Photokina 2012: Nikon talks about the D800's outer AF issue

"I AM here4U" at Photokina 2012

Note: A copy of this blog article has been published in German language too. Please refer to the corresponding version (Deutsch) before posting comments. Thank You.

Prior to Photokina, I had scheduled a meeting with Nikon Germany, partly in order to discuss the known issue regarding the outer AF focus fields of the Nikon D800 camera.

So, on Tuesday I had the pleasure to discuss the topic extensively with both Michael Wollburg (Manager Customer Support Nikon GmbH) and a Nikon technician in charge. The meeting was very friendly and constructive.

LumoLabs: Using FoCal for testing of an AF sensor array

A2C2 (AFMA Array Color Chart) produced by FoCal for a Nikon D800E with Nikkor 24-70/2.8G lens at 24mm f/2.8 at 1.2m distance.

Previously in my blog, I reported about an issue with the D800 AF sensor array to provide accurate enough results when using its left bank of sensors (cf. /2012/05/d800-outer-sensors.html). The issue seems to be rather general, affecting many Nikon cameras with that AF module produced until now, i.e., D800, D800E and D4. E.g., until Nikon ceases to remain silent on the topic, Thom Hogan decided to flag the D800 "Not Currently Recommended" (www.bythom.com/nikond800review.htm).

Pentax K-5 Preview

A cut model of the Pentax K-5 as shown at Photokina 2010
"The engineering masterpiece" (photo © 2010 Falk Lumo)
 

The Photokina 2010 has just closed its doors and I am compiling my findings regarding Pentax into this article. I posted my first impressions a week ago.

Pentax K-5

The most exciting part (besides trying out the 645D) certainly was to find out about the new K-5 camera. I gathered a lot of material from various sources and wrote a full Pentax K-5 hands-on preview report. A short blog article turned out to be the wrong format and therefore, I posted it on my website. Here you go:

-> Pentax K-5 hands-on preview

Don't miss the exciting news. Moreover, I tried to answer as many questions I received as possible in the report and to skip as much redundant info as possible too.

However, a small number of questions didn't fit the K-5 context so I am putting them here:


Lens questions:

DA 35mm f/2.4:

The new DA 35/2.4 introduced short before Photokina is specified here:

• DA 35/2.4 press release by Pentax Japan (English version)

The production capacity is 2/3 of that of the 18-135 lens and will be available as a kit lens for the K-5. Early reports about the 35/2.4 have been very positive (sharpness wide open, bokeh, very fast focussing, maybe 35mm image circle) and as a kit, may have a very attractive pricing.
The kit will be black only and non-black versions are said to be unavailable (at least outside Japan).


DA 18-135mm f/3.5-5.6:

This lens was announced together with the K-5 at Photokina and is specified here:

• DA 18-135 press release by Pentax Japan (English version)

The lens was received with much interest and will be offered as a kit lens for the K-5 too. A number of features I could clarify:

1. The aperture ranges are as follows:
   18-19 mm f/3.5
   20-29 mm f/4.0
   30-79 mm f/4.5
   80-135 mm f/5.6
2. DC focus motor:
   "DC" stands for direct current according to one source, and for dual current according to another. That needs further clarification although my bet is with the direct current camp ;)
3. Focus motor noise:
   The focus motor almost certainly is not ultrasonic. I tried to listen despite the show's background noise. It sounds different than an ultrasonic motor (it's more motor-like, less twitter-like) but isn't necessarily (much) noisier.
4. Focus motor speed:
   The focus motor appeared to be about as fast as a DA*16-50mm ultrasonic motor. Which isn't fast exactly. The DA 18-55 WR kit lens is a lot faster!
   Pentax missed a chance to deliver a higher speed focus motor (e.g. ring type) rather than a lower production cost one.
5. Image quality:
   I don't know. From the specs, it could be better than that of a usual floating aperture lens.

Teleconverters:

Pentax showed its 1.4x autofocus teleconverter at Photokina 2 years ago. It has disappeared. I asked what happened to it. The response I obtained was hard to accept: Pentax Germany requests production of the teleconverter but Japan denies saying that teleconverters aren't required anymore in the digital age. Pentax Germany tried to explain that a digital magnification may actually have a negative impact on image quality (as we all know) but obviously it did not help much. What shall I say...?


Post Photokina Pentax strategy (as I guess it):

Overall, Pentax is a bold performer in 2010: 645D, K-5 and K-r seem to be 3 winners. And the company itself is making profit. So what else should it take to make a Pentaxian a happy Pentaxian? Not much should one think. Esp. as an Olympus E-5 (good-bye gift?), Canon 60D (downgrade?) and Nikon D7000 (Pentax clone?) are not particularly exciting. But the grass never is green enough ;)

It's because many Pentaxians are addicted to their hobby. They want to invest further in their system and are imaginative enough to foresee "their" camera and lenses in one or two years time.

