How do you exercise depth of field in practice?

Question

I'm really frustrated because many of my nicely framed photos are of poor technical quality, mainly because of the wrong depth of field or subject being out of focus. I always thought I know theory but I'm having hard time to put that into practice. How do you learn to set the right focus and depth of field in practice?

Here are some details of how I find hard putting theory in practice.

Playing with aperture. In theory I know that depth of field depends on the matrix size, the focal length, aperture and a distance to a photographed subject, lots of calculators online can compute depth of field for me. However, when I'm outside photographing friends and expect to have them sharp and the wall just behind them blurred I cannot really stop for too long, measure a distance to them, and do calculations. There's no time for that. I tried with 50mm lens doing on f/1.4 to take a photo of a couple standing 2m from me. Only first person, a woman, closer to me is in focus, while the other is blurred, just because, as I calculated later, depth of field was around 9cm. How, the hell, should I know that in real time?

Playing with preview function. Yes, there's a magic button in Canons to preview a frame with actually selected aperture but what I see really small and dark for larger apertures. That doesn't seem very practical to me.

Playing with focus points. Canon body let you choose focus points manually: central or side points. Another option is to chose central point. But when a situation happens I often have little time to update the focus point. The best solution I think is to keep a central focus point and freeze focus and then move lens.

Answer 1

knowing this

Lenses used to be marked with DOD around the scale on the focus ring.

You should learn how shallow it is for large - aperture shooting. Don't memorize inches, but think (nose-to-ear only just fits), (multiple-subjects lax fit), etc.

There are DOF calculator apps you can play with to form your own notes.

live preview

Does that body have a touch screen? My 70D lets me touch where I wish the focus to be. I never got the hang of moving the focal point around using the knobs and do what you say like in the old days when there wa only one: I did learn the "lock" button, in the first EOS bodies. The live preview on anything other than the 70D and 7Dii won't autofocus as fast as the hardware viewfinder. But doesn't the viewfinder have indicators that show what it focused on? You can tell from that if one or both faces are in focus, if you learn to watch it.

what aperture

You might choose something other than the widest possible for the situation, not just because of DOF but the lens' s own performance. If that's a prime lens ("nifty fifty"?) it should be ok wide ooen though.

But figure out what is a good (acceptable) dof for the situation. I noticed a picture taken of my neice at Dallas Comic Con for a local newpaper a few months ago in available light was full-frame f/1.2, and her eye was sharp but the field was so shallow that her ear was blurred.

That pro was maximising light, and knew that he must focus on the eyeball or the photo is junk, and the subject could not be turned or need interesting detail elsewhere on the head.

If you are shooting a coctail party with people gathered at tables, figure you need a dof of a foot or so for people sticking their heads together, etc. and don't go with f/1.4.

bracket and burst

Always shoot bursts. You are doing that anyway. Does the camera have a shift-AE burst? Try Magic lantern firmware and see if it does. Otherwise, rely on the flexibility of a good exposure and use a darker-that-optimum version if the brighter one is too shallow.

Use the knob for shift-AE. After learning the safe aperature values, shift the aperature/speed towards the smaller aperature you want, without leaving P mode.

Answer 2

JDługosz's answer was great. But honestly, the best method is simply to practice. It sounds dismissive, but after lots of practice, you start getting a feel for the right settings in a particular situation.

Use a model (friend, family member, mannequin, etc), and take test shots at every aperture value your lens will let you use. Take the best shots you can for these tests. Then, when you can do A/B comparisons with the shots in your computer, pay attention to what is focused and what is blurred in the shots.

You can spend the time "mathing" your photo knowledge (and believe me, I love the maths and physics of photography), but it's the practice that will most help you develop your intuition and knowledge of the right settings when you approach a particular scene.

Answer 3

There are many variables for calculating the depth of field, but in practice you only have to remember two or three combinations that work for you. To find them out, use DoF tables in a reverse way.

Some scenarios:

• For a headshot with a 50mm lens you decide that a safe DoF would be +/- 8cm. A DoF table tells you not to open more than f/2.8.

• A sports event: you know that the subjects are running around a lot, and you would be comfortable to have +/- 1m DoF. You estimate the distance to be 7m. The lens you bring with you is 85mm. The table would tell you not to open more than f/8.

For other photographers or scenarios this may be different. The point is that you define these scenarios for yourself, examine the DoF tables for your lenses and go to the event with just one number in mind: Not to open more than f/x.

By the way: Professional photographers do this for the shutter speed as well. When they go out to an event for example with a telephoto lens, they know their personal limits for a shake-free photo and don't go beyond.

Edit: If you want to have a blurred background, but cannot use a low f-stop, then increase the distance between subject and background.

Answer 4

I tried with 50mm lens doing on f/1.4 to take a photo of a couple standing 2m from me. Only first person, a woman, closer to me is in focus, while the other is blurred, just because, as I calculated later, depth of field was around 9cm. How, the hell, should I know that in real time?

This comes with experience. In the meanwhile, shoot a series with different f-stops and pick the right one. Seeing the results side by side will also help you estimate the right settings next time you shoot.

Canon body let you choose focus points manually: central or side points. Another option is to chose central point. But when a situation happens I often have little time to update the focus point. The best solution I think is to keep a central focus point and freeze focus and then move lens.

This needs to be used very carefully. Google "focus and recompose" to see diagrams showing why this technique can cause miss-focus.

Speaking of best technique, I think the best is if the photographer pays attention to what is going on and tries to predict what will happen in the next moment. I know this is not always easy, but it gives time to the photographer to think about the focus, exposure and composition a little bit. I think this is how most of the great photographs capturing decisive moments were made.