Ad
Ad
Ad
Pages: « 1 [2] 3 4 »   Bottom of Page
Print
Author Topic: Resolution and alaising challenge  (Read 23618 times)
bjanes
Sr. Member
****
Offline Offline

Posts: 2821



« Reply #20 on: October 19, 2011, 04:52:59 PM »
ReplyReply

The image was taken with studio flash and LV based focusing so I don't really see the probability for random effects. I enclose an unsharpened image.

The file is here: http://echophoto.dnsalias.net/ekr/images/20111019-_DSC1394.dng

Erik,

I downloaded your DNG and rendered in ACR with default settings and repeated my measurements and obtained 96% of Nyquist. I don't think that you can do much better.

On looking at the metadata, I see you have GPS enabled. Very cool. Is this facility built into the camera or are you using an addon? Googleearth gave the following image. Is it accurate?

Logged
Fine_Art
Sr. Member
****
Offline Offline

Posts: 1094


« Reply #21 on: October 19, 2011, 05:11:41 PM »
ReplyReply

The A55 has built in GPS.
Logged
ErikKaffehr
Sr. Member
****
Offline Offline

Posts: 7630


WWW
« Reply #22 on: October 19, 2011, 05:30:59 PM »
ReplyReply

Hi,

Yes and no. It's probably the coordinates were I used the camera last time outdoor.

I seldom use my Alpha 55, most pictures are taken with the Alpha 900. But I use Alpha 55 for long telephoto work where I need LV for critical focus.

The Alpha 55 has GPS built in but the Alpha 900 has not.

Best regards
Erik


Erik,

I downloaded your DNG and rendered in ACR with default settings and repeated my measurements and obtained 96% of Nyquist. I don't think that you can do much better.

On looking at the metadata, I see you have GPS enabled. Very cool. Is this facility built into the camera or are you using an addon? Googleearth gave the following image. Is it accurate?


« Last Edit: October 19, 2011, 05:33:39 PM by ErikKaffehr » Logged

Wayne Fox
Sr. Member
****
Offline Offline

Posts: 2879



WWW
« Reply #23 on: October 20, 2011, 03:07:44 AM »
ReplyReply

It would be interesting to see the results of similar tests with cameras lacking a low-pass filter, but thus far no one has stepped up to the plate. Users of those cameras do not appear to be interested in technical analysis or perhaps they are afraid of what such an analysis would show.


mmmm ... insulting us is certainly a good way to encourage us to do it.  Seeing how this has only been up a couple of days I'm not sure what you expected.

Honestly while this kind of stuff interests me and I enjoy the techie types here on LuLa and read quite a bit of the stuff, it's over my head and I have no interest in spending the effort so it isn't over my head,  and not to sound condescending but I am pretty busy every day out shooting stuff I enjoy.  Maybe when the fall colors fade and winter boredom hits hard in my curiosity will kick in and I'll give it a go, but then I'm sure I'd screw it up anyway since I have no clue what I"m doing.
Logged

BartvanderWolf
Sr. Member
****
Offline Offline

Posts: 3745


« Reply #24 on: October 20, 2011, 05:24:45 AM »
ReplyReply

mmmm ... insulting us is certainly a good way to encourage us to do it.  Seeing how this has only been up a couple of days I'm not sure what you expected.

Honestly while this kind of stuff interests me and I enjoy the techie types here on LuLa and read quite a bit of the stuff, it's over my head and I have no interest in spending the effort so it isn't over my head,  and not to sound condescending but I am pretty busy every day out shooting stuff I enjoy.  Maybe when the fall colors fade and winter boredom hits hard in my curiosity will kick in and I'll give it a go, but then I'm sure I'd screw it up anyway since I have no clue what I"m doing.

Hi Wayne,

The worst that could happen is that you might learn something about your tools you didn't know already, or get a confirmation that there is nothing to worry about. It's so simple to execute, print the target (@600/720 PPI) and shoot an image of it, that shouldn't intimidate the average photographer, and especially not you.

