02-27-2012, 12:29 PM
[quote name='wim' timestamp='1330342579' post='16223']
Hi Frank,
Unfortunately the lw/ih figures provided here on PZ are not usable for photographic resolution calculations, the reason being that these are figures calculated by the software with specific assumptions made which do not reflect photography in itself, but only produce analytical numbers useful for comparison of lenses tested on the same system (i.e., camera).
So, by no means are these system resolution figures, or anything comparable to that. Do also note that some lenses produce "resolutions" higher than the Nyquist frequency (i.e., lp/mm, if you like), which according to the resolution formulas is totally impossible. The system resolution is always lower than the lowest common denominator in the chain, because the formula to calculate the system resolution is
1/system_resolution = 1/lens_resoultion_at_aperture + 1/sensor_resolution
IOW, when lens resolution is infinite, the resulting system resolution is exactly equal to the sensor resolution. BTW, this has been a bone of contention for me with the PZ figures all along, and i have discussed this a few times already. It is also why I suggested quite a while ago to replace the lw/ih figures with just a quality type scale. However, demand was such apparently, that the lw/ih were reintroduced.
If you take into account that because of the diffraction limit, using the Rayleigh criterion, the maximum resolution is 1600/N, and for MTF-50 760/N, where N equals the numerical aperture value, e.g. 4 for F/4, it will become clear that the figures provided in PZ test reviews bear no relationship to anything we see in the photographic image as produced by the camera and lens.
Furthermore, there are only incredibly few normal photographic lenses which are diffraction limited at an aperture wider than F/4, maybe 2 I know of (if the reports are true), and potentially a bunch more around F/4, and most of the best lenses out there at F/5.6. This due to optical aberrations which cannot be entirely eliminated. At F/4 and wider apertures, optical aberrations tend to control the maximum resolution rather than diffraction limits.
These limits are valid for the optical centre, however, there always is fall-off in resolution towards the edges and corners of a lens, although that may not be a nice curve due to lens design, e.g., because of the use of aspherical surfaces or special glass. The fall-off is due to optical projection, effectively, which also causes optical vignetting. Without going into too much detail, in simple terms the same natural forces are at work here which cause vignetting, and on top of that there also is the fact that optical aberrations are stronger towards the edge of the glass, generally speaking.
If you would like to see proper lp/mm system resolution results, I'd suggest you take a look at lenstip. They produce system resolution figures, still at MTF-50 though.
BTW, there is an additional effect limiting system resolution, and that is the AA-filter. I estimate the effect of the AA-filter on average to lower resolution by about 5% to 8%, based on quite a few reviews I charted, but this does vary from system to system, or rather, from camera body to camera body. F.e., D3X system reolution figures are essentially the same as those from a Canon 5D II, which seems to indicate that the AA-filter on the D3X is slightly stronger than that on the 5D II.
I have attached a quick overview of diffraction limits and maximum theoretical system resolutions at different apertures for a few camera, for both Rayleigh criterion and MTF-50.
HTH, kind regards, Wim
[/quote]
Thank you very much, Wim. Your explanation is very clear and very comprehensive. I appreiciate it very much. I think I understand it now.
The optical resolution derived by me simply exceeds the Rayleigh limit and this is impossible. Now I know that the PZ resolution numbers are not the system resolution, they are just some numbers generated by the software. Following your suggestion, I compared the resolution numbers of the Nikon AF-S 35mm f1.4 tested on D3x from PZ and lenstip. At F4, the PZ center resolution is "translated" to 82 lp/mm, while the lenstip resolution is only about 46 lp/mm (MTF-50). So, indeed the PZ resolution number is not the system resolution.
I guess there is no simple way to convert the PZ resolution number to the system resolution since otherwise the system resolution will be used in displaying the test result.
Best regards,
Frank
Hi Frank,
Unfortunately the lw/ih figures provided here on PZ are not usable for photographic resolution calculations, the reason being that these are figures calculated by the software with specific assumptions made which do not reflect photography in itself, but only produce analytical numbers useful for comparison of lenses tested on the same system (i.e., camera).
So, by no means are these system resolution figures, or anything comparable to that. Do also note that some lenses produce "resolutions" higher than the Nyquist frequency (i.e., lp/mm, if you like), which according to the resolution formulas is totally impossible. The system resolution is always lower than the lowest common denominator in the chain, because the formula to calculate the system resolution is
1/system_resolution = 1/lens_resoultion_at_aperture + 1/sensor_resolution
IOW, when lens resolution is infinite, the resulting system resolution is exactly equal to the sensor resolution. BTW, this has been a bone of contention for me with the PZ figures all along, and i have discussed this a few times already. It is also why I suggested quite a while ago to replace the lw/ih figures with just a quality type scale. However, demand was such apparently, that the lw/ih were reintroduced.
If you take into account that because of the diffraction limit, using the Rayleigh criterion, the maximum resolution is 1600/N, and for MTF-50 760/N, where N equals the numerical aperture value, e.g. 4 for F/4, it will become clear that the figures provided in PZ test reviews bear no relationship to anything we see in the photographic image as produced by the camera and lens.
Furthermore, there are only incredibly few normal photographic lenses which are diffraction limited at an aperture wider than F/4, maybe 2 I know of (if the reports are true), and potentially a bunch more around F/4, and most of the best lenses out there at F/5.6. This due to optical aberrations which cannot be entirely eliminated. At F/4 and wider apertures, optical aberrations tend to control the maximum resolution rather than diffraction limits.
These limits are valid for the optical centre, however, there always is fall-off in resolution towards the edges and corners of a lens, although that may not be a nice curve due to lens design, e.g., because of the use of aspherical surfaces or special glass. The fall-off is due to optical projection, effectively, which also causes optical vignetting. Without going into too much detail, in simple terms the same natural forces are at work here which cause vignetting, and on top of that there also is the fact that optical aberrations are stronger towards the edge of the glass, generally speaking.
If you would like to see proper lp/mm system resolution results, I'd suggest you take a look at lenstip. They produce system resolution figures, still at MTF-50 though.
BTW, there is an additional effect limiting system resolution, and that is the AA-filter. I estimate the effect of the AA-filter on average to lower resolution by about 5% to 8%, based on quite a few reviews I charted, but this does vary from system to system, or rather, from camera body to camera body. F.e., D3X system reolution figures are essentially the same as those from a Canon 5D II, which seems to indicate that the AA-filter on the D3X is slightly stronger than that on the 5D II.
I have attached a quick overview of diffraction limits and maximum theoretical system resolutions at different apertures for a few camera, for both Rayleigh criterion and MTF-50.
HTH, kind regards, Wim
[/quote]
Thank you very much, Wim. Your explanation is very clear and very comprehensive. I appreiciate it very much. I think I understand it now.
The optical resolution derived by me simply exceeds the Rayleigh limit and this is impossible. Now I know that the PZ resolution numbers are not the system resolution, they are just some numbers generated by the software. Following your suggestion, I compared the resolution numbers of the Nikon AF-S 35mm f1.4 tested on D3x from PZ and lenstip. At F4, the PZ center resolution is "translated" to 82 lp/mm, while the lenstip resolution is only about 46 lp/mm (MTF-50). So, indeed the PZ resolution number is not the system resolution.
I guess there is no simple way to convert the PZ resolution number to the system resolution since otherwise the system resolution will be used in displaying the test result.
Best regards,
Frank