Therefore, it's a bit irritating that all signs emanating from Pentax at Photokina (foremost but not only from Pentax Germany staff) cry out: we're not going upstream with the K mount. Not now. The 645D digital medium format is as professional as one may actually desire. "Unfortunately" however, cameras in the tradition of the *istD or the higher end film bodies attracted quite a few rather professional photographers into Pentax. Over the last couple years, their fraction is shrinking though and this Photokina won't halt it. It's no fortunate coincidence either that no new Star-lenses are announced and FA Ltds. are tagged "end of life" (whatever that means...) in Germany. Personally, I also don't expect a revival of non-standard lenses (like 85/1.4, 135/1.8, 300/2.8, 600/4, maybe not even a 400/5.6), based on a statement that the rarer lenses aren't currently profitable for Pentax to make. And I hear a bold "no" to full frame. Pentax doesn't see their enthusiast user base go full frame.

To be fair, nobody knows about the future prospect of mirrored cameras. On the other hand, Pentax seems still to be in an evaluation phase with respect to mirrorless cameras and meanwhile anticipates to sell the K-r against mirrorless cameras from other vendors. This worked out fine in the past (with the K-x) but Pentax may start to realize that this won't continue for too long. So, it may be understandable that in this atmosphere of change, Pentax may not invest in the $1500+ segment in order to re-establish their share in that market segment. On the other hand, Pentax learns that what was the $1500 segment (K-7, D300) now is the $1200 segment (D7000) and soon will be in a region with aggressive price competition. As will be mirrorless cameras.

So, some kind of either upstream or innovation strategy was expected and missed from Pentax at this year's Photokina. That's the one complaint. But I hear it from many, from Pentaxians and all over the press watching the market: "where is Pentax heading?".

As a side note, there has been staff from Hoya visiting Pentax and Photokina. E.g., Ned Bunnell was spotted at Photokina and Eguchi Kazumichi (head of marketing imaging systems division) gave an interview. So, if anybody (not me) had a chance to meet with Hoya Int'l management at Photokina and exchange some ideas, please drop a comment... ;)


Summary and verdict:

• K-5: Very solid and satisfying performance. (as known so far) 
• Hoya Pentax: In good shape but tight on resources. Communication skills leave some room for improvement.

P.S.
I activated some ads on my blog. It enables a certain guy to pay reparations:) (just joking!)

K-7 and its precision autofocus

Pentax claims to have improved the autofocus system in the K-7. It is now called SAFOX VIII+ and has an additional sensor to improve accuracy in tungsten light. So, I had a close look at the autofocus system in AF.S mode, the mode normally used to achieve highest precision.

A thing which I didn't do: I did not look into automatic selection of AF points. I've always used the center AF point (the K-7 has 11 AF points, which 9 are cross sensor of).

In order to have a scale of reference, I quantitatively compared with the Pentax K-m which is rumored to have a snappier AF over the earlier K20D. I also played with a Nikon D300 in similiar low light situations to get a better feeling about what I see...


1. Raw technical improvement

The K-7 turns the screw drive focus motor faster than the K-m. A complete drive-thru for the kit lens at 55mm is only 0.18s! This is 40% faster then the 0.25s of the K-m which is fast already (cf. table given below, first row). A overall cycle of the autofocus hunting for impossible focus is 0.81s (down from 0.94s). Interestingly, the K-7 still waits for the exact same 0.45s before hunting back (0.81s = 2 x 0.18s + 0.45s).

So, the K-7 has a stronger focus motor to start with. It will not have a big impact in most situations where the focus won't travel much. And SDM lenses with their built-in ultrasonic motor won't see the difference anyway.


2. Accuracy

I have been extremely pleased with the accuracy in AF.S mode (e.g., in the focus series we are going to talk about below, all images are perfectly sharp). I did comparisons between manual live view in 10x magnification, contrast AF and phase detect AF with a DA 70mm Ltd. and I wasn't able to beat phase detect AF accuracy by manual focussing. Contrast AF and manual live view in 10x magnification have been very close, though and would be a good option with a less evident subject than a test chart ;)

When going from 6x to 8x/10x magnification in live view (only possible with manual focussing), the quality of the magnified image is enhanced (e.g., less noise!) and visibly outperforms the quality known from the K20D.

Other lenses like 40mm, 18-55mm kit lens, 16-50mm, 300mm have all been focussed to "the point". At least for my test camera (which may have been manually adjusted, of course) I have been very happy with accuracy. This includes shots in tungsten light which produced correct focus as well.