I recently could help a fellow photographer who purchased a new lens, and he wanted to know if it was a good copy. One typically searches for clues of decentering because uneven performance over the focus plane can be very disturbing (and hard to correct in postprocessing). His first copy showed an issue, especially in the bottom right hand corner, and his second copy was much better (top left hand corner was only slightly less than the other corners, well within expectations). He is now very happy and confident with his lens, and he quantifiably knows what to expect compared to his other lenses (when switching from zoom to fixed focal length makes sense).

Bill was perhaps a bit enthousiastic, and surprised there was seemingly no response, but time constraints are a common issue. Take your time, it would be nice to get some feedback over time with regards to different camera models and lenses. The expectation is that many systems will be able to resolve close to maximum (Nyquist frequency), and that occasional lens issues will be revealed.

The fact that in practice so many Bayer CFA camera systems resolve luminance so well (>90% of maximum/Nyquist), while many quote figures of 70% of Nyquist, is already interesting to know. Chroma resolution will be worse than luminance resolution, but that's somewhat similar to our eyes. Also the effect of an AA-filter is perhaps not as detrimental as some are led to believe. A question that remains to be resolved/demonstrated is how much of a difference does a MFDB without AA-filter (due to cost) make?

By processing a single Raw file with a capture of the target with different Raw converters, one can also reveal useful differences. The current ACR/LR Raw converter e.g. has narrowed the gap with Capture One considerably for my 1Ds3 files. That could be an interesting thing to know when making a workflow choice or considering a switch. Ease of use is one thing, but quality may be another.

It would be nice if people share their findings. It's not a contest, but an attempt to build some more practically founded collective knowledge of how our tools of the trade perform. It also doesn't hurt to improve some of the equipment comparisons we've seen of late, with more objective measurements but without the need to dedicate a test wall or fixed setup to do it.

Cheers,
Bart
Logged
Cem
Newbie
*
Offline Offline

Posts: 28



WWW
« Reply #25 on: October 20, 2011, 08:40:23 AM »
ReplyReply

Hi,

...I recently could help a fellow photographer who purchased a new lens, and he wanted to know if it was a good copy. One typically searches for clues of decentering because uneven performance over the focus plane can be very disturbing (and hard to correct in postprocessing). His first copy showed an issue, especially in the bottom right hand corner, and his second copy was much better (top left hand corner was only slightly less than the other corners, well within expectations). He is now very happy and confident with his lens, and he quantifiably knows what to expect compared to his other lenses (when switching from zoom to fixed focal length makes sense)...

First of all, a belated hello to all of you. I have been reading the forums regularly for some 6 years but I haven't had the chance to actively participate till now. I am the "fellow photographer" Bart was referring to above.

As he mentioned, I have recently bought an EF 70-200L f4 IS zoom lens for my Canon 5D MkII body. I had the possibility to exchange the lens within 8 days so naturally I have set out to test it as best as I could. Bart's resolution target has helped me in achieving this quickly. Looking at the results, the 1st copy seemed to have an issue in the bottom right hand corner. I have exchanged it for another copy and retested. This one was a better performer and I have kept it. Obviously, I have also taken real life pictures to test the lens in the field. But the aberration I have discovered in the 1st lens would probably have gone unnoticed that way.

The pictures were taken with the camera on a tripod, using mirror lock up and contrast focusing in live view. At first, I have used the 2sec timer but the resulting images have indicated that this was not long enough to get rid of the residual shaking. I have then switched to using the remote control and the 10sec timer.

I took pictures of the target at the center of the frame and also at the 4 corners. The target was taped to a window pane and the camera was set up at a distance of around 25x of the focal length used. The lens axis was set to be as perpendicular as possible to the target plane since I was also testing the corner performance. I wanted to be able to see whether the distortions in the corners would be symmetrical or not. For all pictures, I have left the camera fixed on the tripod and have moved the target around instead. This has been repeated using 4 main focal lengths (70mm, 100mm, 135mm and 200mm).

