Olympus-OM
[Top] [All Lists]

Re: [OM] Oly supplimentary lenses

Subject: Re: [OM] Oly supplimentary lenses
From: "Terry and Tracey" <foxcroft@xxxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 11 May 1998 18:13:24 +1000
>
>The formula to calculate the exposure factor is very simple.  It is the
>bellows length squared, divided by the lens focal length squared.  So if I
>have a 50mm lens and 100mm of bellows (or other means) extension, then the
>exposure factor is 100 squared divided by 50 squared, or 10000 / 2500 = 4.
>An exposure factor of 4x is two stops more exposure than would be indicated
>by a flash meter positioned at the subject.  So if your flash meter
>indicated 1/60 sec at f/11, you would need to adjust your lens to f/5.6 to
>get the right amount of light on the film.
>

This is true if the lens is symmetrical. Most macro ones are, and other ones
are way out. Measure the apparant iris diameter from the rear of the lens
and the front. If the same, they are symetrical.

Another way I use is effective aperture = aperture *(magnification + 1). A
50mm lens with 100mm extension is 2 times. +1 =3. Times 11 is ~ 32. This is
3 stops from 11. I've lost 3 stops of light. This gives a different answer
to your equation. I've tested this by metering at zero extension and adding
to 1 times. The calculated result and TTL meter have always been the same.

Foxy


< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz