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Author Topic: Dxo tests canon/nikon/sony 500mm's  (Read 16811 times)

Mika

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #75 on: July 25, 2013, 04:34:02 PM »
As far as I know, digital image sensors are a bit more complicated case than the classical sampling theorem would predict. First of all, it is important to understand the full meaning that the captured image is a two three dimensional signal (x, y and intensity) and how the eye sees it.

Using classical sampling theorem, a maximum resolvable frequency could be found by taking the inverse of (2*pixel pitch), which would lead to Nyqvist cut-off frequency. However, this is not the case, as in the measurements the image sensor tends to see further, as explained in [1] and published in [2]

As a short version, if one is able to align the pixel array exactly in the direction of bar patterns, the classical Nyqvist frequency holds. However, it is very difficult to do this, and thus what is actually seen is a result of sub-pixel sampling, which is then averaged by the eye and interpreted as a distinguishable bar. If one would only take a single line of the image, I'm not sure if the result in that case would be classified as distinguishable.

Add on top of that the fact whether we want to represent the actual shape of the subject at the maximum resolvable frequency despite the fact if it lands between the pixels, it can be seen that there can be a need for three to five times oversampling. I don't unfortunately have a good link to show this, I'll try to look for it and post it whether I can find it. However, this tends to be a way of selling more pixels too.

EDIT: Ah, found it, the PDF was by Andor [3]. What I want to say with all this, is that it is actually not that well defined what is meant by "resolving something" with the image sensors.
« Last Edit: July 25, 2013, 04:59:49 PM by Mika »

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #75 on: July 25, 2013, 04:34:02 PM »

Pi

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #76 on: July 25, 2013, 06:33:20 PM »
As far as I know, digital image sensors are a bit more complicated case than the classical sampling theorem would predict. First of all, it is important to understand the full meaning that the captured image is a two three dimensional signal (x, y and intensity) and how the eye sees it.

Using classical sampling theorem, a maximum resolvable frequency could be found by taking the inverse of (2*pixel pitch), which would lead to Nyqvist cut-off frequency. However, this is not the case, as in the measurements the image sensor tends to see further, as explained in [1] and published in [2]

As a short version, if one is able to align the pixel array exactly in the direction of bar patterns, the classical Nyqvist frequency holds. However, it is very difficult to do this, and thus what is actually seen is a result of sub-pixel sampling, which is then averaged by the eye and interpreted as a distinguishable bar. If one would only take a single line of the image, I'm not sure if the result in that case would be classified as distinguishable.

Add on top of that the fact whether we want to represent the actual shape of the subject at the maximum resolvable frequency despite the fact if it lands between the pixels, it can be seen that there can be a need for three to five times oversampling. I don't unfortunately have a good link to show this, I'll try to look for it and post it whether I can find it. However, this tends to be a way of selling more pixels too.

EDIT: Ah, found it, the PDF was by Andor [3]. What I want to say with all this, is that it is actually not that well defined what is meant by "resolving something" with the image sensors.

Those links have nothing to do with the sampling theorem. The latter does not care whether you image bars, etc., it tells you how to sample an a priori band limited signal (the bars are NOT that), and how to reconstruct it. The modification needed that I mentioned is simple and must have been done by somebody already. In short, if your image is band limited already (this is what the AA filter does, together with the lens), and you have a good estimate what that limit is, you know how many pixels you need.

Do not confuse a convenient resolution test (bars) with the sampling theorem. 

Mika

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #77 on: July 25, 2013, 07:12:19 PM »
As far as I know, digital image sensors are a bit more complicated case than the classical sampling theorem would predict. First of all, it is important to understand the full meaning that the captured image is a two three dimensional signal (x, y and intensity) and how the eye sees it.

Using classical sampling theorem, a maximum resolvable frequency could be found by taking the inverse of (2*pixel pitch), which would lead to Nyqvist cut-off frequency. However, this is not the case, as in the measurements the image sensor tends to see further, as explained in [1] and published in [2]

As a short version, if one is able to align the pixel array exactly in the direction of bar patterns, the classical Nyqvist frequency holds. However, it is very difficult to do this, and thus what is actually seen is a result of sub-pixel sampling, which is then averaged by the eye and interpreted as a distinguishable bar. If one would only take a single line of the image, I'm not sure if the result in that case would be classified as distinguishable.

