About central (diaphragm) shutters at high speeds/large ap..

On 20 août, 06:23, Peter <pir....RemoveThis@ktb.net> wrote:
>
> The problem can only arise if you are using fast film (Say EI 100
> or faster) in full daylight. The shutter makers didn't think you
> would normally be doing that, and certainly not with reversal
> film. If you were shooting 32 speed Ektachrome, you would
> never run in to the problem - at 1/500 second you would be using
> f/4 under full sun.
>
> Nowadays EI 100 is a normal speed for reversal film, and if you
> shoot under full sunlight, you may have to keep shutter efficiency
> in mind.

Definitely...

I take the opportunity of this thread for another question: some time
ago I read in an old manual that transmission losses in lenses without
multicoating were not negligible... I remember something like 30%
losses for a 4-element lens (Tessar). Can someone confirm that ?

>
> I've never shot B&W reversal in a still camera. What film and
> processing are you using?

Actually I used to shot with Agfa Scala, which is now discontinued.

I still have some rolls, however, and some stores still have some
also. I do not process it on my own, but rather send them to a lab (a
single one in France is still processing this film).

--
pehache

On Aug 20, 12:28 pm, pehache-tolai <pehach... DeleteThis @gmail.com> wrote:>
>
> I take the opportunity of this thread for another question: some time
> ago I read in an old manual that transmission losses in lenses without
> multicoating were not negligible... I remember something like 30%
> losses for a 4-element lens (Tessar). Can someone confirm that ?

For an uncoated lens with six air-glass surfaces
(as in a Tessar or other lens with three groups)
you get around 74% transmission (0.95^6),
of image forming light so your figure is about right.
A single coated Tessar will pass more than 90%
of the light (say 0.985^6).

But, some of the reflected light hits the film in the form
of flare, so in practice your exposure hardly changes
at all. The exposure recommendations for Kodachrome
did not change with the advent of coated lenses.
The case may be different in complex lenses with
more than 3 groups, but with Triplets, Tessars and
Sonnars it is close enough not to matter.

Even the difference between 74% (for uncoated)
and 90% (for single coated) is less than a third-stop,
and since at least some of the reflected light does appear
as flare, one shouldn't expect very much exposure
difference.

> Actually I used to shot with Agfa Scala, which is now discontinued.

It is too late for me to try Scala now. I have been planning to try
my own reversal processing, but I have never got around to it.

Peter.
--
pirwin DeleteThis @ktb.net