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From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: net.rec.photo
Subject: Re: Flashes and telephoto lenses
Message-ID: <14794@onfcanim.UUCP>
Date: Tue, 11-Mar-86 12:58:18 EST
Article-I.D.: onfcanim.14794
Posted: Tue Mar 11 12:58:18 1986
Date-Received: Wed, 12-Mar-86 05:07:02 EST
References: <1587@watmath.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: ONF, Montreal
Lines: 33

In article <1587@watmath.UUCP> rayazwinski@watmath.UUCP (Rick Yazwinski) writes:
>I have heard that when using a flash and, say a 135mm lens you have to change
>you f-stop to allow for the light loss through the extra glass.
>I have also heard that f4.5 is f4.5 no matter which lens you use.

It is, strictly speaking, true that f/4.5 is always f/4.5, since the f/number
really is just the ratio of the focal length of the lens to its
effective aperture.

But the f/number does not tell you how much light the lens passes - it tells
you how much light the lens would transmit if there was no loss of light
due to reflection or absorption or scattering in the lens.  In general,
the more complex the lens, the more light lost, and the more exposure
needed.

But a 135mm fixed lens may lose no more light than a 50mm fixed lens, because
it may have no more elements.  Zooms and extreme wide-angle lenses are most
likely to have loss-of-light problems.

For most purposes, I doubt if the extra light loss will matter (particularly
with flash, where it is difficult to set exposure exactly anyway).

Some lenses are marked with "T" stops rather than "F" stops.  In this case,
the markings really are accurate, because the transmission of the lens
has been measured and the light lost is taken into account.  I only know
of such lenses for specialized applications though (cinematography, view
cameras).

One thing you *do* have to watch out for are zooms that change aperture
as they zoom - the real aperture may be one full stop below the marking on
the ring.

Through-the-lens flash metering avoids all of these problems.