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From: hofbauer@utcsri.UUCP (John Hofbauer)
Newsgroups: net.rec.photo
Subject: Re: Flashes and telephoto lenses
Message-ID: <2310@utcsri.UUCP>
Date: Tue, 11-Mar-86 18:45:59 EST
Article-I.D.: utcsri.2310
Posted: Tue Mar 11 18:45:59 1986
Date-Received: Tue, 11-Mar-86 19:47:31 EST
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Organization: CSRI, University of Toronto
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> 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.
> 
Not true, in principle. In practice, though, you may find it necessary
to open up the lens further, but not because of the extra glass in the
lens. Indeed, really long lenses (>400mm) may only have two or three
elements.

When using a flash in manual mode and calculating the exposure from
first principles you may compute an incorrect f-stop because of
violated assumptions. Flashes are rated by a Guide Number -- the
higher the number, the more powerful the flash. This is not a very
scientific measure but good enough in practice. The f-stop can be
determined by dividing the Guide Number by the distance. The Guide
Number, though, assumes a normal size room with average reflectance.
Therefore if you shoot outdoors at night you should open up a stop
or two to compensate for the lack of reflectance since the flash
essentially becomes a point source of light.

Another problem, which is more likely to arise with the use of
telephoto lenses, is using the flash beyond its normal range.
A flash can only deliver so much illumination and this, of
course, is governed by the inverse square law. It never fails
to amuse me to go to a grand outdoor event, such as a rock
concert, and watch people popping off their flash cubes. It
makes for a lovely light show but doesn't do the would-be
snapshooters a bit of good. With a long lens you are likely
photographing a subject which is farther away and so it may
be necessary to open up the lens more to obtain a correct
exposure.

Underexposure is generally due to a combination of these two
factors.

P.S. I hope no one brings up the subject of T-stops.