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05-09-2010, 04:43 PM   #1
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f stop versus t value

I have only seen this value refered to a few times but can anyone explain the difference between a lenses f stop value and its light transmission speed (t value)? for example, say you have two 135mm f2.8 lenses. One with a 55mm filter, and one with a 72 mm filter. Will the 72mm filter expose correctly at a faster shutter speed? I've only seen the T value listed in relation to mirror lenses which may shoot at a DOF of f-8, but expose more like an f-11. As if finding fast glass wasn't tricky enough there may be more variables then first thought. heck the coatings alone may reduce or enhance a lenses speed. I think when i have time I may try a little test with my 3 135 f2.8 primes and my DA*50-135 and see if exposures and shutter speed are the same for all. I imaging there are enough gurus who know far more than me who can help clear up the mystery. (thinking of falconeye here obviously.

shu

05-09-2010, 05:52 PM   #2
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I've only heard T values used in relation to film, and I'm pretty vague about them.

I was told it is similar to f value, except it relates to the amount of light actually hitting the film, not necesarily the size of the aperture. So maybe sticking a polariser on the front alters the T stop, but doesn't change the f stop?
05-09-2010, 06:47 PM   #3
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T stop is the ACTUAL transmission of the lens(the amount of light passing through it), F stops are calculated by focal length and the size of the iris* they don't take into account the amount of light absorbed by the glass. Ruining one frame because of lens based exposure inconsistencies is one thing; ruining several hundred feet of film is another matter entirely.

for example the Pentax K 50mmm f/1.2 aperture value is f/1.2 however the amount of light the lens actually transmits is T= 1.2.4

the 31mm f/1.8 limited aperture is rated at f/1.8 the transmission of the lens is T=1.8.7

*to the best of my knowledge

Last edited by Digitalis; 05-09-2010 at 09:35 PM.
05-09-2010, 06:53 PM   #4
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see the attached

F-number - Wikipedia, the free encyclopedia

it explains the development in a little more detail than Digitalis, but it essentually comes from the movie industry and film cameras with turret lenses. they had to be identically calibrated in transmission because the turret woudl be rotated to change focal length.

note that on a telephoto lens the F stop is dictated by the diameter of the front element, not the filter diameter

A 135 F2.8 lens only needs a 55 mm filter to fit the glass comfortably inside.

the diameter of the filter itself is not relevant that will be a selection by the lens builder based upon the element mounting techniques etc.

additionally with respect to light transmission. the difference in transmission between a super takumar and SMC coatings was to go from 98% to 99.9% transmission factor. By the time of SLRs, the lenses were pretty good. Otherwise flash photography would not work because you need to know accurate F stops to do the manual calculation

05-09-2010, 09:16 PM   #5
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QuoteOriginally posted by shuie Quote
I have only seen this value refered to a few times but can anyone explain the difference between a lenses f stop value and its light transmission speed (t value)? for example, say you have two 135mm f2.8 lenses. One with a 55mm filter, and one with a 72 mm filter. Will the 72mm filter expose correctly at a faster shutter speed? I've only seen the T value listed in relation to mirror lenses which may shoot at a DOF of f-8, but expose more like an f-11. As if finding fast glass wasn't tricky enough there may be more variables then first thought. heck the coatings alone may reduce or enhance a lenses speed. I think when i have time I may try a little test with my 3 135 f2.8 primes and my DA*50-135 and see if exposures and shutter speed are the same for all. I imaging there are enough gurus who know far more than me who can help clear up the mystery. (thinking of falconeye here obviously.

shu
The most striking difference between f/ and t/ is with a mirror lens. They are often a full stop short on light transmission. I would suspect it has to do with the hole in the doughnut - the area covered by the secondary mirror that obscures the light from the primary.
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