June 19, 2009

Comparative resolution study K-7 vs. K20D

This blog article was initially meant to be part of the noise article. However, the matter turned out to be much more intricate than I thought (and somehow, it still is). So, I made it a separate article again owned by falconeye.

I would have loved to make it a short answer. However, I am going to tell you a story instead. I am sorry that Falk Lumo will have to wait with his catwalk beauties ;)

In the top of the article, you see a (synthetic -- more on this later) resolution test shot for the K-7. Please, click on it and open it in original size, and scroll to make the center visible. Note that what you see is the inner part of a special edition of the ISO 12233 test chart. Special because it contains insets of 4x resolution. I made this edition to be able to measure today's dSLRs. It is printed in A2 size and the printer's dots start to form Moiré at figure "8.5" in the chart insets, i.e., between "8" and "9". No big deal, just something to keep in mind.

For more information about the ISO 12233 test chart, you may want to visit this link:

Also, note that the test chart only fills 86.7% of the picture height. As a consequence, the chart figures have to be multiplied by 460 (rather than 100) to yield LW/PH (line widths / picture height). Read, the K-7's Niquist limit is at chart figure "6.7". There shouldn't be any structure beyond this number.

Now, look at fields labelled "7" or "8" (we ignore "9" and higher for the reason given above). There still is structure but it is just Moiré, not the proper line pattern which is printed. So the lens plus AA (anti-alias) filter outresolve the sensor and we pick up a pseudo pattern which basically is a sampling error (which is what is called a Moiré pattern). Would lens plus AA filter be at the sensor resolution or below, than we would have uniform gray fields labelled "7" or "8".

First result:

The K-7 has an AA filter which allows to outresolve the sensor. It better has :)

Now comes the tricky part. Because we want to compare with the K20D which we must assume does have an AA filter which allows to outresolve the sensor, too. How on earth shall we measure a resolution beyond the sensors' capabilities? If I a had the accuracy and software of a testing lab at my disposal, I would extract these numbers from subtle details in the respective MTF figures; but I don't.

First, let's verify that the K20D is close indeed. Here is the corresponding synthetic resolution test shot for the K20D:

Oh yeah, just as nice :)
(About 3 visible fine white rings in the center circle.)

Ok, let me get serious now (which means that most likely, you're gonna run away before the end of the article -- just make sure to join in for the verdict ;) ). In order to study the AA filter's effects, I will apply a technique called superresolution. This technique involves taking multiple test shots (16 in this case) and creating a synthetic image at twice the native resolution, i.e., an 58 MPixel image! In order to construct this image, a "kernel" (deconvolution with a native pixel's PSF (point spread function)) is involved. By pure accident, I happen to have a kernel which rendered me good service with my K20D. Let me call this kernel "Falk1". I refrain from describing the exact procedure here... Let's just accept that, by some miracle, we now have test images at twice the resolution.

Because they are so large, I will only present crops (200% crops = 100% crops of superresolved image). Let's call them supercrops. First, for the K-7, next for the K20D.

Now, we see 5 rings and we suddenly see a difference, too! However, let's be cautious. The K-7 image has some artifacts. In the K-7 supercrop, the lines have more false color and more interestingly, the end of lines have a bubble artifact. E.g. look at the "5". From my experience with the superresolution method, this is a strong hint that the kernel (Falk1) overcorrected (similiar to the artifacts from oversharpening). While undercorrection just leaves some softness, overcorrection can have more funny effects ;) The assumed PSF was too wide. With a narrower PSF (Falk2, to be developped now ;) ), the supercrop would have looked better.

Second result:

From this, I am going to conclude that the AA filter in the K-7 is a tad weaker than in the K20D. This would also easily explain the pick-up of some false color in the demosaicing process.

But let's assure that this is a very subtle effect. Just any microscopic change in focus will add the softness required to make K-7 supercrops look like K20D supercrops. Here is an example from an earlier (premature) series I shot while doing the noise tests. Now, the difference is gone.

The images are with a Pentax DA 70mm Ltd. at f/5.0. Except for the last, shot with a Zeiss 50mm at f/4.0. And I did verify that a Zeiss 50mm at f/4.0 produces the same bubble artifacts when in precise focus.

One last thing ...

The images shown so far have all been synthetic. I.e., they are downsampled from the superresolved images. Here are three unaltered sample resolution images as shot:

Unaltered resolution test shot, K-7 + DA 70mm Ltd. @ f/5.0
(Note: underexposed on purpose)

Unaltered resolution test shot, K20D + DA 70mm Ltd. @ f/5.0

Unaltered resolution test shot, K-7 + Zeiss 50mm @ f/4.0 (from "premature series", 4.5% larger)

Interestingly, the unaltered sample image from the K-7 looks just a tad softer than from the K20D. Which is why I showed a third sample (with the Zeiss) to verify that this is within a range of insignificant variation. This is the reason why I consider the matter to be intricate still. Some samples tell a different story than other samples ... But it is all in the subtle details, anyway.


Pentax K-7 and K20D both have an AA filter of very similiar strength. It is pretty hard to say which one is weaker. I take my bet and say the K-7 has the weaker AA filter.

But by just so little that in almost all normal shooting conditions, I expect to see no significant difference in practice. The above image summarizes the data this verdict is based upon.

These and more test images are in the following image gallery:
Resolution and Noise of K-7 vs. K20D
Please, visit for further details.

Still here?

Well, one more last thing ...

Myself (and I know of at least one other alpha tester as well) have been wondering why the images on the rear screen, when magnified after the shot, don't look as crisp as with some other makes. One could think that the images are a bit soft. So, I made a test and compared the magnified image on the rear screen with a 1:1 crop as developed by Lightroom with default settings.

As you can see, the image on the rear screen, when magnified after the shot, does indeed look a bit soft. Even if it is perfectly crisp on a computer screen. I verified that this holds true for in-camera JPGs as well (to a slightly lesser extent). I think that this is something to keep in mind when working with the K-7. Even though it has a VGA screen and goes up to 32x magnification, it never shows the pixels. Probably a feature rather than a bug;) This may have been part of early rumors that the K-7 produces softer images than the K20D. My tests have not been able to confirm this.

Still here?

Then you must be waiting for the catwalk models ;)
Thanks for your patience and interest.

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