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Messages - Mika

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16
Lenses / Re: Dxo tests canon/nikon/sony 500mm's
« 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 µm 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.

17
Lenses / Re: Dxo tests canon/nikon/sony 500mm's
« 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.

18
Lenses / Re: Dxo tests canon/nikon/sony 500mm's
« 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.

19
Lenses / Re: Dxo tests canon/nikon/sony 500mm's
« 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.

20
Lenses / Re: Dxo tests canon/nikon/sony 500mm's
« on: July 24, 2013, 04:50:48 PM »
I personally think this is the weirdest lens test article I have ever read. It is not a lens test, but a system test.

What it comes to Hasselblad measuring the MTF of the superteles, not all information is included there. Typically reduced secondary spectrum doublets achromats have a better MTF than equal element number apochromatic designs, but the color error is then visible with areas of high contrast difference. I'm also interested in the Hasselblad's methodology of doing the test, could you send me the link, please?

It is not always clear to me whether Nikon is reviewed with software corrections enabled on the color aberrations in these tests or not. For that reason, I'd be a bit suspicious on using current Nikon objectives with film.

Even in the Digital Pictures crops, it is not clear to me, how the crops were obtained from RAWs. But for me it seems that the Canon lens performs actually better than Nikon's at this distance for all tested field points. Whether this difference is significant in real life photography is another story.

By looking at the manufacturers MTFs for 200/2.0s, it seems Canon has optimized the 10 lp/mm performance while slightly sacrificing 30 lp/mm performance compared to Nikon, and averagely the impression seems to be that the Canons 200 mm performs better (to my eye), and considerably better close to the edge. This is nothing new, it is known that images taken with a lens that has a better macro contrast tend to look better than the ones taken with a lens with less contrast but higher micro-contrast. Only in the center crop we can see a bit of the micro-contrast at play, but it isn't clear to me what causes aliasing (or Moire?) on the Canon crop, is it the lens or the sensor.

The MTF tests that we do actually does not include the camera body, but a microscope objective and a known image sensor. It is arguable whether this gives full information since there is further processing due to the software, but it allows checking which lenses are better corrected to begin with.

21
Some people already mentioned the lighting. Usually, tungsten filaments are relatively color stable, but compact fluorescents or fluorescents are not. I did have a minute differences in the color balance when using available light inside a room, until I realized the effect of the CFLs. Does this color balance issue happen outside (when the illumination is stable)?

22
Lenses / Re: Poll: Most Wanted New Lenses of 2013.
« on: May 08, 2013, 05:01:06 PM »
Voted for a new 50/1.4. Although, if Canon doesn't get it done soon, I'm pretty sure Sigma will.

The other thing I'd like to see is either 400/5.6 IS or 300/4 IS II. I'd love to have an affordable F/4 telefoto, but 400/4 would probably be far too expensive, hence 400/5.6 IS or 300/4 IS II (and this time, make that IS 4 stops)

I'd also prefer 14-24/4, not 2.8. Up till then, Sigma 12-24 is hopefully sufficient.

23
I'm a bit late from this bandwagon, but I'd also venture to guess it's the heat waves & sand in the air (seems a bit yellowish to me). Focusing through these is difficult to any AF-system, they cannot detect the presence of heat haze. The other possibility is a bad filter, try it out at home on mid day to see whether it degrades the image or not, I have had it happen to me with telephotos, but not with wide-angles. Your luck might be different.

I haven't figured a way of getting good landscape shots during mid-day, but that's my limitation, there's bound to be someone who can, but I'm not also a full time pro. I have climbed on top of a volcano to shoot about a hundred of photos there at the height of 3 kilometres, not realizing that the metering is different from sea-level due to 30 % more solar radiation and considerably less atmospheric haze - you can imagine the success rate... Should've brought CPL and a ND grad there, but that thought simply didn't cross my mind. But you learn something new every day.

I don't know, it's just a thought of mine, but occasionally it also pays off to exaggerate the natural phenomena. In your case, I don't know how it would have looked like if shot wide open and slightly closer to the camels so that they would be in focus and the background not. But then it wouldn't have been a landscape shot to begin with.

I wouldn't strictly adhere to rules of thirds, it's sorta saying that averagely stuff looks better if placed here, keyword being averagely. When on vacation and I was carrying a 28/1.8, I think I did have to apply a bit more contrast on the scenes to get them more to my liking compared to 85/1.8. But that's just a tad more, and I haven't tried 28/2.8 either.

24
EOS Bodies - For Stills / Re: 5Dc a good option?
« on: April 08, 2013, 11:59:48 AM »
It's not the first time I hear 5D would have better low ISO performance. Haven't seen that many 5DII images to make the call.

But if OP is a professional, then 5DII is pretty clear answer. I can live with the risk of 5D coming apart any day. Professional can't, and that would be enough for me to turn towards 5DII, the probabilities work in its favor. For amateurs I would say to get the original 5D.

25
EOS Bodies - For Stills / Re: 5Dc a good option?
« on: April 08, 2013, 11:34:32 AM »
I've been going with 5Dc for some time now, got it around 450 € second hand. If you aren't a professional, I wouldn't put my money on the bodies, and I don't know whether there is that big a difference between 5D and 5DII.

