Thanks to froeschle, I now had a chance to have a deeper look into the test.
First, the test is more complete and consistent than I would have thought. It certainly is the best test published in the print media about AF accuracy that I am aware of. But it isn't without its own flaws.
First, what the test does right:
- First, they work with experts.
Image Engineering - test charts . camera testequipment . image quality analysis is who did the test and I guess, it wasn't cheap. Image Engineering is on the better side of testers. I know about one who did his thesis there and I've seen his diploma thesis about the efficiency of shake reduction. Good work overall.
It measures sharpness in an objective way. The test method is described here:
Der neue Kameratest - Testversion 1.6 - colorfoto - Magnus.de (section "Auflösung").
Basically, they measure the spatial frequency where MTF drops to 10% aka MTF10. That's a more intelligent measure than MTF50 and they use a star chart rather than edge blur to do so. Their method may therefore be superior to Imatest's (and photozone's or mine). For an AF test, it is a bit overkill though. More cumbersome, it is a resolution test not allowing an easy mapping into focus accuracy (what it more easily possible in my tests based on edge blur).
. - They use a focus series to determine best focus
. - They use a multitude of lenses and focal ranges (however, they seem to use each lens wide open only).
. - They make 1300 shots overall, 10 shots at each setting. That's good enough statistics.
. - They defocus the lens prior to every test shot.
Second, the results in a bit more detail:
Based on this setup, they find that Pentax performs best in the phase AF camp. As I said, they declare Panasonic to be the winner (overall). But they also declare Pentax to be position 1 (Platz 1) in the phase AF camp. It isn't a headline in the text though.
But the opening double page shows six cameras with bar charts and it clearly shows Pentax to be ahead of everybody except Panasonic who is the clear winner.
Pentax wins #2 because most of the lenses performed strong.
- Strong in particular: DA21, DA14, Sigma70-300, DA50-200
- Average (which is good): DA15, Sigma10-20, Sigma18-250, DA12-24
- Weak: DA*55, Sigma50/1.4
For the zooms, the average lenses were strong at some focal lengths. They tested 10 lenses, 20 focal lenghts and 200 shots for Pentax.
It was 410 shots for Canon and 270 for Nikon. Canon only had two strong lenses. But the 16-35 (average at the wide end) was very strong at standard and tele end. Similiar for Nikon: no strong lens, but e.g., the Sigma70-200 at 135mm was strong. So, YMMV.
They made 30 shots with a NX11 50-200 and 10 shots with a NEX-5 16mm but did not rank them. Both were very good.
Olympus ranked behind Pentax (despite contrast AF) because of a few weak lens/focal length combinations, such as 14-42 at 42mm.
Sony SLT was good overall, but the 70-200 showed catastrophic results, with obviously some totally blurred images.
BTW, Pentax was second fastest in the phase AF camp too, with 390ms for 1000 Lux (Sony SLT was fastest with 330 ms). Actually, the Panasonic GH2 won the speed comparison too (280ms!, despite contrast AF, 450ms for the GH1). That's pretty amazing and another headline, maybe. Almost twice as fast as a D7000 (460ms). But at such good lighting conditions, static AF speed isn't the real problem anymore anyway.
Originally posted by Class A the quote
This is the table Class A posted, complemented by the two contrast AF entries:
My rank | Type | Camera | % sharp | % acceptable | % not sharp | speed 1000lx |
1 | contrast | Panasonic GH2/1 | 85.3% | 14.0% | 0.7% | 280/450ms |
2 | phase | Pentax K5 | 62.4% | 28.9% | 8.6% | 390ms |
3 | contrast | Olympus EP1 | 57.0% | 29.0% | 14.0% | 1040ms |
3 | phase | Sony Alpha 55 | 62.1% | 16.4% | 21.4% | 330ms |
5 | phase | Nikon D7000 | 35.2% | 40.0% | 24.8% | 460ms |
5 | phase | Canon 7D | 40.2% | 29.7% | 30.0% | 440ms |
Third, the flaws in the test:
IMHO, there are 4 major flaws in the test:
- The sample variation of bodies is large. They did not write that they did AF fine tuning for each lens tested, prior to the test. For the K-5, I found this to have a large impact on results for such a test. A 5% drop in MTF10 is a rather fragile measure.
This means that the tested K-5 had good calibration out of factory. Which is a combination of luck and better than competition quality control. Quite some surprise
. - They make an error in their scale:
They measure sharp to be 95% spatial frequency resolution of best focus. IMHO, that's nonsense and a result of the difficulties to map MTF10 to focus accuracy. I.e., a soft lens to start with, which has a small aperture, will have an advantage because the criterion allows for much less accurate to be still considered sharp.
This explains why many mediocre lenses (like DA50-200) performed nicely in the test. Worse even, it introduces an arbitrary factor into the test: test enough bad lenses (or closed down lenses) and you'll win the AF competition. I don't see that they did this. But it is a flaw of the test methodology.
Moreover, a body with high resolution is at disadvantage too, because then the 100% measure is higher. That certainly was an extra burden for the 7D.
They should have recomputed focal plane accuracy (in µm/sensor size) from image quality. This is doable as I've shown in my K-5 low light focus test.
. - They don't select from even lens categories, like one fast portrait lens, two standard zooms, a wide and a tele prime etc. With equal aperture settings in the test.
And they don't predetermine aperture across similiar lens tests.
. - They only tested at 1 luminosity: 1000 Lux.
.
One may argue that a higher resolving lens or camera requires better focus accuracy and that the test automatically accounts for it. However, I dismiss this argument. If I want to know what to do to produce a sharp photo when hanging on a wall, I want to know what camera and lens to choose. And not the other way round.
Overall
Despite the methodological flaws, I think that the result holds true. They print bar charts for all 1300 shots! And their entirety speaks a clear language:
The tested K-5 outperforms the tested 7D, D7000 and A55 in terms of phase AF accuracy (and speed actually).
If this only holds true for the tested samples or is a general result on the market remains to be seen. But Pentax always had a good reputation for focus accuracy and maybe, for a very good reason. I think overall, the test result is credible.