Originally posted by W.j.christy I have been bothered for some time about the depth of field of a lens for aps-c and have been doing some research on the subject. What I have come up with is of course, the longer the lens, greater the aperture, or closer to the subject you get the shallower the depth of field.
I get the larger aperture and closer to the subject bit, what I don't understand is why a longer focal length can create shallower depths of field more easily that wider. I honestly thought that is was the reverse due to the distance from the subject being shorter in order to maintain the same field of view (i know the angle will stay the same or it should for a given focal length) .From my readings the reason for this is how the light is bent for the shorter focal lengths that cause it to create fairly large depth of fields where as the light is straighter going into longer focal lengths and this somehow corresponds to shallower depths of field.
My question is, can someone explain this to me on perhaps a 9th grade physics level so i can understand what is actually causing the shallow depth of field in the longer lenses?
the answer can be a little math intensive, but it all comes down to what people call an acceptable degree of sharpness.
the old standard is 1/100 of an inch, when viewing an image on 8 x 10 enlargement. this same definition also applies to camera shake, and image blur for camera shake.
it is referred to as the circle of confusion, and when applied to a point source of light, it refers to the limit where it no longer appears to the unaided eye, viewing from a reasonable distance as a point .
without getting into too much math, think about the difference between a telephoto and a wide angle lens, when you take a photo of a subject, from the same place, using the same aperture with both lenses. the wide angle lens appears to have very high depth of field, only because it magnifies the image less. zoom into or crop to the field of view of the telephoto, and then look at the two images,
aside from perhaps loss of definition in the wide angle image due to lens and camera resolution, you should see the same subject with the same depth of field.
depth of field as it applies to image sharpness is all about magnification.
it does not care whether the magnification is between lens and sensor, or in the final enlargement of the image, although where this magnification takes place is of interest in terms of the bokeh (the subjective quality of the out of focus area) a simple way to evaluate this is to simply take a shot using a wide angle lens and zoom in on your monitor. ghe bigger the enlargement the less sharp things look
i hope this truly answers the question