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Adding slightly to John Corns' response and explanation, and adding to the
previous posts that started this one, an easy to remember rule of thumb on
this subject is:
Wide Angle increases the size of the foreground in relation to the background
Telephoto increases the size of the background in relation to the foreground
Also keep in mind that the term "normal" in reference to focal length means
that it resembles the magnification of your eyes, (not because most normal
lenses are 50 or 55 mm) and that angle happens to be the diagonal measurement
across the image you're making. Normal for a 35mm frame is about 43 mm; the
50 mm lens actually offers some magnification / cropping. For a 6x7 normal is
about 90mm, and 4x5 is about 162 mm. So if you shoot in multiple formats the
105 lens doesn't offer the same magnification in different formats -- the
focal length means that the center of the lens is 105mm from the film plane --
put a 50mm lens for 35mm film 50mm from a sheet of 4x5 and the image won't
cover the entire negative or transparency.
Also keep in mind that the F-stop is a ratio of the aperture vs. focal length
-- that number tells you how big the front element is in relation to its focal
length (how much light it gathers). That number has nothing to do with
quality of the glass or mirrors or whatever. Now, when you shoot with a wide
aperture, light passing through the very edges of the front element is
reaching the film. A small part of the reason why images shot "wide open"
with normal lenses often look that way is because the perspective at the edges
of the lens is slightly different from that at the center. A bigger part is
spherical aberation (imperfect parabolic shape of the bigger lens, result,
fuzzy image). I have no doubt that if Canon is marketing a 50mm f1.0 that
it's a high-quality lens. How big is the front element? -- if it's bigger
than 50mm and Canon is using just the center of the lens at wide open, the
resolution at f1 should be quite good.
John Corns is absolutely correct that compression has nothing to do with the
lens, but the camera's relationship to the subject in the frame. You can
stand next to a 500mm shooter shooting the same scene normal with the
locomotive in the same spot relative to the scenery. While the 50mm scene
will have more scene and the train will be distant, the compression in that
portion of the frame will still be there.
I missed some of the posts on this, so I hope I didn't duplicate anything.
....Mike Del Vecchio
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