Add on top of that the fact whether we want to represent the actual shape of the subject at the maximum resolvable frequency despite the fact if it lands between the pixels, it can be seen that there can be a need for three to five times oversampling. I don't unfortunately have a good link to show this, I'll try to look for it and post it whether I can find it. However, this tends to be a way of selling more pixels too.

EDIT: Ah, found it, the PDF was by Andor [3]. What I want to say with all this, is that it is actually not that well defined what is meant by "resolving something" with the image sensors.

Those links have nothing to do with the sampling theorem. The latter does not care whether you image bars, etc., it tells you how to sample an a priori band limited signal (the bars are NOT that), and how to reconstruct it. The modification needed that I mentioned is simple and must have been done by somebody already. In short, if your image is band limited already (this is what the AA filter does, together with the lens), and you have a good estimate what that limit is, you know how many pixels you need.

Do not confuse a convenient resolution test (bars) with the sampling theorem. 

There is a misunderstanding somewhere here, for me it sounds like we are talking about different things or use different terms. I'm well aware of the different nature of the problem described in [3]. However, what I meant to say with that is related to your earlier PSF considerations, when characterizing the PSF, the energy in the typical photographic objective spot is typically within the region of 1-3 camera body pixels, with a central core of the energy (something like 80 %) in a single pixel.

So in that case, you would be quite subject to errors in estimating the PSF due to the effect shown in [3]. And you really don't know the PSF beforehand. Only at the proximity of image edges (or using fast lenses) the PSF may become large enough to be sampled well by the camera sensor. If you are using a different bench for estimating the PSF with magnification, you'll then lose the effect of the AA filter as well.

Also, the photographic objective MTF isn't typically evaluated from a PSF (haven't seen this being used in many places), but from an edge or line spread function which then allows sub-pixel sampling and is more robust against positioning with respect to sampling grid. Astronomical telescopes may be a different thing, I don't have experience in designing them.

The point of [1] was to show that for example, depending on the angle the camera is mounted with respect to the bar chart target, your micro-contrast figures may change slightly.

None of this actually matters to the actual photography, though. I don't know whether we should continue with private messages, I suppose this is going to get technical and lots of people aren't probably interested in seeing this.

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #78 on: July 25, 2013, 07:48:31 PM »
As far as I know, digital image sensors are a bit more complicated case than the classical sampling theorem would predict. First of all, it is important to understand the full meaning that the captured image is a two three dimensional signal (x, y and intensity) and how the eye sees it.

Using classical sampling theorem, a maximum resolvable frequency could be found by taking the inverse of (2*pixel pitch), which would lead to Nyqvist cut-off frequency. However, this is not the case, as in the measurements the image sensor tends to see further, as explained in [1] and published in [2]

As a short version, if one is able to align the pixel array exactly in the direction of bar patterns, the classical Nyqvist frequency holds. However, it is very difficult to do this, and thus what is actually seen is a result of sub-pixel sampling, which is then averaged by the eye and interpreted as a distinguishable bar. If one would only take a single line of the image, I'm not sure if the result in that case would be classified as distinguishable.

Add on top of that the fact whether we want to represent the actual shape of the subject at the maximum resolvable frequency despite the fact if it lands between the pixels, it can be seen that there can be a need for three to five times oversampling. I don't unfortunately have a good link to show this, I'll try to look for it and post it whether I can find it. However, this tends to be a way of selling more pixels too.

EDIT: Ah, found it, the PDF was by Andor [3]. What I want to say with all this, is that it is actually not that well defined what is meant by "resolving something" with the image sensors.

Those links have nothing to do with the sampling theorem. The latter does not care whether you image bars, etc., it tells you how to sample an a priori band limited signal (the bars are NOT that), and how to reconstruct it. The modification needed that I mentioned is simple and must have been done by somebody already. In short, if your image is band limited already (this is what the AA filter does, together with the lens), and you have a good estimate what that limit is, you know how many pixels you need.

Do not confuse a convenient resolution test (bars) with the sampling theorem. 

There is a misunderstanding somewhere here, for me it sounds like we are talking about different things or use different terms. I'm well aware of the different nature of the problem described in [3]. However, what I meant to say with that is related to your earlier PSF considerations, when characterizing the PSF, the energy in the typical photographic objective spot is typically within the region of 1-3 camera body pixels, with a central core of the energy (something like 80 %) in a single pixel.

