Actually, it is do to the increase in focal length at closer distances. For
example, a 50mm f2.8 macro stretchs to 100mm when it is focused at 1:1, and
becomes an f5.6. We usually ignor this fact and just say you need to open it up
two stops at 1:1, but that is just a convient rule of thumb.
--graywolf
----- Original Message -----
From: Rofini <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, September 24, 2001 1:03 PM
Subject: Re: lens brightness
> Shel wrote:
>
> >Well, Martin, there's a fly in your intellectual ointment: What
> >happens to exposure calculations when one is not using a camera
> with
> >TTL metering? Sure, the TTL camera may meter the scene
> properly
> >regardless of what the actual aperture is, but if one is using
> a
> >hand-held meter, or a non-ttl metered camera, such as an older
> model
> >or certain rangefinders, then your theory is worthless, since
> even in
> >those cases proper exposure's are made using the aperture
> stated on
> >the lens.
>
> A factor affecting light transmission will be lens extension.
> Set to any given aperture, charts typically show that a 50/1.4
> lens will need about 1/3 stop more exposure when focused at 18"
> than it needs at infinity. This may be due to the size of the
> image circle being cast over the negative at varying extensions.
>
> Mark Rofini
> -
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