mpmendenhall wrote:
Too bad lenses don't typically come with drop-in slots for fixed aperture stops. With a precision circle --- or, even better, a bit of apodization for a soft-edged aperture --- you should be able to make a lens much more "well behaved" at small apertures, so all the lower-frequency effects could be cleanly reversed in software (instead of leaving irregular streaky artifacts with overly touchy deconvolution kernels).
There is a lensmaker that uses drop-in slots for fixed aperture stops, which are a precision circle (or you can make your own aperture shape)
It's called Lensbaby. It may not meet your standards for lens sharpness though
kwalsh wrote:
As to the above, I think this is an expected result. Remember, MTF curves are always shown linear and those of us that obsess about window functions look at everything in dB usually. Once you move away from rectangular to almost any other window function that attempts to suppress side-lobes you have a similar affect on the main lobe. The differences between the various non-rectangular window functions is primarily in the exchange between close and distant side-lobes - but those side-lobes are very low in a linear scale regardless of the function chosen. So I'd suspect that in a linear MTF we'd see all practical non-rectangular window functions looking about the same. ...Show more →
That makes sense.
I would also suspect, that as a practical matter, no real lens has macro-contrast at all affected by diffraction side lobes or anything else to do with the aperture. Baffles, optical coatings, etc. etc. are likely a big part of the game in very low spatial frequency contrast.
That also make sense, if we define macro contrast as the contrast at 1 line pair per 36 mm. I was thinking about macro contrast and micro contrast in a rather loose sense as the contrast at low and high spatial frequencies, respectively, without putting numbers to these frequencies.
Lens performance documented by Zeiss and Leica has 10 and 5 lp/mm as the lowest frequency, respectively. At these frequencies f/22 diffraction lowers the MTF of good lenses by a considerable margin. I don't know at which frequency flare becomes the limiting factor.
Oh, one last thing. I came across a link to this article in another forum recently. With regards to what we perceive as "sharp" and the effects of diffraction on MTF it is very interesting. It removes the question of aberrations by only dealing with pinholes but is still a good read. It is a short read despite the page count - giant text and lots of figures.
Toothwalker wrote:
Lens performance documented by Zeiss and Leica has 10 and 5 lp/mm as the lowest frequency, respectively. At these frequencies f/22 diffraction lowers the MTF of good lenses by a considerable margin. I don't know at which frequency flare becomes the limiting factor.
Ah, that's a good point. Those are actually probably spatial frequencies long enough that we'd perceive them as "contrast" in a macro or even nearly a global sense. Visual system is actually pretty insensitive to very long spatial frequency contrast, so these more moderate spatial frequencies are probably a good indicator of perceived contrast. And as you point out, are affected by diffraction in the more extreme aperture settings.
When measuring surveillance lenses (mostly quite "cheap" C-mount stuff, "cheap" being less than 1000$) I get a factor of about 1:4 in difference between different lenses in absolute contrast. That's about 2Ev difference in flare limited DR. With absolute contrast I mean the flare limited contrast at very low frequencies, 1-5 lp/mm. My flare target is a square pattern checkerboard backlit diffusion screen, about 2x bigger than the image field - so that you get some light pollution from outside the image as well.
The calculated MTF times diffraction is set as the ground truth, and when you divide by the the measured result you get the flare / random dispersion loss.
What's interesting about the tapered aperture is that if you build an F2.8 lens like the Sony/Minolta STF, the portion of the MTF curve that gets a noticeable increase in contrast transfer is well above the resolution limit of any modern APS or FF sensor. You get a good increase in contrast at 65lp/mm, and the result isn't "worse" than the normal circular aperture until you get to about 135lp/mm.
24MP APS is about 125lp/mm.
Considering that the apodization element is gradually phased out in effect as you stop the mechanical aperture down, the tapering effect is lowered by about 41% per each full stop of aperture actuation. At F5.6 the tapering is close to non-measureable in the end result.
theSuede wrote:
When measuring surveillance lenses (mostly quite "cheap" C-mount stuff, "cheap" being less than 1000$) I get a factor of about 1:4 in difference between different lenses in absolute contrast. That's about 2Ev difference in flare limited DR. With absolute contrast I mean the flare limited contrast at very low frequencies, 1-5 lp/mm. My flare target is a square pattern checkerboard backlit diffusion screen, about 2x bigger than the image field - so that you get some light pollution from outside the image as well.
The calculated MTF times diffraction is set as the ground truth, and when you divide by the the measured result you get the flare / random dispersion loss. ...Show more →
That is interesting and good to know. 2Ev is quite significant.
What's interesting about the tapered aperture is that if you build an F2.8 lens like the Sony/Minolta STF, the portion of the MTF curve that gets a noticeable increase in contrast transfer is well above the resolution limit of any modern APS or FF sensor. You get a good increase in contrast at 65lp/mm, and the result isn't "worse" than the normal circular aperture until you get to about 135lp/mm.
24MP APS is about 125lp/mm.
Considering that the apodization element is gradually phased out in effect as you stop the mechanical aperture down, the tapering effect is lowered by about 41% per each full stop of aperture actuation. At F5.6 the tapering is close to non-measureable in the end result....Show more →
Yup.
The benefit of tapering at large apertures would also depend on the level of correction of lens aberrations. The F/1.4 lenses for demanding users, announced by Zeiss, are interesting candidates for apodization, featuring a boost of MTF up to 260 lp/mm (from the diffraction point of view) and very attractive bokeh. The price paid is a loss of light corresponding to about one stop at full aperture. In case the lens is also apochromatic, such a lens may aptly be called Apo-Apo-Planar (or -Distagon, -Sonnar, or whatever). I want one