So in that case, you would be quite subject to errors in estimating the PSF due to the effect shown in [3]. And you really don't know the PSF beforehand. Only at the proximity of image edges (or using fast lenses) the PSF may become large enough to be sampled well by the camera sensor. If you are using a different bench for estimating the PSF with magnification, you'll then lose the effect of the AA filter as well.

Also, the photographic objective MTF isn't typically evaluated from a PSF (haven't seen this being used in many places), but from an edge or line spread function which then allows sub-pixel sampling and is more robust against positioning with respect to sampling grid. Astronomical telescopes may be a different thing, I don't have experience in designing them.

The point of [1] was to show that for example, depending on the angle the camera is mounted with respect to the bar chart target, your micro-contrast figures may change slightly.

None of this actually matters to the actual photography, though. I don't know whether we should continue with private messages, I suppose this is going to get technical and lots of people aren't probably interested in seeing this.

I love this kind of stuff, and have been reading your discussion so far. Instead of private messages, maybe just start a new thread, and link to the conversation here? So far, my understanding is more in line with yours, Mika...but I'd like to see what Pi has to say on the subject, as perhaps there is something new to learn.

Pi

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #79 on: July 26, 2013, 02:22:14 AM »
There is a misunderstanding somewhere here, for me it sounds like we are talking about different things or use different terms. I'm well aware of the different nature of the problem described in [3]. However, what I meant to say with that is related to your earlier PSF considerations, when characterizing the PSF, the energy in the typical photographic objective spot is typically within the region of 1-3 camera body pixels, with a central core of the energy (something like 80 %) in a single pixel.

So in that case, you would be quite subject to errors in estimating the PSF due to the effect shown in [3]. And you really don't know the PSF beforehand. Only at the proximity of image edges (or using fast lenses) the PSF may become large enough to be sampled well by the camera sensor. If you are using a different bench for estimating the PSF with magnification, you'll then lose the effect of the AA filter as well.

Also, the photographic objective MTF isn't typically evaluated from a PSF (haven't seen this being used in many places), but from an edge or line spread function which then allows sub-pixel sampling and is more robust against positioning with respect to sampling grid.

Exactly. The slanted edge test averages over a relatively long edge, and I do not know of a single test which would try to look at a single point, or would try to align the bars exactly, etc. But that allows you to restore the actual PSF (convoluted with something but let us keep it simple) by a simple calculation.


Quote
None of this actually matters to the actual photography, though. I don't know whether we should continue with private messages, I suppose this is going to get technical and lots of people aren't probably interested in seeing this.

It matters enough for so many labs and companies to measure the MTF, for academics to write papers and books, etc.

Mika

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #80 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.
« Last Edit: July 27, 2013, 12:22:29 PM by Mika »

Pi

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #81 on: July 27, 2013, 12:27:29 PM »
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.

It does but not of the PSF directly - of the PSF convoluted with "something", derived from the pixel size and the AA filter. That "something" is a known quantity, depending on the angle as well. From there, you can get the PSF. Again, the computation is NOT the same as deconvolution but still, it is not going to be too accurate if the PSF is too concentrated compared to one pixel but the instability is far from the (exponential) one for deconvolution. The lens PSF convoluted with the effect of the AA filter however is not "too concentrated" (the reason AA exists in the first place), and can be well reconstructed. Factoring out the effect of the AA filter itself if trickier but if you keep the same sensor, you want to keep that effect in place. 

Quote
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 (if this isn't done, there will be uncertainty in the slant angle and the sub-pixel sampling is affected).

Look at the actual targets. They consists of squares which are not "too large" but "large enough". In any case, you are measuring some average of the PSF along the edge, hoping that it does not change wildly.

Quote
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 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.

They use black slanted squares in the target. In the good old times, DXO would report horizontal and vertical resolution (and they usually were different enough).

Quote
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.

Do not underestimate the power of computed PSF vs. the measured one. In most modern medical imaging techniques, for example, the image is computed from the data with serious math methods, vs. just displaying pictures as with the traditional X-ray tomography, for example. BTW, my work is related to that.

