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| Re: Alt Lens FAQ |
I think "tiny prism" may still be an appropriate description of what a focusing screen consists of --- angled facets (much smaller and more varied orientation than the ones in a microprism focusing aid) for redirecting light rays. Old-school focusing screens were ground glass, producing somewhat randomly oriented scratches with a variety of angles; now the process is more controlled with laser cutting, but I believe the principle is the same. I say "prism" instead of "lens" because the focusing screen surface elements aren't aiming to form an image by converging a large bundle of rays to a point, but rather to grossly change the direction of rays coming from a particular angle; given the extremely small aperture that the focus screen elements have, attempting to truly focus in a lens-like way would be futile against diffraction. Do you know whether modern, laser-cut focus screen facets are curved or just flat? Admittedly, a small wedge prism can be though of as an off-axis fragment of a lens (and a lens as a carefully ordered array of prisms).
Have a look at the flickr link at the bottom of this page: http://forums.dpreview.com/forums/read.asp?forum=1019&message=26635036
This is informative as well: http://forums.dpreview.com/forums/read.asp?forum=1019&message=38132287
I didn't find a really conclusive answer here. Perhaps electron microscopy images are needed to find out how the surface elements of *s screens are really shaped.
I also added CA, LCA, LoCA to the glossary
I find LCA and LoCA a bit confusing. LCA could mean both longitudinal and lateral CA. Why not LaCA to be more specific? (Frankly, I don't think I've ever seen "LCA" used outside of your posts.)
Anyway, nice additions. You didn't really have to credit me at the top of the OP.
One thing that could perhaps be changed a bit is to mention the use of adapters, and what they are in a single sentence, in the earlier sections (what are alt lenses or how do I use alt lenses). Alt lenses are very much about using lenses that have a different mount than the camera, by a simple mechanical adapter that allows the lens to be attached to the camera, as a rule with fully manual operation of aperture and focus.
Maybe you could make two sub-sections of "How do I use alt lenses", namely for adapting and converting.