In this final tip on getting “clear” images, we’ll consider depth of field (also known as DOF). Depth of field just refers to how much of your image or “field” is in focus. If there is a very narrow zone of focus, so that most of the image is blurred out, we call that a shallow depth of field.
Notice how this image has blur both in front of and behind the candle. It doesn't matter where the blur is - blur is caused by a large aperture and shallow depth of field.
In order to get a picture that is sharp and in focus from front to back (a deep depth of field) two things are necessary:
1. To use a small aperture (f16 or above is a good place to be but will vary depending on your focal length)
2. To focus toward the end of the lens’s focusing range.
Let’s look at #1 first.
Using a smaller aperture causes more of the scene to be in focus from front to back. Sharp focus, even with the same aperture, is enhanced by using a shorter focal length.
Check out these shots of the lantern for instance.
f2.8 200mm f29 200mm
So for example, if I want a sharp image from front to back, I might choose an aperture of f16 or higher.
As a side note, if I’m using a 200mm lens, even at f29, I might still see some blur in the back (or front) of the image. However, if I use a 16mm lens at a small aperture of f22, I probably won't notice the blur as much there (mainly because the background will appear farther away). On both of these shots, I'm focused at the near side of the lens's focusing range. (This leads to point #2, by the way).
200mm @ f29 (notice the minor blur) 16mm @ f29 (minor blur not as noticeable)
Now let’s look at #2.
This point is a minor one. It can help enhance sharp focus, but if you don’t get it, just make sure you get #1!
Every lens has a focusing range. That is, it can focus close up, and it can focus far away. The range of each lens is different. For instance, my non-macro 16-35mm lens has a close focusing distance a few inches from the subject.
My non-macro 200mm lens has a close focusing distance of 4 feet from the subject. So in other words, the closest I can get to the subject and still have my lens focus on it is a few inches for the 35mm lens, and 4 feet for the 200mm lens.
Now each lens also has a far focusing limit. This is the point after which everything behind that point is also in focus - in other words there’s no more “layers” of focus to be had. The lens is at the end of it’s focusing range. This is called infinity. Infinity is designated by the little sideways ∞ symbol on your lens barrel by the focusing ring (if it's marked). If your nearest object to you in the frame is in focus at infinity, then everything behind that object will be in focus too.
This shot is not focused at infinity. It is focused at the "minimum focusing distance" of a few inches from the lens. As a result, my hand is in focus, and the entire rest of the scene is very out of focus. 27mm @ f2.8
This shot is focused at infinity ∞. Notice that now my hand is blurred in the very near foreground, but the entire rest of the image is in focus.
Now notice that I've removed my hand, and the car is the main subject closest to the lens. However, even though the car is only about 10 feet away, the lens is focused at infinity, and the car PLUS everything behind it is in focus even at f2.8.
In summary, depth of field sharpness is helped by framing your shot so that objects in it are farther away from you than the lens’s near focusing point. For the 200mm lens, that might mean 20 or 30 feet away. For the 35mm lens, that might only mean 10 feet away.
There’s no set number or distance that will work all the time, because each lens has it’s own characteristics. The best way to figure it out is - yup - just to go experiment. The more you play with your lenses, the more you’ll figure out what they can (and can’t) do.