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #81 on: July 27, 2013, 12:27:29 PM »

Mika

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #82 on: July 29, 2013, 06:34:48 AM »
I checked through some of the late night photos I have taken with 28/1.8 and 40D to see how large the star points actually are on the sensor. With 8-15 second exposure, I'm seeing that the star spots are within 4x3 pixels or 5x4 (the extension along the other dimension is the most dominant here, and is due to exposure time and Earth's rotation).

This is measured from a straight out of camera JPEG, with F/3.5 and with a lens that had slight amount of frost on the front element. And even then the spots are quite limited. I do think that had the conditions been better (no frost and better lens with equal aperture and better ISO than 40D has) and had I taken RAWs, most of the star images would fall within 3x3 region (as I said earlier) - spread is mainly because of the AA filter, otherwise stars should fall within a single pixel assuming any kind of reasonable performance of the objective and with F-numbers less than 5.6. With a PSF of size 3x3 pixels, it is hard for me to see how this could be used to compute the MTF even with sub-pixel sampling without averaging over larger area.

For example, 100 pixels with a 5 ┬Ám pitch respresents about 0.5 mm, which is significant. If we are talking about smaller averaging distance, for example 30 pixels, the uncertainty of the slant angle itself would be about 2 degrees. I haven't seen many error estimations to the slanted edge method, but I'm afraid I'll have to do that myself in the near future in a publication.

It is great to hear about your background in the tomography, it helps me understand how you think about these issues. But I have to remind you that we are talking about optical systems within visual wavelength range, where things are quite bit different from radio waves (MRI) or THz region or ultrasound. From my hazy memory, MRI actually measures time differences on different detectors, but it's 11 years since I have needed to think anything related to NMR? These medical wavelengths are used mainly because of the requirement of non-invasiveness and that's why you need to deal a lot with image processing techniques.

I think a better equivalent would be to compare image processing techniques in astronomical telescopes to get the state-of-the-art results in the visible wavelength range. Adaptive optics correction to the PSFs allows ground based telescopes just to match them with Hubble over a smaller field of view on good nights.

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #83 on: August 04, 2013, 02:54:13 PM »
With a PSF of size 3x3 pixels, it is hard for me to see how this could be used to compute the MTF even with sub-pixel sampling without averaging over larger area.

And still they do it. Here is a good read:

http://www.imatest.com/docs/sharpness/
http://www.imatest.com/docs/sharpness/#calc

Quote: Briefly, the ISO-12233 slanted edge method calculates MTF by finding the average edge (4X oversampled using a clever binning algorithm), differentiating it (this is the Line Spread Function (LSF)), then taking the absolute value of the Fourier transform of the LSF. The edge is slanted so the average is derived from a distribution of sampling phases (relationships between the edge and pixel locations).

Of course, this measures the combined lens+AA filter MTF (a bit more complicated than that, actually). Different lenses show different enough results to make the test meaningful. They do average over some area, that is the whole idea of the slanted edge test vs. measuring what happens near a single pixel.

Quote
It is great to hear about your background in the tomography, it helps me understand how you think about these issues. But I have to remind you that we are talking about optical systems within visual wavelength range, where things are quite bit different from radio waves (MRI) or THz region or ultrasound.

My background is in math, which (fortunately for me) is useful regardless of the whether you have microwaves, or visual wavelengths, etc.
« Last Edit: August 04, 2013, 02:56:00 PM by Pi »

Mika

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #84 on: August 08, 2013, 08:46:18 AM »
Sorry again about the delay, I was on a vacation trip.

I have not said that the MTF computed using the slanted edge method isn't useful. However, I have said that the MTF calculated with this method isn't scientifically accurate if you want absolute accuracy. The problem with the averaging is that it tends to lose information of the spot itself, while the average along two orthogonal directions is computed with sufficient sampling, pretty much nothing is said about what happens between the orthogonal directions.

For this reason, I don't believe it would be possible to reconstruct an accurate PSF with the slanted edge method and thus the measured MTF must be slightly invalid as well. You can think of this from the dimensional reduction point of view; it is generally not possible to recreate a 3D function from two 2D functions. Higher order aberrations do give rise for all sorts of interesting spot shapes and orientations with element decentering.

But as I said, slanted edge method allows comparable MTF measurements and is very good at that, but it does not allow absolute measurements where you have to guarantee the results.