Besides using the resolution target for identifying the possible aberrations, I have also taken shots at various apertures from f4 to f22. This has helped me experimentally identify the aperture at which diffraction became an issue and how much of it I could normally tolerate.

Below is one of the center of the frame test pictures. Exposure details are: 5D Mk II, EF 70-200L f4 IS, 200mm, f6.3, 1/13s, ISO 100. Let me point out that the target has a serious color shift since the old Canon printed I have used to print it would not play ball. But talking to Bart on this, we have concluded that it would not be a problem for the test and I have left it at that. The conversion from raw is done in LR3, using all neutral settings. No sharpening, no noise reduction, no lens corrections, no clarity. The image was then taken into PS where I have added the 92 pixel Nyquist limit circle as a layer.

Full image at 100%:



Cropped image when zoomed in at 300% (not a real/permanent resizing, this was just screen captured from PS):



Cropped image when zoomed in at 300%, including the 92 pixel Nyquist limit circle (not a real/permanent resizing, this was just screen captured from PS):








« Last Edit: October 20, 2011, 08:49:21 AM by Cem_Usakligil » Logged

Kind Regards,

Cem

Photographs
bjanes
Sr. Member
****
Offline Offline

Posts: 2821



« Reply #26 on: October 20, 2011, 10:52:17 AM »
ReplyReply

First of all, a belated hello to all of you. I have been reading the forums regularly for some 6 years but I haven't had the chance to actively participate till now. I am the "fellow photographer" Bart was referring to above.

As he mentioned, I have recently bought an EF 70-200L f4 IS zoom lens for my Canon 5D MkII body. I had the possibility to exchange the lens within 8 days so naturally I have set out to test it as best as I could. Bart's resolution target has helped me in achieving this quickly. Looking at the results, the 1st copy seemed to have an issue in the bottom right hand corner. I have exchanged it for another copy and retested. This one was a better performer and I have kept it. Obviously, I have also taken real life pictures to test the lens in the field. But the aberration I have discovered in the 1st lens would probably have gone unnoticed that way.

Besides using the resolution target for identifying the possible aberrations, I have also taken shots at various apertures from f4 to f22. This has helped me experimentally identify the aperture at which diffraction became an issue and how much of it I could normally tolerate.

..where I have added the 92 pixel Nyquist limit circle as a layer.

Cem,

A good illustration of the utility of Bart's target. A nice feature of the method is that the object distance is not important. For reasons I don't understand, it seems as if the Nyquist limit is always at 92 pixels, regardless of the pixel pitch of the sensor. Perhaps Bart can explain.

The importance of this fact is that one does not have to go through the equations Bart posted if one does not need the actual resolution in cycles/mm. One can simply measure the blur diameter.

The effect of demosaicing is of interest. I used Iris to demosaic my original image, white balanced on the white area of the target, and multiplied the pixel values by 4 to convert from 14 bits to 16 bits/pixel and saved as a TIFF. No gamma correction was applied, so the gamma is 1.0. The result is shown. Much more color aliasing is evident. The blur circle is best identified by looking for the phase change as Bart commented above.

Regards,

Bill

Logged
BartvanderWolf
Sr. Member
****
Offline Offline

Posts: 3745


« Reply #27 on: October 20, 2011, 05:13:57 PM »
ReplyReply

A good illustration of the utility of Bart's target. A nice feature of the method is that the object distance is not important.

Yes, this is deliberate. One of the easiest mistakes to make is to shoot targets that are sensitive to shooting distance (=magnification) at the wrong distance. Besides, it is not always clear how to measure that distance when setting up for a test. Do we need to measure from the film/sensor plane, or from the front of the lens (and where, entrance pupil, nodal point, filter threads, etc.). We can only figure out the actual resulting magnification after shooting, by measuring the size of the target on the sensor (microscope on film, or pixels times sensel pitch for digicams or scanners) divided by the size of the original. When there are fixed markings on the target, then we need to re-adjust the focusing distance iteratively to arrive at the intended magnification. This is such boring work that most either skip this calibration step of don't do the test in the first place.

