20mm f1.7 for Night-time Street Photography

Started Feb 15, 2014 | Discussions thread
Ontario Gone
Ontario Gone Senior Member • Posts: 4,183
Re: 20mm f1.7 for Night-time Street Photography

Conor OBrien wrote:

So really the differences in sizes of the AF squares just relate to the accuracy in picking up the contrast of the subject, in this case the wall. So if there are identical photographs, one using the small AF square and one using the large AF square, both can produce the same photograph with the same amount of area in sharp focus.

Yes, this is correct. The amount of area you see in focus is called the depth of field (DOF). This DOF is determined by the aperture you set your lens at, and how close you are to your subject. The focus box just determines how small or large of an area the camera will search for something to lock on to.

Is this correct? If so, I think I will use a smaller AF area when photographing in the future. I assumed the smaller the AF square, the smaller the area of the photograph that will be in focus - good to know that is not the case.

Correct. The smaller boxes are designed to allow you to zero in on what exactly you want in focus. If you use a larger box on a person's face, it might lock onto the nose, or the mouth. Most people want the eyes to be the sharpest part, so a smaller box might allow you to point only at the eyes, assuring that is what is the point of focus.

A larger box is more likely to lock, but you might not get it exactly where you wanted. A smaller box gives you more precision. Hope this helps.

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