01-04-2011, 12:18 PM
[quote name='mst' timestamp='1294131755' post='5332']
That's just a matter of reporting, isn't it? Canon cameras display the physical aperture, while Nikon cameras show the effective aperture. Of course, both (have to) use the effective aperture for metering.
Anyway: no issue here. The Tamron already starts to report slightly different effective apertures at the distance we measure MTF at, but I made sure to use the correct physical aperture settings. f/22 in the charts is reported as an effective aperture of f/25 by the lens (which means "fully stopped down").
Yes, I know, that LL masterpiece ... but the total amount of magnification will probably hardly make any impact in our measurements. And of course, as I understand it, this happens at large apertures, not at f/22.
-- Markus
[/quote]
How does the reporting happen then? Yes, when we set aperture to "f22" on the camera, the aperture is still at f2. The lens is not reporting anything. When we make a photo, the lens gets closed to "f22" on Canon.
So what goes on with Nikon? We set an aperture, but the lens remains wide open. Yet, when we focus closer, we see the aperture "reporting" a smaller set aperture. The aperture mechanism is not reporting that. So how does that happen? What is driving that? The lens is not closing its aperture mechanism more, and if you have an unknown lens, the reported aperture is not changing obviously.
It is merely the fact that it changes that is interesting.
So, now back to the DXO/LL wide open trickery. We now also know that cameras apparently can cheat with amplification.
Now I am combining the two... apparently, lenses can get a Nikon camera to report a different aperture than set, probably from measurements done by the manufacturer during design. So, with macro lenses and the bellows factor for instance, they can get the camera to show smaller apertures than originally set, when we focus closer.
When we also take into account that ISO-related amplification can be used to make exposure match what we are expecting, we could imagine a scenario here where the Tamron on Nikon does not really close to f22, but that the amplification is being used to let the exposure time match a real f22.
I know, this sounds like a big "why would one want to do that", especially since smaller f-values with macro photography are not used for longer shutter times, but rather for more DOF. It would have been done with resolution tests in mind, to show better results on review sites....
Anyway, what is the most curious here is the difference of measurements between the Canon EOS 50D (measurements are the way we expect) and the Nikon D200 (measurements make little sense).
That's just a matter of reporting, isn't it? Canon cameras display the physical aperture, while Nikon cameras show the effective aperture. Of course, both (have to) use the effective aperture for metering.
Anyway: no issue here. The Tamron already starts to report slightly different effective apertures at the distance we measure MTF at, but I made sure to use the correct physical aperture settings. f/22 in the charts is reported as an effective aperture of f/25 by the lens (which means "fully stopped down").
Yes, I know, that LL masterpiece ... but the total amount of magnification will probably hardly make any impact in our measurements. And of course, as I understand it, this happens at large apertures, not at f/22.
-- Markus
[/quote]
How does the reporting happen then? Yes, when we set aperture to "f22" on the camera, the aperture is still at f2. The lens is not reporting anything. When we make a photo, the lens gets closed to "f22" on Canon.
So what goes on with Nikon? We set an aperture, but the lens remains wide open. Yet, when we focus closer, we see the aperture "reporting" a smaller set aperture. The aperture mechanism is not reporting that. So how does that happen? What is driving that? The lens is not closing its aperture mechanism more, and if you have an unknown lens, the reported aperture is not changing obviously.
It is merely the fact that it changes that is interesting.
So, now back to the DXO/LL wide open trickery. We now also know that cameras apparently can cheat with amplification.
Now I am combining the two... apparently, lenses can get a Nikon camera to report a different aperture than set, probably from measurements done by the manufacturer during design. So, with macro lenses and the bellows factor for instance, they can get the camera to show smaller apertures than originally set, when we focus closer.
When we also take into account that ISO-related amplification can be used to make exposure match what we are expecting, we could imagine a scenario here where the Tamron on Nikon does not really close to f22, but that the amplification is being used to let the exposure time match a real f22.
I know, this sounds like a big "why would one want to do that", especially since smaller f-values with macro photography are not used for longer shutter times, but rather for more DOF. It would have been done with resolution tests in mind, to show better results on review sites....
Anyway, what is the most curious here is the difference of measurements between the Canon EOS 50D (measurements are the way we expect) and the Nikon D200 (measurements make little sense).