Quote
For reasons I don't understand, it seems as if the Nyquist limit is always at 92 pixels, regardless of the pixel pitch of the sensor. Perhaps Bart can explain.

I'll give it a try. At any diameter, the target always has 144 full cycles per circumference. The only thing that changes with distance is the magnification factor. Well, some lenses perform a bit better at some distance than at others, but in the suggested range of 25x to 50x focal length the differences are not likely to be significant.

We can know how high the spatial frequency is along the circumference at any given diameter, the circumference of a circle is 2 x Pi x radius (or Pi x diameter), and there are always 144 cycles at that circumference. So by dividing 144 cycles by the circumference we know the number of cycles per pixel. Since Nyquist is at 0.5 cycles per pixel (or 1 cycle per 2 pixels), the equation becomes 144 / (Pi x Diameter), and when the diameter is expressed in pixels we multiply by 2 to find the Nyquist frequency.

BTW, I prefer to write the formula as Cy/px = (144 / Pi) / Diameter, which is the same, but it allows to pre-calculate 144 / Pi once and change the diameter as we take different measurements. A very basic calculator suffices.

Now, as to why we also find similar values regardless of shooting distance. As shown, we can calculate the Cy/px for any diameter (and thus circumference) on the target, or a projection of it. The diameter is expressed in pixels (1 sensel or sample per output pixel), and the sampling frequency in pixels is constant for any sensor. The only thing we change with shooting distance is magnification, but the sampling density remains constant. Therefore we shoot a different radius on the target itself (e.g. larger radius at longer distance, but also with equally smaller magnification, thus with the same resulting diameter or circumference), and the frequency we can resolve per pixel remains the same. So the distance and magnification factor result in a constant projected blur diameter, although the origin is sampled at different diameters on the target. The Nyquist frequency of a sensor is always at 2 sensels per cycle, and thus remains constant between comparisons of different sensors when expressed in pixels.

Quote
The importance of this fact is that one does not have to go through the equations Bart posted if one does not need the actual resolution in cycles/mm. One can simply measure the blur diameter.

That's right, expressed in cycles/pixel, the diameter is all that's needed. Only when one wants to calculate things like magnification potential does it make sense to add the physical sensel pitch into the equation, by changing the diameter to pixels x sensel pitch in mm. When we e.g. know that our output medium can resolve 5 cycles/mm, and our optical system resolves 78 cycles/mm, then we know we can magnify our sensor size by 78/5= 15.6x to find the uncompromised maximum output dimensions, fit for reading distance inspection (5-8 cycles/mm at a normal reading distance is at the verge of human visual acuity). Larger output should be viewed at a proportionally larger distance for the same quality impression.

Cheers,
Bart
Logged
Fine_Art
Sr. Member
****
Offline Offline

Posts: 1094


« Reply #28 on: October 20, 2011, 10:05:56 PM »
ReplyReply

That is a good explanation. Great chart.
Logged
ejmartin
Sr. Member
****
Offline Offline

Posts: 575


« Reply #29 on: October 20, 2011, 10:40:27 PM »
ReplyReply

A nice feature of the method is that the object distance is not important. For reasons I don't understand, it seems as if the Nyquist limit is always at 92 pixels, regardless of the pixel pitch of the sensor. Perhaps Bart can explain.

Always useful to consider the symmetries of the situation.  In this case, the target is scale invariant -- take the central half of the image, magnify it 2x, and it looks just like the original.  Same for any other fraction 1/X and magnification factor X.  Same thing at work on the sensor -- scale the sensor uniformly down or up by any factor and the sensor's magnified/shrunk pixel array sees the same image.  So if the resolution craps out at 92 pixels on one sensor, it will do the same on any other, modulo the effects of the AA filter, lens softness, etc.
Logged

emil
bjanes
Sr. Member
****
Offline Offline

Posts: 2821



« Reply #30 on: October 21, 2011, 06:49:24 AM »
ReplyReply

I'll give it a try. At any diameter, the target always has 144 full cycles per circumference. The only thing that changes with distance is the magnification factor. Well, some lenses perform a bit better at some distance than at others, but in the suggested range of 25x to 50x focal length the differences are not likely to be significant.