My worst deal ever was to buy 20D new. Now I could only get about 10 % of the price I paid back, and this hasn't changed. I don't want to buy 5DIII for that reason only. If you plan on using 5DII, be prepared to sell it after 5DIV or 6DII comes out, that way you'll probably get most of it back.

Currently I'm just using 5D and 20D until they break, after that it will be the next cheapest FF frame available.

26
Lenses / Re: Lens purchase strategy
« on: March 04, 2013, 07:16:26 PM »
Heh, I suppose the lens purchase strategy depends on whether you are a professional, what things you like to shoot at, and what is financially possible. Or if you happen to decide that particular objective would be interesting for your "research" (hey, I do optics in my day job). I checked a 200/1.8 recently, but decided against that since it really wouldn't work with the way I shoot. Too bad for the large aperture though.

Seriously though, since I live in Scandinavia where light is scarce for four months of a year, I do sort of need two sets of lenses. I use fast primes for the winter time, and zooms in the summer, though you won't find me carrying a zoom on vacation.

But I'd recommend not going for the most expensive thing first, but just something on the focal length range that you think could be interesting. For example, Sigma 12-24 is completely sufficient for me in the ultrawide area, while 120-400 does the trick for the 400 mm stuff. Most of the things I take photos of reside between 24 and 200 mm, typically between 24-70 inside and 70-200 outside. 120-400 is more for small animals.

For a vacation, if I think I need a camera with me, I'll probably go with a 20D and 28/1.8 and 85/1.8. That tends to cover all I need, up to night time too.

27
Lenses / Re: Canon EF 12-24 f/2.8L [CR1]
« on: February 28, 2013, 06:08:05 PM »
You may want to hold your horses on this one.

The more recent 14-24 mm patent actually contained a 14/2.8, a 17/4.0, and 14-18/4-4.7, I don't recall the fourth embodiment just now, it had a zoom factor of 1.2 and was in the 20ish millimeter range. This is not to say 14-24 does not exist on Canon - they are surely trying to find a way to do that and determine whether this would sell enough.

Canon seems to trade corner sharpness for less distortion at their wide angles, this is the reason why they don't attain high drawing capabilities - I'm sort of glad they do when using film cameras. I'm not sure on how they deal with field curvature in their ultrawides.

The question I have is what is the point of 14-24/2.8? I understand 14-24, but I don't understand the F/2.8 part in it - the purpose of F/4.0 is much more clear to me.

28
Lenses / Re: Landscape tips needed on shooting the Grand Canyon
« on: November 18, 2012, 07:42:25 AM »
If the haze bothers you, I think there is one comparatively cheap option to try. You can try diminishing that effect by using a circular polarizer. What is harder to know is how much difference it makes, as this depends on the scene.

It is not like I'd typically nitpick about this stuff, but the scientific term for cause of the blue haze is scattering, not diffraction. Other than that, the explanation was correct, the blue haze is caused by stratospheric scattering, and since blue is the shortest wavelength, it is the most affected.

29
Lenses / Re: 24-70 F/4L IS - Why I will Buy/Not Buy this lens.
« on: November 06, 2012, 06:00:04 PM »
Actually, I think Canon has done a comparatively smart move with this one. If the price level drops to around 1000-1100 €, I'm actually very tempted. I'll still keep the 24-70/2.8 mk I for different purposes, it is just that awesome lens in my hands  ;D 24-70/2.8 would remain in my bag just because of the very good bokeh, not to mention other capabilities!

But this new F/4L, I could not believe my eyes when I saw macro 0,7x ratio! That's just insane and shows the guys in the lens department at Canon Inc. have indeed been working! For a general walk around lens, I always thought 24-105 just did not have the close focusing capability like the 24-70 does.

What's even better, the new 24-70 lens is a lot lighter than 2.8 variants and does provide image stabilization for some lower light photography. For me, this would be the thing I'd take for trekking or travelling, especially since it is smaller and lighter. Not to mention the massive improvement in macro capabilities. It sounds like Canon has indeed listened to our collective whining :)

I'll wait for them to work out the kinks and the price to drop some tens of percents, then it is most likely a deal. Any ideas of the distortion of this one, though?

30
Lenses / Re: Can a UV filter affect IQ (sharpness) on a lens?
« on: November 05, 2012, 06:42:41 PM »
The UV filter will not only affect the sharpness of a lens, but also the whole performance, including bokeh. This is specifically true for telephoto lenses, following are shots taken with Sigma 120-400 immediately after buying it (sorry, I had to resize to 70 % of the original due to attachment size limit).

I wondered about the performance of the lens as I thought it really should be better than what I got, and contacted the importer. It turned out that the cause for image quality degradation was a comparatively bad UV filter.

I actually do have some interferograms of the filter; the filter shape error can be determined from bokeh outline (diagonal lines). Without the UV filter, the lens worked much better. I still use UV filters in front of normal and short telephoto lenses, but not in the 120-400. The only reason why I still keep them on is mainly the ease of cleaning a separable glass plate.

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