01-04-2011, 01:42 AM
[quote name='mst' timestamp='1294076651' post='5322']
That was my idea, too. In fact, the aperture not fully closing is the only reason that seems to make sense. However, I checked durations and "unfortunately" they are flawless:
f/5.6: 1/40s
f/8.0: 1/20
f/11: 1/10
f/16: 1/5
f/22: 1/2.5
The resulting files show no shift in exposure (f/22 files aren't any brighter).
Honestly, I'm absolutely clueless about the reason for the high f/22 resolution. Will be interesting to see if the lens shows the same behaviour on the D7000.
-- Markus
[/quote]
Quite puzzling. But there might be a (strange) explanation.
With Canon cameras, aperture settings are only done in the lens, there is no stuff going on between body and lens (well there is, but more on that later). With Nikon, that is different. We all know that with focussing closer, the bellows factor makes macro lenses less "fast": The closer we focus, the smaller the real aperture is. With Canon, the camera keeps on saying "f2.8", where on Nikon we can see the aperture figure in camera go to f3.5, f4 even, even though we do not change the aperture setting.
This means that, while the aperture mechanism does not change, the camera and lens together determine a different f-value.
And with Canon... only the user set f-value is being "used", not the "real" aperture.
There is something else going on too, "recently" discussed on internet. Wide aperture lenses seem to be too fast for sensors to deal with, and there seems to be tampered with ISO amplifications to make the exposure times match the f-value.
For instance, if I put my 35mm f2 lens on my 450D normally, the photo taken is a bit lighter than when I mount the lens in such a way that the lens/camera electrical contacts are not connected.
In other words... same lens, same lens opening, so same exposure, but different amplification of the signal.
This not only can be observed with Canon, also Nikon (and Pentax and others).
Now, combining these two things, one can imagine that maybe Tamron's lens does strange things on Nikon because "it can", closing down less and letting the camera "adjust ISO" accordingly?
It can not be the optical design, as that is identical for the different mounts, with identical aperture placement.
That was my idea, too. In fact, the aperture not fully closing is the only reason that seems to make sense. However, I checked durations and "unfortunately" they are flawless:
f/5.6: 1/40s
f/8.0: 1/20
f/11: 1/10
f/16: 1/5
f/22: 1/2.5
The resulting files show no shift in exposure (f/22 files aren't any brighter).
Honestly, I'm absolutely clueless about the reason for the high f/22 resolution. Will be interesting to see if the lens shows the same behaviour on the D7000.
-- Markus
[/quote]
Quite puzzling. But there might be a (strange) explanation.
With Canon cameras, aperture settings are only done in the lens, there is no stuff going on between body and lens (well there is, but more on that later). With Nikon, that is different. We all know that with focussing closer, the bellows factor makes macro lenses less "fast": The closer we focus, the smaller the real aperture is. With Canon, the camera keeps on saying "f2.8", where on Nikon we can see the aperture figure in camera go to f3.5, f4 even, even though we do not change the aperture setting.
This means that, while the aperture mechanism does not change, the camera and lens together determine a different f-value.
And with Canon... only the user set f-value is being "used", not the "real" aperture.
There is something else going on too, "recently" discussed on internet. Wide aperture lenses seem to be too fast for sensors to deal with, and there seems to be tampered with ISO amplifications to make the exposure times match the f-value.
For instance, if I put my 35mm f2 lens on my 450D normally, the photo taken is a bit lighter than when I mount the lens in such a way that the lens/camera electrical contacts are not connected.
In other words... same lens, same lens opening, so same exposure, but different amplification of the signal.
This not only can be observed with Canon, also Nikon (and Pentax and others).
Now, combining these two things, one can imagine that maybe Tamron's lens does strange things on Nikon because "it can", closing down less and letting the camera "adjust ISO" accordingly?
It can not be the optical design, as that is identical for the different mounts, with identical aperture placement.