TranquilPhotos.com

This came up tonight, so thought it may help someone else as well.

What determines what your depth of field is? Most think that the aperture size is the most important factor for the size of the field. This isn’t true at all.

The three factors that determine your depth of field are:

1. Your final focal length, by which I mean your lens’s focal length x your sensor magnifier.

   If you have a crop sensor, for example a 40D, your magnifier is a 1.6. So a 50mm lens is actually a 80mm lens.

   The factor is that the longer the focal length the smaller your depth of field is.

   Example: With everything else equal, a 50mm lens has a depth of field of 5 feet, but a 200mm lens at exactly the same situation will have 3.6 inches of depth.

2. The distance from you to the subject.

   How close the subject is to you is also a very important factor, or rather your focus point is the determining factor.

   The closer your focus point (past your minimum focusing distance) is to your lens the smaller the depth of field is going to be.

   Example: A 50mm lens focusing at a subject 8.6 feet away (100 inches) will have a depth of ~45 inches. If the same subject was moved closer to the lens, say 16 inches away the depth of field would be reduced from 45 inches to 1 inch.

3. Lastly, the aperture size of your lens.

   The last option is the size of your aperture, as the smaller the aperture is the more depth of field you will have. Same goes true for the opposite, the larger your aperture size is, the less depth of field you will have.

   Example: At f/7.1, the depth of field could be 1.3 feet. However at f/2.8 the depth is going to be reduced to 0.5 feet, and f/32 5.8 feet of depth.

This is also in order of most effectiveness, the longer your focal length the less the aperture size matters. Same is also true for the distance to subject and your aperture. The focal length and the distance to subject are just about neck and neck as far as importance. Both have a huge effect on the final result, however again the change in size of the focal length does more to effect the size of the field than the distance to the subject.