[rollei_list] OT: T-Stops
- From: Marc James Small <marcsmall@xxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Thu, 19 Nov 2009 16:32:29 -0500
At 04:14 PM 11/19/2009, Richard Knoppow wrote:
> About the 1950's some makers of lenses for professional
>motion picture cameras began to offer "T" stops. These were
>f/stops calibrated for the actual transmission of the lens.
>The idea being that for motion picture use, where scenes
>have to be intercut, its important to match the exposure. T
>stops were offered on single coated lenses. While multiple
>coating increases transmission somewhat there is still
>absorption in the glass and more complex lenses tend to have
>lower transmission than simpler lenses. The difference
>between ideal f/stop transmission and actual transmission
>can be considerable, especially in un-coated lenses. Some
>older uncoated lenses had nearly a stop of loss. Some
>aftermarket calibration was also available.
T-stops were originally developed by Bouwers for
use on his Old Delft catadioptric lenses as a
means for accounting for the light loss caused by
the central obstruction (the reflecting spot) in
such lenses. This was then taken over by the
astronomical community and is still very much
alive there today. Questar, for instance, rates
its telescopes in t-stops rather than by a focal
ratio. Even most classical reflecting telescopes
(Newtonians, Cassegrainians, and true Gregorians) are now rated in T-Stops.
The brief use of T-Stops by the motion picture
industry in the US is most interesting but the
concept is a decade older and survives to this day in rating telescopes.
Marc
msmall@xxxxxxxxxxxx
Cha robh bàs fir gun ghràs fir!
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