10-27-2010, 01:25 PM
[quote name='_sem_' timestamp='1287995098' post='3769']
Have they adjusted metering perhaps? [/quote]
According to my tests so far, it doesn't seem so.
[quote name='_sem_' timestamp='1287995098' post='3769']
My guess regarding the exposure issue with this lens is that the aperture works as designed, but that there is another rim in the lens blocking the light more than the aperture diaphragm at inf and intermediate focus (not at close focus, you get F/2 there).[/quote]
Certainly a point to consider. So far I did all formal exposure testing at infinity. However, the issue was clearly visible to me in the field, too. At any distance.
[quote name='_sem_' timestamp='1287995098' post='3769']
And that the autoexposure issue is due to the lens "thinking" it meters at F/1.8 (the metering aperture position is a bit wider than that of F/2), which in fact gives no more light than say F/2.5.
[/quote]
This is the part that I don't understand, really. In my opinion, all "thinking" should be done by the camera. And, actually, there's not really much "thinking" to it anyway, the camera simply has to meter the light coming through the lens with wide open aperture (and it doesn't really make any difference if it does so with a single cell light meter or a thousand dot matrix metering ... it's the camera in the end that meters and matters). It actually doesn't really need to know what that aperture is used (that's how stopped down metering works with old manual lenses, right?), unless it needs this information to "scale" the metering for other apertures (read: any modern lens).
But to be honest: I have no clue how the metering and information and parameter exchange between lens and camera actually works. I just find it strange that especially Tamron seems to struggle quite a bit in this regard, but only with F mount.
To be fair, I just got the Sigma 70-300 OS back from service. It seems to show very similar behaviour as the Tamron 70-300 VC ... which is significant overexposure especially at the short end.
-- Markus
Have they adjusted metering perhaps? [/quote]
According to my tests so far, it doesn't seem so.
[quote name='_sem_' timestamp='1287995098' post='3769']
My guess regarding the exposure issue with this lens is that the aperture works as designed, but that there is another rim in the lens blocking the light more than the aperture diaphragm at inf and intermediate focus (not at close focus, you get F/2 there).[/quote]
Certainly a point to consider. So far I did all formal exposure testing at infinity. However, the issue was clearly visible to me in the field, too. At any distance.
[quote name='_sem_' timestamp='1287995098' post='3769']
And that the autoexposure issue is due to the lens "thinking" it meters at F/1.8 (the metering aperture position is a bit wider than that of F/2), which in fact gives no more light than say F/2.5.
[/quote]
This is the part that I don't understand, really. In my opinion, all "thinking" should be done by the camera. And, actually, there's not really much "thinking" to it anyway, the camera simply has to meter the light coming through the lens with wide open aperture (and it doesn't really make any difference if it does so with a single cell light meter or a thousand dot matrix metering ... it's the camera in the end that meters and matters). It actually doesn't really need to know what that aperture is used (that's how stopped down metering works with old manual lenses, right?), unless it needs this information to "scale" the metering for other apertures (read: any modern lens).
But to be honest: I have no clue how the metering and information and parameter exchange between lens and camera actually works. I just find it strange that especially Tamron seems to struggle quite a bit in this regard, but only with F mount.
To be fair, I just got the Sigma 70-300 OS back from service. It seems to show very similar behaviour as the Tamron 70-300 VC ... which is significant overexposure especially at the short end.
-- Markus
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