[quote name='clkirksey' timestamp='1299682020' post='6636']
I am trying to reconcile the differences in MTF50 values between PZ and dpreview.com site say for the Canon 70-200mm f/2.8L IS II on an APS-C format body. To what extent does the PZ results reflect postprocessing sharpening via Imatest?
[/quote]
We are using a minor degree of post-sharpening simply to compensate the effect of the AA filter.
The AA filter as well as USM-type sharpening are both near linear in terms of sharpening.
So let's say that we've got a signal X. Assuming the AA filter dampens the signal by say 20% we try to use USM to equalize this to some degree. +25% would be perfect here.
e.g.
signal = 1 -> 1 * 0.8 (AA) = 0.8 -> 0.8 * 1.25 (USM) = 1
It is, of course, not possible to find the perfect compensation values which is why we're generally sticking relatively close to the default USM settings in the RAW converter. After all that's exactly what most users do.
If we didn't do some USM tuning the differences between the brands would be quite pronounced at times resulting in the illusion that the lenses of brand A are generally better than the lenses from brand B which is, of course, nonsense - they're all very close in reality. This issue is already a problem regarding the different megapixel base which is why I would actually prefer to go for an abstract scale altogether. Markus has kept me from doing so so far. :-)
Just to mention - the quality of the demosaicing engine (of the RAW converter) is also affecting the resolution. You may argue that you could use the same engine for all brands (e.g. dcraw or so) but these engines are never static. They are getting better (thus different) over time.. e.g. ACR has changed its engine at least 3 times as far as I can tell. So some sites may argue that they're sticking to the same engine but if they've a naive approach they'll get very different results after certain updates (Adobe did one change even in a minor version update for instance).
I am trying to reconcile the differences in MTF50 values between PZ and dpreview.com site say for the Canon 70-200mm f/2.8L IS II on an APS-C format body. To what extent does the PZ results reflect postprocessing sharpening via Imatest?
[/quote]
We are using a minor degree of post-sharpening simply to compensate the effect of the AA filter.
The AA filter as well as USM-type sharpening are both near linear in terms of sharpening.
So let's say that we've got a signal X. Assuming the AA filter dampens the signal by say 20% we try to use USM to equalize this to some degree. +25% would be perfect here.
e.g.
signal = 1 -> 1 * 0.8 (AA) = 0.8 -> 0.8 * 1.25 (USM) = 1
It is, of course, not possible to find the perfect compensation values which is why we're generally sticking relatively close to the default USM settings in the RAW converter. After all that's exactly what most users do.
If we didn't do some USM tuning the differences between the brands would be quite pronounced at times resulting in the illusion that the lenses of brand A are generally better than the lenses from brand B which is, of course, nonsense - they're all very close in reality. This issue is already a problem regarding the different megapixel base which is why I would actually prefer to go for an abstract scale altogether. Markus has kept me from doing so so far. :-)
Just to mention - the quality of the demosaicing engine (of the RAW converter) is also affecting the resolution. You may argue that you could use the same engine for all brands (e.g. dcraw or so) but these engines are never static. They are getting better (thus different) over time.. e.g. ACR has changed its engine at least 3 times as far as I can tell. So some sites may argue that they're sticking to the same engine but if they've a naive approach they'll get very different results after certain updates (Adobe did one change even in a minor version update for instance).