« on: July 27, 2013, 11:57:47 AM »
Sorry about the delay in replying, the weather has been (almost too) good in this week.
What it comes to slanted edge testing, this is where I disagree (partially). If we consider a slanted edge test with a body+lens setup, there are several issues in that what I'd think as a deal breaker for recovering the real point spread function as I know it.
First, the pixel pitch typically does not actually support sufficient sampling. Second, the slanted edge is considerably larger and thus the average of the line spread functions is taken over a comparatively large image block where PSF has probably changed by some amount - this is typical for wide angle constructs where there are several aspherical surfaces. And if the length of the slanted edge isn't long enough, there will be uncertainty in the slant angle and the sub-pixel sampling is then affected. Third, given the slant angle is small, this test methodology cannot differentiate between imaging quality of tangential and sagittal axes and can miss changes in the averaging direction completely.
For an extreme example, it would report the MTF of a cylinder lens system equal to a spherical lens system if it was aligned along the imaging axis. This mistake of course, is hard to imagine happening in real life, but extending the thought for a bit, it is easier to understand that decentered elements along one axis could be missed with this. For this reason, lens would need to be turned 90 degrees to determine both directions.
The bar chart quality assurance benches that I have seen are used as OK/NOK step in quality control. The actual MTF measurement benches magnify the known spot with a high quality microscope objective, and thus this measurement of the MTF is much more local, and for that reason I accept it as a representative PSF. The only people who I do know to have sampled the PSF directly are astronomers.
What I'm saying here is not that the slanted edge method in lens+body setup isn't useful in determining MTF (with certain error bounds), it is. It is also very useful in relative comparisons if all systems are measured in the same bench. But what it does not do is provide scientifically accurate MTF values, and additionally, the online reviews are usually about resolving power of a body+lens combination, but the macro-contrast level is not that often reported.
So I suppose it all boils down on what is accepted as a PSF.