Night Photography Tips–The 30-Foot & 15-Foot Rules

If you’re shooting a night-sky scene that has a foreground, you might be tempted to stop the aperture down several stops from wide open so you’ll have enough depth of field (DOF) to keep everything sharp from near to infinity. But if you do this, you’ll sacrifice a lot of the light you need to get a good exposure in the sky. Chances are, you don’t need to do it, anyway. Assuming you’re using a wide-angle lens, which is likely in most scenarios involving Earth-sky images, you have tremendous DOF to work with, even at wide apertures.

The first thing I recommend is getting a good DOF calculator, either a printed chart or a phone app, so you can determine the DOF at any given focal length and focus distance. My favorite is an app called TrueDoF. It tells me that with a 24mm lens on my full-frame Nikon D800, I have full DOF from 13 feet to infinity when the aperture is at f/2.8. That’s all you need for most any night-photography scene. With the 14mm that I often use, you have full DOF from about 5 feet to infinity when focused at 10 feet. And that’s when shooting wide open at f/2.8! So you can forget any notions of having to stop down to f/16 or f/22 to get enough DOF.

You’ll need to consult the DOF charts when you’re shooting with a very close foreground to make sure you focus at the proper point (the hyperfocal distance), but if the foreground is not real close, you can get by with estimating. That’s where the 30-foot and 15-foot rules come in. As long as the foreground is at least 30 feet away and you’re using a focal length between 18mm and 24mm, you can safely focus on the foreground and have full DOF from it to infinity when using an aperture of f/2.8. If you’re using a focal length between 14mm and 18mm, the foreground can be only 15 feet away. Of course, if you stop the lens down to f/4 or farther, you’ll have even more DOF. (This is based on a full-frame image format.)

When I’m using a 14mm or 17mm focal length, I rarely shoot Earth-sky scenes that have a foreground closer than 15 feet, and when I’m shooting a 24mm focal length, the foreground is rarely closer than 30 feet, so these rules work very well for me. In those situations when I do shoot with closer foregrounds, I use the DOF calculator to determine the proper focusing distance. If the foreground is extremely close, like only one or two feet away, there just isn’t enough DOF at f/2.8 to get everything sharp. In those cases, I shoot two exposures, one where I focus on the sky and the other where the focus is on the foreground, and blend these in Photoshop.

Night Photography Tips is a nightly feature in which I give a quick tip for night photographers. It’s a way to share all the little things I do and think about in my night shooting, without boring you with my usual wordiness. If you have a tip you’d like to share, please email it to me and I might include it here, along with a link to your blog or website.

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