Actually the focal length increases - physically - as you crank the lens out when
focusing closer. That�s why the f-stop and focal length are correct only at infinity.
All the best!
Raimo
Personal photography homepage at http://personal.inet.fi/private/raimo.korhonen
-----Alkuper�inen viesti-----
L�hett�j�: Rofini <[EMAIL PROTECTED]>
Vastaanottaja: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
P�iv�: 24. syyskuuta 2001 19:48
Aihe: 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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