12-01-2010, 10:46 AM
[quote name='genotypewriter' timestamp='1291160485' post='4618']
It's a really difficult thing to measure, especially because stopping a lens down reduces optical vignetting that reduces the transmission difference between center and borders. This effect interferes with what they're trying to measure at the sensor level, simply put.
[/quote]
Exactly the point, gtw. I think that the light loss they measured is indeed the increase of vignetting at larger apertures. A vignetting would decrease the overall sum of light if one does not distinguish the distribution of light within the optical cross section. Optically, if the light in the outer regions of the lens would be lost at wide open aperture, then it must be similar to an additional aperture directly in front of the pixel, something they seem to assume by bringing in the angle at which the light hits the photon buckets. But in this case, the out of focus blur circles wouldn't show an increase in diameter as well when using wide open aperture.
And I guess that the increase in ISO for compensation was not measured but back calculated: they assumed that the amount of recorded photons has to be constant for the photo not to be underexposed, then brought in the measured light loss and calculated, what the "real" ISO must have been for the equation to be valid.
Christian
PS: one last idea: if one uses an old f/1.4 lens and does not program the data into the camera (or uses false data), the camera should not be aware of when to compensate and for how much. I wonder if the images would show the amount of under exposure that has been predicted.
It's a really difficult thing to measure, especially because stopping a lens down reduces optical vignetting that reduces the transmission difference between center and borders. This effect interferes with what they're trying to measure at the sensor level, simply put.
[/quote]
Exactly the point, gtw. I think that the light loss they measured is indeed the increase of vignetting at larger apertures. A vignetting would decrease the overall sum of light if one does not distinguish the distribution of light within the optical cross section. Optically, if the light in the outer regions of the lens would be lost at wide open aperture, then it must be similar to an additional aperture directly in front of the pixel, something they seem to assume by bringing in the angle at which the light hits the photon buckets. But in this case, the out of focus blur circles wouldn't show an increase in diameter as well when using wide open aperture.
And I guess that the increase in ISO for compensation was not measured but back calculated: they assumed that the amount of recorded photons has to be constant for the photo not to be underexposed, then brought in the measured light loss and calculated, what the "real" ISO must have been for the equation to be valid.
Christian
PS: one last idea: if one uses an old f/1.4 lens and does not program the data into the camera (or uses false data), the camera should not be aware of when to compensate and for how much. I wonder if the images would show the amount of under exposure that has been predicted.