.
• Since perfect focus may be more elusive at night you have another reason to perhaps use smaller apertures with their greater depth of field.
• If your camera has a “live view” feature, you may be able to manually focus in light light levels at which neither AF nor traditional manual focusing will work – highly recommend for night photography work!

Precise exposure time is less critical than you think – When shooting in the daytime very small changes in absolute exposure time can make a bit difference. If you shoot at 1/100 second, increasing the exposure time by 1/100 second doubles the exposure. But at night things are a bit looser. If you are making a 6 minute exposure, you’d have to be off by six minutes to double the exposure or by three minutes to cut it in half. With that in mind, a few seconds hardly matter at all. I don’t bother with automated timers. I just count seconds (or chimpanzees, or hippopotamuses, or whatever…) or use my watch.

Daytime exposure tricks often work just as well at night – When photographing high dynamic range scenes during the day I often make two exposures – one for shadows and one for highlights – and combine them using masks in post. The same situation often arises at night, especially if you have a shot that includes dark areas and artificial light sources. You may need to make one exposure that gets some detail out of the shadows and a second that avoids blowing out the light source.

Lens hoods are extra important with night photography – That may seem completely counter-intuitive, but when shooting at night you often have light coming from many more directions than when you shoot in sunlight, and the potential for flare increases. (You also may be a bit more likely to bump your lens into something…)

That’s a quick list. Hope you find it useful!

G Dan Mitchell :: Feb.11.2009
Technique
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