We can know how high the spatial frequency is along the circumference at any given diameter, the circumference of a circle is 2 x Pi x radius (or Pi x diameter), and there are always 144 cycles at that circumference. So by dividing 144 cycles by the circumference we know the number of cycles per pixel. Since Nyquist is at 0.5 cycles per pixel (or 1 cycle per 2 pixels), the equation becomes 144 / (Pi x Diameter), and when the diameter is expressed in pixels we multiply by 2 to find the Nyquist frequency.

Bart,

An excellent explanation. Many thanks.

MTF at Nyquist is very low and may not be helpful for practical photography. Norman Koren has demonstrated a way to determine MTF using the ImageJ Plot Profile function with a sinusoidal linear chart.  Unfortunately, Image J does not plot the profile for a circular path.  Do you have any way to calculate MTF from your test chart?

Regards,

Bill
Logged
BartvanderWolf
Sr. Member
****
Offline Offline

Posts: 3745


« Reply #31 on: October 21, 2011, 02:37:43 PM »
ReplyReply

MTF at Nyquist is very low and may not be helpful for practical photography.

Hi Bill,

That's true, but it's significantly higher with AA-filterless sensors. It's also where deconvolution sharpening can do a targeted restoration, as far as there is still some contrast available.

Quote
Norman Koren has demonstrated a way to determine MTF using the ImageJ Plot Profile function with a sinusoidal linear chart.  Unfortunately, Image J does not plot the profile for a circular path.

There is an Oval Profile Plugin available for ImageJ. However, it's a lot more work to extract good data from my target than from Norman's version. One needs to get very good centering and sufficient oversampling (>576 points) and then use a copy of the list data from an "Along Oval" analysis to further process in e.g. MS Excel. A quick impression follows from a maximum and minimum from such a dataset, and they tend to converge as they get closer to the Nyquist frequency. And that is for a single spatial frequency. The same plug-in also allows to make an 'EquiCircumference' analysis, but I'm not very confident that it does what's needed. But don't forget that also Norman's target requires calibration for meaningful output values. In fact input images of both need linearization to linear gamma space before meaningful comparisons can be made.

But then the star part of my target was not directly intended to extract an MTF. My target does allow to visualize the effects of aliasing and contrast loss much more realistically and accurately, because it covers many angles instead of one, and it stresses a Raw converter quite well. Norman has been searching for a method to quantify sensitivity for aliasing for a long time, and I've done suggestions such as a ratio of the integral of the MTF curve from 0 to Nyquist versus Nyquist to 2 x Nyquist, but a visual impression is perhaps more meaningful than a single abstract number.

Quote
Do you have any way to calculate MTF from your test chart?

Yes, there is another feature in my target that is better suited for numerical analysis (and even some nice graphs), the slanted edges. Of course Imatest software can easily deal with those.

For a very fast quick-and-dirty Edge profile plot (very useful for detecting sharpening halos) one can make a horizontal and/or vertical single pixel wide linear profile plot almost along the almost horizontal or vertical slanted edge. The profile plot is then an oversampled representation or a high contrast edge transition. When plotted along the slant, the original target has a 10x oversampled edge transition. Depending on the actual rotation of that edge versus the sensel grid the oversampling may be a bit less or a bit more, one needs to estimate the average number of pixels per phase (line) transition.

The Edge Spread Function (ESF) that resulted is the basis for an MTF. One differentiates the ESF, which becomes the Line Spread Function (LSF). One then performs a Fourier transform on the LSF, and the Modulus (=Absolute value) of that Fourier transform is the MTF. Not an exercise one wants to do without an application for the math involved.

Frankly, and while I'm used to interpreting MTFs, an ESF is already quite telling about how sharp an optic is, and whether sharpening introduced halos. Imatest also plots that in a 10% to 90% edge plot and derives a number for comparison from those percentage of response points.

