Hi all.
As I am reading threads with two days lag (the web version is
_that_ slow), it all might have been answered already. I address
the issue of "bellows factor" and its difference in tele or
wideangle slr lenses, as well as T stop.
1) f/stop can't be just focal length/front diameter, because front
diameter is much larger in retrofocus wideangles (my 2.8/20 has
~65mm dia, but it isn't f/0.3 lens <g>).
Also, to even more complicate it, f/stop isn't just physical
diaphragm/aperture opening dividing focal length...
I can't find the exact calculation at the moment, but I
remember that it included such values like entrance pupil
diameter and exit pupil diameter, and that the diaphragm
diameter wasn't included directly!
2) T/stop (true transmission, in practice, than only computed,
theoretical, geometrical transmission). It is measured not
computed. e.g. one 4/400mm movie lens was stated as T/4.5
transmission stop. It is slower because of light losess at each
air-glass surface (due to reflectance of uncoated glass, even SMC
glass reflects some light). With formula on Boz's KMP, we get at
least 5% loss of light in complex 2.8 pro zoom (usually 12 groups
or more), with SMC. With advances in SMC this might be less. That's
T/2.94 instantly. Add to it manufacturers tolerances (about 5%
too), and 2.8 lens might test actually as 3 (like some 300mm are
tested to be less, ~280mm).
3) with retrofocus and tele designs, the usuall formula for close up
loss of light changes too (or rather we should use formula with
this in mind). Normally, lens "losess" 2 stops at 1:1
magnification AFAIK. With tele designs, lens loses up to 1.5x more!
(this is often cured by using some IF design, which sometimes
lessens the focal length). OTOH, retrofocus (wide) lenses loose
less light at 1:1 ! Maybe just 1 stop compared to normal 2 stops.
That's because again, the entrance and exit pupils come into it.
Entrance pupil of telephoto lens is large, but exit pupil is small.
Vice versa in retrofocus wideangle lens. The entrance divided by
exit (or vice versa?) pupil diameter is in the proper equation for
loss of light with closer focus than infinity.
So a plain tele lens without any fancy IF or REAR design might loss
even almost stop at very close portrait distance. Example
would be old 4/300 (not *) SMC K, or K 5.6/400 lenses.
If I find the proper formulas, I will post them, but I guess they
must be in any good photo encyclopaedia or on web (my focal press
encycl. is now packed away in crates, as is majority of my library
:(
Frantisek
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