Originally Posted by
abruzzi
Correct me if I’m wrong, but as I read that he says that the fresnel optics cause a focus shift due to the focal length of the fresnel lens, and the whole thing should be shimmed back 1/3 the thickness of the fresnel. But what isn’t clear, and the author glosses over very quickly, is that 1/3 focus shift assume the ground glass is in the correct position. So if the fresnel is placed where the GG usually goes, then the GG is placed on top of it, it is already being shifted back 1x the thickness of the fresnel, so in fact, you’d want to shift the the two forward by 2/3 the thickness of the fresnel, something many cameras can’t do.
For this reason, the rear mount, between the glass and you, seems a more practical solution. It also has the benefit of being easily removable. I’ve only found fresnels to be useful on wide lenses. Normal and long lenses, even f9 lenses, seem fine to me without fresnels.
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