[mst]

Usually, if you set aperture and shutter time to corresponding values, you can expect the final exposure (picture brightness) to be constant. So, if you close the aperture by one stop and double the exposure time, the resulting image should show the same brightness as before.



This is true for the Tamron, also, except for f/2. So, if you go from, say, f/2.8 with 1/125s shutter time to f/2.0 with 1/250s, the f/2 shot is darker than the f/2.8 shot.



[quote name='curriguy' timestamp='1297457039' post='6065']

Also, when you say visibly darker - how much is it underexposing by? a stop? two stops?

[/quote]



Around a half stop, I'd guess. The visible impression is a bit higher, since vignetting also increases at f/2.



[quote name='curriguy' timestamp='1297457039' post='6065']

Do you know why this is happening? Or do you have a reasonable guess as to what's going on?

[/quote]



Sorry, no idea. Except that f/2 might be really f/2 geometrically, but it's obviously not f/2 in terms of transmission.



[quote name='curriguy' timestamp='1297457039' post='6065']

(I'm sure you're sick to death of hearing about this unusual lens)

[/quote]



Not yet <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='' /> Apart from this issue, it's actually a nice piece of glass. I still own it, but that's mainly for the upcoming D7000 reviews. I'll probably sell it afterwards.



-- Markus