It is relatively easy to think that there isn't differences between the behavior of rays when shifting from a wavelength range to another. I hear this argument quite often, and this may sound like blasphemy for some, but I disagree with that. For example, there is a considerable difference between the ray propagation physics between a visual wavelength range camera (typically not diffraction limited) and a THz system and you have to take them into account when designing them.

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #85 on: August 08, 2013, 10:21:24 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.

It does but not of the PSF directly - of the PSF convoluted with "something", derived from the pixel size and the AA filter. That "something" is a known quantity, depending on the angle as well. From there, you can get the PSF. Again, the computation is NOT the same as deconvolution but still, it is not going to be too accurate if the PSF is too concentrated compared to one pixel but the instability is far from the (exponential) one for deconvolution. The lens PSF convoluted with the effect of the AA filter however is not "too concentrated" (the reason AA exists in the first place), and can be well reconstructed. Factoring out the effect of the AA filter itself if trickier but if you keep the same sensor, you want to keep that effect in place. 

If you are reverse engineering a PSF which is the convolution of lens + AA filter, then that would be the exact issue I was trying to point out before. The AA filter is designed to limit the resolution of the image that reaches the sensor plane by filtering out higher frequencies, while leaving lower frequencies in tact. If you are reverse engineering the image post-AA, then it has an intrinsic upper limit on resolution. The lens could very well (and in the case of a good lens like the EF 500/4 L II, most likely does) resolve more than what the lens+AA convolved resolve.

If you knew the exact nature of the AA filter, you could probably exclude its effect from the PSF, and arrive at a result much closer to what the lens itself is actually capable of. If you leave the convolution with the AA filter in, then you haven't really reverse engineered the lens MTF, you've just reverse engineered the lens+AA filter MTF. That might be useful for comparison, but it really doesn't tell you all that much about the lens itself.

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #86 on: August 08, 2013, 11:45:31 AM »
Sorry again about the delay, I was on a vacation trip.

I have not said that the MTF computed using the slanted edge method isn't useful. However, I have said that the MTF calculated with this method isn't scientifically accurate if you want absolute accuracy. The problem with the averaging is that it tends to lose information of the spot itself, while the average along two orthogonal directions is computed with sufficient sampling, pretty much nothing is said about what happens between the orthogonal directions.

For this reason, I don't believe it would be possible to reconstruct an accurate PSF with the slanted edge method and thus the measured MTF must be slightly invalid as well. You can think of this from the dimensional reduction point of view; it is generally not possible to recreate a 3D function from two 2D functions. Higher order aberrations do give rise for all sorts of interesting spot shapes and orientations with element decentering.

We are getting here to more philosophical questions. In science, we use modeling. We make some a priori assumptions, ignore this and that, and then build a model which we analyze. That model is never perfect, and it can't be. We must be aware of its limitations. But saying - you can never have a perfect model, so why bother with science at all - is not the right thing.

Long time ago, Riemann suggested to model anisotropic phenomena by a quadratic form. The level curves (in 2D) of that form are ellipses. 2 measurements then are enough. In some sense, this is equivalent to taking a truncated Taylor expansion of a more complicated function.

Going back to photography - when the PSF is well concentrated, approximation by a quadratic form is OK. When it is not, you can see it, no need of sophisticated methods (like my 35L at f/1.4 in the corners, wide open).  Those are some of the limitations in this case.

So when I say - you can get the the PSF from the MTF, I always assume some reasonable model in place. Also, I do not mean that you must use the DXO test, necessarily. If you are curious enough, you can rotate your target, and get get more directions.

Quote
It is relatively easy to think that there isn't differences between the behavior of rays when shifting from a wavelength range to another. I hear this argument quite often, and this may sound like blasphemy for some, but I disagree with that.

In the good old times, DXO published MTF charts on each of the (RAW) RGB channels. They were different enough, indeed. Actually, sometimes, too much. They reported some kind of average, weighed heavily towards greens, if I remember well, because people want simple answers. If you dig deeper in that, the spectral decomposition of the light in the test would play a role, too, etc. In principle, the camera projects an infinitely dimensional color space to a 3D one, so full spectral information is lost anyway.

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Re: Dxo tests canon/nikon/sony 500mm's
« Reply #86 on: August 08, 2013, 11:45:31 AM »