Depth-of-field (DOF) simplified
Well, maybe that’s a tall claim. But I’ll try to cover DOF in a way that gives you enough knowledge to make better pictures without getting into the nitty-gritty of this highly complex topic. If you’re a math person you can find plenty of interesting stuff to read elsewhere on the web.
Depth-of-Field: The portion of a photograph which is acceptably sharp
Let’s say your camera focuses on a subject that is 10 feet away. Let’s also say that the picture includes objects off to the side that are closer than 10 feet and that there are many objects behind your subject that are various distances away – say 12, 15, 20, 40 and 500 feet way from the camera. Some of the closer objects will also be in focus but those nearest the camera will not be. Maybe the objects behind your subject as much as 40 feet away are also in focus but everything beyond that gets a little blurry. Then, for that image, with that camera, with that lens and with that f-stop, the depth-of-field is from maybe 6 feet to 40 feet.
So, it’s a simple concept but the rub is that DOF is affected by the camera, the lens and the f-stop. (For you picky technical types all the lenses have the same DOF but due to magnification the practical effect is that they yield much different results).
Here are a few things to keep in mind:
- Point and shoot cameras have more DOF than DSLR’s
- The wider the lens the more of the whole scene will appear sharp. With a really wide, wide angle lens almost everything will appear sharp at any f-stop.
- The longer the lens the more critical DOF becomes. I have shot water lilies with a 250mm lens where the DOF is only inches – the front of the flower is sharp and the back isn’t.
- The more wide open the lens (f1.4, f2.0, f2.8) the less depth of field and of course the smaller the opening (f16, f22,f32) the more depth of field.
- The closer the subject the less DOF and the further the subject the more DOF. This means that in macro photography, where the subject is only inches from the lens, DOF is extremely shallow. You may very well need a tripod so that you can use a slow shutter speed and very high f-stop (f32 etc.) You will also get the best results with manual focus. You will want to focus on the part of the subject that yields the most DOF. This might be right on the subject or it might be an inch in front of the subject. Your cameras autofocus will sometimes get that right but just as often will get it wrong.
That may be a lot to digest so let’s see what that means in practice. Note that these numbers are for my camera, a Canon T1i. Your camera will yield different results. There’s a handy DOF calculator at www.dofmaster.com/dofjs.html where you can check your own camera and lenses.
Normal shooting – subject at 10 feet
|Lens||f-stop||Near limit||Far limit||Total DOF|
Take a look at the 18mm lens stopped down to f/22. Everything will be in focus from 2 feet to Infinity. Take a look at the 200mm lens at f/8. Only objects between 9.89 and 10.1 feet will be sharp.
I mentioned that very wide angle lens have extreme DOF. My 10-22mm when set to 10mm and focused at 10 feet provides DOF from 1.78 ft to infinity.
Now let’s look at what happens with a macro at 1 foot with 60mm lens
|f-stop||Near limit||Far limit||Total DOF|
As you can see not much in front of or behind the subject will be in focus but you will get better results with smaller apertures.
Of course lots of DOF or a little isn’t automatically good or bad; it depends on what you’re trying to achieve.
Here’s a shot I took with an 85mm lens set to f/5.6. Depth of field is quite shallow but that’s okay; I like the blurred background and much of the subject is acceptably sharp.
Now here is a shot with a 17mm lens set to f11. The entire scene is acceptably sharp. The camera chose the following autofocus point.
I’m guessing that was about 4 feet away. Going back to our DOF calculator at www.dofmaster.com/dofjs.html we see that DOF extends from 2.11 feet to 37 feet. Had I chosen a f-stop of f/2.8 only things between 3.27 feet and 5.15 feet would have been in focus.
I mentioned that point and shoot cameras have lot of DOF. Consider this shot from a few years ago with a point and shoot.
Even at f/4 this 11mm shot is sharp from front to back. With my first few cameras I was surprised at how much DOF there was compared to the film cameras I had used in the past. I pretty much gave up trying to achieve out-of-focus backgrounds with my first few digital cameras.
So what should you do with this information? Here are a few simple things
- Forget about DOF with wide angle lens and concentrate on it more as you use longer lenses.
- The closer the subject the more you should pay attention to DOF.
- If maximum DOF is your goal, shoot at small apertures like f/16 and f/22. This may be especially important for macro work.
- Use the DOF preview button on your camera
- Pay close attention to the indicator in your camera that shows you where the camera has set the autofocus point (mine is a red dot). Decide if that makes sense and if not either recompose or learn to use selective autofocusing if your camera supports it. Here’s just one example where it matters. For people head shots you have to focus on the eyes. The camera might pick out the nose or chin or even shoulder but don’t got with that. Make sure the focus point is on the eyes.
There’s one more concept I just want to mention. It’s called hyperfocal distance. This fancy term just means the point at which you should focus your lens to achieve maximum depth of field. It may be that you can achieve more depth of field by focusing on something other than your subject. I’m not going to try to cover that whole subject here because it just happens that Dave Johnson covers it in these two articles:
He also talks about depth of field calculators that are available for your smart phone.