Hi Paul,


Quote Originally Posted by peterborough_photography
I fully understand DOF, what I cant understand is I only get the front of the image doing this on 70-200, I have several lenses that are as open as the 70-200 and never get the same effect.

Several lenses with as wide an aperture...but do they have as long a focal length as 200mm (or longer)?


As I'm sure you understand, there are three main factors that affect DoF:
  1. aperture - wider aperture with same focal length and subject distance means shallower DoF
  2. subject distance - closer subject with same aperture and focal length means shallower DoF
  3. focal length - longer focal length with same aperture and subject distance means shallower DoF



Factor #1 is pretty well understood by everyone - wide aperture means shallow DoF.


Factor #2 is partly why the fence behind your zebra is much more OOF than the fence behind your rhinos - you're a lot closer to the zebra than to the rhinos (subject-to-background distance is obviously a factor as well). Macro shooters have a clear understanding of this factor, which is why they struggle with DoF so much - shooting at working distances of a few inches means very shallow DoF, in the millimeter range even at narrow apertures.


Factor #3 means that longer lenses have shallower DoF - for example, that's why bird photographers shooting at 400mm often need to stop down (f/6.3 or f/8) to get even a relatively small bird completely in focus. Here's an example with my 7D at 400mm, f/6.3:





The oriole is in focus, but the branch in the lower right that also extends up through the frame is very blurred. That blurred branch was only a few inches in front of the branch on which the bird is perched.


If you've not had a lot of experience shooting at longer focal lengths, you might not have experienced the effect you're seeing in terms of foreground blur.


The other possibility, of course, is that you've got a defective lens, but I doubt that for two reasons. One, you stated you've tried three copies with the same result, and it really stretches the bounds of probability that you got three defective copies. Two, the platform that the lions are resting on, which is low in the frame like the grass in the rhino foreground, is in focus (but unlike the grass, the vertical part of the platform would be within/near the DoF for the shot).