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lens marking question


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#1 AdamBray

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Posted 09 June 2007 - 05:02 AM

This lens is supposed to have a speed of f3.8, yet I can turn it so the f3.8 marking goes past the normal marking, almost to the red dot as shown in the picture. Is this normal? And what does the red dot represent anyways?

http://img174.images...dscn0442tu4.jpg

Edited by AdamBray, 09 June 2007 - 05:03 AM.

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#2 Laurent Andrieux

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Posted 09 June 2007 - 06:23 AM

The red dot shows the actual T stop, photometric. The lens is engraved in F stop, geometrical. This is why it's given for a 3.8 (T) stop. As to actually have a photometric stop of 3.8, you actually need to open it up a bit more (geometrically).

The difference beetween the two is the light loss caused by the lens. So that when you use an independant lightmeter, you should use the red dot as to expose correctly. Then, you can read the geometrical F stop you actually get, for depth of field calculation for instance.

If you use a through the lens lightmeter (TTL), just don't bother, the light loss will be taken in account, since it meters through the lens... then again, you can read the actual F stop if needed.
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