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Matt Brandon of The Digital Trekker did such a great job of explaining how depth of field does not change with focal length (considering aperture stays the same) that I am not going to type it all up again, but instead, link directly to his post.
What Matt says in this post is fairly simple for some, but mind-bendingly difficult for others to conceptualize. We’re all wired differently but I think Matt’s way of explaining the concept works well. Basically he’s pointing out, through a clickable image, how depth of field does not change with varying focal lengths.
The important caveat here is that he kept the subject the same proportions in the field of view. You have seen this in Hollywood films and it is known as the Vertigo or Dolly Zoom effect because the camera is moved closer, or further, from the subject on a dolly to keep proportions the same while changing focal lengths. Such as shown here:
This can further be verified by playing with a depth of field calculator such as DOF Master.
If I go to the site and put in Canon 7D, 20mm lens, f/8 and a subject distance of 3′, I get a total depth of field of 2.31′.
If I change my focal length to 30mm, my depth of field shrinks to .92′. It also means the subject has become larger in the frame, filling more of it.
So if I want the subject to remain the same size in the frame, I need to physically move back. In testing, I can find that I now need to be 4.65′ away from the subject to give the same proportion in the frame. This also gives me a depth of field of 2.31′.
If you don’t believe me, take a look at Matt’s post and then try it yourself. One of the aspects of photography that I love the most is the ability to test and confirm things. There is no black magic in photography, it’s just all light and physics. And art, it’s art too.