Originally Posted By: siberia37
Well diffraction comes into effect on the other side of the scale- the high f numbers.

Oh, so I did not have the physics right in my mind, there. laugh In any case, I understand.
So, This must be due to different refraction indexes of the various elements in the lens itself, I suppose.

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So you will find most lenses don't allow you to stop down the aperture past say f/22, even though it is mechanically possible to go higher they don't want you too because you will get unsharp results. Just for a historical aside, Edward Weston had one of his lenses modified to go to f/256, he then complained to Ansel Adams (in a letter) of unsharp results with the said lens. Ansel Adams then explainined that modifying lenses in such a way wasn't a good idea because of diffraction problems.

Yes, that'd be nice to see some more in depth explanation on what exactly causes diffraction at low aperture. My (wrong) assumption before was that a greater aperture, the incidence of light rays would be such, especially at the edges of the lens, to create high diffraction, but clearly there's something more complex happening while rays cross the various components of the lens.

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Simply put it's because if you take two lenses and frame a shot at the same magnification (in this case 1:1) then depth of field is identical. In other words the picture you setup with both lenses would be identical, same framing, same subject size, same aperture. So depth of field is also the same. The only thing that affects Depth of field in this case is the aperture you use and/or the Image Format (digital vs film, cropped digital vs full frame).

Oh right, I was assuming different focal length=>different aperture, but that's not necessarily the case. In other words, same aperture, same DoF regardless of the focal length...

Again, this is so good. Thanks all for the explanations! smile
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