Cheers,
Bart
« Last Edit: March 17, 2012, 09:41:12 PM by BartvanderWolf » Logged
bjanes
Sr. Member
****
Offline Offline

Posts: 2821



« Reply #32 on: October 22, 2011, 10:42:24 AM »
ReplyReply

There is an Oval Profile Plugin available for ImageJ. However, it's a lot more work to extract good data from my target than from Norman's version. One needs to get very good centering and sufficient oversampling (>576 points) and then use a copy of the list data from an "Along Oval" analysis to further process in e.g. MS Excel. A quick impression follows from a maximum and minimum from such a dataset, and they tend to converge as they get closer to the Nyquist frequency. And that is for a single spatial frequency. The same plug-in also allows to make an 'EquiCircumference' analysis, but I'm not very confident that it does what's needed. But don't forget that also Norman's target requires calibration for meaningful output values. In fact input images of both need linearization to linear gamma space before meaningful comparisons can be made.

Bart, thanks for the detailed reply. The methods you describe are beyond my expertise, and I will not attempt to use them. I have used Normans linear charts, but have not bothered to calibrate them, so I was only viewing relative resolution. Sometimes, it is nice to have a visual impression of resolution rather than using quantitative data only.

But then the star part of my target was not directly intended to extract an MTF. My target does allow to visualize the effects of aliasing and contrast loss much more realistically and accurately, because it covers many angles instead of one, and it stresses a Raw converter quite well. Norman has been searching for a method to quantify sensitivity for aliasing for a long time, and I've done suggestions such as a ratio of the integral of the MTF curve from 0 to Nyquist versus Nyquist to 2 x Nyquist, but a visual impression is perhaps more meaningful than a single abstract number.

Yes, there is another feature in my target that is better suited for numerical analysis (and even some nice graphs), the slanted edges. Of course Imatest software can easily deal with those.

Yes, I did post an Imatest analysis previously in this thread. Your comment about Norman's searching for a method to quantify aliasing is relevant to the discussion. Aliasing and over-sharpening can easily spurious resolution beyond Nyquist with Imatest. The latter can be prevented by omitting sharpening, but some sharpening is needed when the sensor utilizes a blur filter. One should avoid excessive overshoot on the edge plot, but this is subjective. Aliasing is readily observed with repeating high contrast patterns, but its effect on the usual field photographic images continued to be debated.

Regards,

Bill
Logged
Fine_Art
Sr. Member
****
Offline Offline

Posts: 1094


« Reply #33 on: October 23, 2011, 06:33:18 PM »
ReplyReply

Here is my center at 300% -attached.


Sony A55 is 4912 pixels/23.5mm 4.78um
My circle diameter was 317pixels /3 [for the 300%] or 0.5055mm

Clearly the AA filter is very weak.
Logged
Fine_Art
Sr. Member
****
Offline Offline

Posts: 1094


« Reply #34 on: October 23, 2011, 09:32:46 PM »
ReplyReply

I spent the day backing up onto a new hard-drive so I re-did this just in case the 100 was hitting the printer limits. Had to kill time.

This is with the 50mm on 1.5 crop sensor. I put in a 96pixel circle for nyquist.
Logged
BartvanderWolf
Sr. Member
****
Offline Offline

Posts: 3745


« Reply #35 on: October 24, 2011, 06:54:53 AM »
ReplyReply

Clearly the AA filter is very weak.

Hi Fine_Art,

That's useful for owners of that camera. The AA-filter is indeed very mild. However, the interesting thing is that the resulting aliasing is not all that bad, although a potential issue. Which Raw converter did you use?

The aliasing will of course rear it's ugly head when we can least use it. But armed with the knowledge about that, one always has an option to use a small enough aperture to function as an AA-filter. That's another benefit of doing tests like these, one comes to the battle prepared.

Cheers,
Bart
Logged
bjanes
Sr. Member
****
Offline Offline

Posts: 2821



« Reply #36 on: October 24, 2011, 08:15:33 AM »
ReplyReply

The aliasing will of course rear it's ugly head when we can least use it. But armed with the knowledge about that, one always has an option to use a small enough aperture to function as an AA-filter. That's another benefit of doing tests like these, one comes to the battle prepared.

