Another exception is mirror lenses. I did some measuring a while ago and
a lengthy post. the quick summary is that all 4 mirror lenses (350, 500,
600 & 1000mm) I checked (and the Zuiko, according to Gary's test) were
marked noticeably slower than the f-stop calculated from the actual
clear aperture with central mirror mount subtracted. It appears they are
generally marked with t-stop.
A lot of lens tests note the difference between f and t stop.
Dizel wrote:
Always ;) f/stops are different than real stops, called t/stops.
The f-stop is a geometric relationship between focal length and
effective aperture and does not take into account light loss within a
lens. Each air to glass surface reflects some light. So real
'opening', called t/stop is different than f/stop. This difference
depends of lens construction (number of elements, coatings used..)
and usually is NOT given. Only some companies, most notably Kiron,
callibrated their diaphragm scale in t/stops.
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