11-08-2010, 03:19 PM
Sorry for the delayed response...
[quote name='mst' timestamp='1288185934' post='3817']
According to my tests so far, it doesn't seem so.
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]
I think in the field there are two things involved. First is the naked effect of the aperture. Second is the meter (autoexposure, and also M mode indirectly via you watching the meter).
I can clearly see the aperture blades move looking from the front using the DoF preview button. It certainly does get wider when decreasing the F stop. The metering position is even a bit wider than F/2.0. But at inf focus, watching the resulting histogram, there is no more light when going from F/2.4 to F/2.0. But there is at close-focus. Watching through the detached lens from the rear playing with the aperture lever observing the rims seems to confirm this.
[quote name='mst' timestamp='1288185934' post='3817']
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.
[/quote]
I'm sorry if I can't write clearly <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt=':blink:' />
I think the camera is aware at what aperture it is metering, which is at a bit wider than F/2.0, say F/1.8. But I assume the exposure will be miscalculated if what the camera in fact sees at position F/1.8 is no brighter than at say F/2.5 (the point where the aperture diaphragm actually becomes the limiting rim - its movement earlier being a kind of a dead-zone, onyl with some effect on bokeh). I would assume this could be a problem with adapting the lens to the F-mount, and the kind of thing Tamron could have a problem admitting.
Another thing is the light meter which seems "rigged". I mean, the lens is obviously metering always at the same widest aperture position. But changing same-exposure combination in M mode (dec F-number and inc shutter time, both 1 stop at a time) with the same white-wall scene the meter readout changes when approaching wide-open (not taking a shot at all, so the aperture stays in place all the time). It seems to cover up for the less light at widest aperture. I was very surprised to see such behaviour and I'd kindly ask someone knowledgeable to have a look into it. Also noticed this with Nikkor 18-200VR but not with 50/1.8D. I guess this could be reasonable for more user-friendly experience with superzooms, but a questionable practice with primes.
[quote name='mst' timestamp='1288185934' post='3817']
According to my tests so far, it doesn't seem so.
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]
I think in the field there are two things involved. First is the naked effect of the aperture. Second is the meter (autoexposure, and also M mode indirectly via you watching the meter).
I can clearly see the aperture blades move looking from the front using the DoF preview button. It certainly does get wider when decreasing the F stop. The metering position is even a bit wider than F/2.0. But at inf focus, watching the resulting histogram, there is no more light when going from F/2.4 to F/2.0. But there is at close-focus. Watching through the detached lens from the rear playing with the aperture lever observing the rims seems to confirm this.
[quote name='mst' timestamp='1288185934' post='3817']
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.
[/quote]
I'm sorry if I can't write clearly <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt=':blink:' />
I think the camera is aware at what aperture it is metering, which is at a bit wider than F/2.0, say F/1.8. But I assume the exposure will be miscalculated if what the camera in fact sees at position F/1.8 is no brighter than at say F/2.5 (the point where the aperture diaphragm actually becomes the limiting rim - its movement earlier being a kind of a dead-zone, onyl with some effect on bokeh). I would assume this could be a problem with adapting the lens to the F-mount, and the kind of thing Tamron could have a problem admitting.
Another thing is the light meter which seems "rigged". I mean, the lens is obviously metering always at the same widest aperture position. But changing same-exposure combination in M mode (dec F-number and inc shutter time, both 1 stop at a time) with the same white-wall scene the meter readout changes when approaching wide-open (not taking a shot at all, so the aperture stays in place all the time). It seems to cover up for the less light at widest aperture. I was very surprised to see such behaviour and I'd kindly ask someone knowledgeable to have a look into it. Also noticed this with Nikkor 18-200VR but not with 50/1.8D. I guess this could be reasonable for more user-friendly experience with superzooms, but a questionable practice with primes.