To demonstrate the effects of diffraction on aliasing and resoluton, I shot the target at f/4 and f/32 with the D3 and my 60 f/2.8 AFS. Resolution is maximal at f/4 to f/5.6 and falls off dramatically beyond f/11 due to diffraction. At f/32, aliasing is completely eliminated. At f/4 resolution is approximately 57 cy/mm or 96% of Nyquist and at f/32 the corresponding figures are 48 cy/mm and 81% respectively. A glance at the lower frequencies shows that the contrast is markedly decreased at f/32. A useful exercise would be to plot the response at intermediate f/stops.

One can use Imatest and the slanted edge to look at MTF, which is markedly better at f/4. The MTFs may appear low, but no sharpening was used to counter the effect of the low pass filter.

Logged
Fine_Art
Sr. Member
****
Offline Offline

Posts: 1094


« Reply #37 on: October 24, 2011, 12:58:40 PM »
ReplyReply

Hi Fine_Art,

That's useful for owners of that camera. The AA-filter is indeed very mild. However, the interesting thing is that the resulting aliasing is not all that bad, although a potential issue. Which Raw converter did you use?

The aliasing will of course rear it's ugly head when we can least use it. But armed with the knowledge about that, one always has an option to use a small enough aperture to function as an AA-filter. That's another benefit of doing tests like these, one comes to the battle prepared.

Cheers,
Bart

The top one was IDC, the software that comes with the camera. The second one was RT, all detail enhancement was turned off.
Logged
Guillermo Luijk
Sr. Member
****
Offline Offline

Posts: 1291



WWW
« Reply #38 on: October 24, 2011, 01:14:05 PM »
ReplyReply

At f/32, aliasing is completely eliminated.

Fantastic exercise Bill, this confirms the practical usefulness of difracction as an AA filter. This can be interesting for interiors and arquitecture shooters: it they find some area of the scene where aliasing might be a problem once at home, it's worth to do some diffracted extra shooting to use in that area. I would always prefer some diffraction against straight defocusing of the area, because diffraction affects equally the entire image (so it can be used on any aliased area) and is easier to achieve (just change aperture).

At a risk of being boring, could you please show one RAW channel (dcraw -v -d -r 1 1 1 1 -4 -T file.nef + 50% nearest neighbour) of the f/32 shot in order to find out it there still exists aliasing in a single RAW channel? that would be a nice evidence of the RGGB pattern + demosaicing to be much less prone to aliasing than the individual channels.

Regards
« Last Edit: October 24, 2011, 01:20:10 PM by Guillermo Luijk » Logged

bjanes
Sr. Member
****
Offline Offline

Posts: 2821



« Reply #39 on: October 24, 2011, 02:07:34 PM »
ReplyReply

Fantastic exercise Bill, this confirms the practical usefulness of difracction as an AA filter. This can be interesting for interiors and arquitecture shooters: it they find some area of the scene where aliasing might be a problem once at home, it's worth to do some diffracted extra shooting to use in that area. I would always prefer some diffraction against straight defocusing of the area, because diffraction affects equally the entire image (so it can be used on any aliased area) and is easier to achieve (just change aperture).

At a risk of being boring, could you please show one RAW channel (dcraw -v -d -r 1 1 1 1 -4 -T file.nef + 50% nearest neighbour) of the f/32 shot in order to find out it there still exists aliasing in a single RAW channel? that would be a nice evidence of the RGGB pattern + demosaicing to be much less prone to aliasing than the individual channels.

Guillermo,

As requested, the ACR rendering is on the left and the  (dcraw -v -d -r 1 1 1 1 -4 -T file.nef + 50% nearest neighbour) after a levels adjustment is on the right. I converted the ACR to monochrome for the composite.

Bill

Logged
Pages: « 1 [2] 3 4 »   Top of Page
Print
Jump to:  

Ad
Ad
Ad