Note that many Pentax users are more keen in exact focus rather than fast focus because this is what renders the ultimate image quality the K-7 with its almost 15 mega pixels is capable of if combined with some of the good glass Pentax is known for.

The only caveat is the rear display which will not create the illusion of overly sharp images except if taken with very high contrast. It requires a bit of experience to verify focus. However, the quick view zoom is fast and this is not a big issue at all.

Side note:
Sometimes, the contrast AF achieves focus where the phase AF doesn't and vice versa. E.g., the contrast AF is able to focus onto a white wall or a computer screen displaying a white surface only. Obviously, it is able to find sufficient micro contrast there! Phase detect AF systems are known to fail in these situations.


3. Low light and speed

In order to measure the speed one has to define the amount of light a scene is lit with. It is a knwon fact that below a certain threshold, phase detect AF systems cease to work. Therefore, they may feature an AF assist light illuminating a very dark scene. The K-7 has a green one and it is bright enough to disturb if used towards people. Fortunately, the K-7 only uses it in very dark situations, a lot darker where a Nikon D300 would already invoke its own assist light. In all my tests below, I switched it off!

When comparing to a K-m it becomes obvious that the AF algorithm is almost the same, only faster. It would still focus in 1, 2, or 3 tiny steps to aquire perfect focus.

I've made tests in five different situations of lighting:

1. 3 LED torch behind flash diffusor in 1m distance to target
2. 12 LED torch behind flash diffusor in 1m distance to target
3. Dim tungsten
4. Indoor with indirect daylight
5. Outdoor with subject in shadow

The table below will give exact light values (LV) and temperatures of the lighting, in the same order. You can read more about light values here: Wikipedia on LV .

At ISO 100, exposure values EV and light values LV are the same. LV 0 is very dark and not normally met indoors. I measured LV by spot-metering a white sheet of paper (the target) and adding 2.47 stops to adjust for 100% rather than 18% gray. The correctness of exposure was verified by inspection of the histogram. Color temperature as given where 5500 Kelvin is daylight and less is warmer.

The subject was a white sheet (A4) of paper about covering the frame, with a big cross in the middle painted by a 0.5mm wide black marker. The lens was the 18-55mm WR kit lens at f/5.6. I didn't check if the new WR kit lens is faster (has a shorter AF screw) than the previous kit lens.

The AF speed was measured by analyzing the audio recording and taking the time from start of focus motor (the half button press is inaudible) till focus confirmation beep. The actual shutter noise you hear on the audio tapes is from the full button press at some arbitrary later time.

-- Begin of update --

I have decided to measure the latency between the shutter button press and begin of focus motor activity. I've activated the shutter with a little hammer (button was shielded ;) ). So, one can hear shutter press as well. Below is the sound at 9 EV, daylight. The big sound at the beginning is the shutter button being hit ;)

mp3 audio (shutter press to shutter release)

As one can see, the focus motor delay is 135 ms and complete operation (shutter button to shutter release) is 515 ms (0.38s after focus motor activity). focus motor delay is unnoticeable in practice. In all following figures, the additional (short) delay before focus motor activity is ignored. Add 0.14s to all figures if you need the complete release latency.

-- End of update --

In all experiments, the lens was manually focussed to 35cm prior to first AF operation, then twice from the respective previous position. In all timings marked "~0s", there was no activation of the focus motor. The timings from such a series of up to 3 AF operations are separated by a semicolon (;) in the table below. Timings separated by comma (,) are for the same operation, but from different series. The subject was 1m (or more) away.

For every result, you find a link to an audio tape of the sample, always exactly starting at the first button half press. The amplitude in those samples has been normalized. Don't use to compare shutter noise.

Table of results:

LV	Kelvin	Pentax K-7	Pentax K-m
	Hunt	0.18s / 0.45s / 0.81s	0.25s / 0.44s / 0.94s
0.4	5800	1.71s ; ~0s ; 9.42s mp3 audio	-fail- mp3 audio
1.8	7600	0.81s ; ~0s ; ~0s mp3 audio	0.96s ; 0.45s ; ~0s mp3 audio
2.1	2500	1.47s mp3 audio	0.99s mp3 audio
5.4	5400	0.42s, 0.48s ; 0.29s mp3 audio	0.38s, 0.35s mp3 audio
10.2	5500	0.40s mp3 audio	0.53s mp3 audio

Discussion:

Most remarkably, the K-7 was able to focus at 0.4 LV without focus assist light! It also seems to be about its limit as can be seen by the second refocus operation which succeeded (sharp image) but took almost 10s ;) The K-m couldn't focus at so low light and it seemed to need a full stop more light.

At about 2 LV, AF was already quick and snappy needing about 1s for both cameras.

In normal lighting conditions, AF was even faster, requiring about 0.4s where K-7 was somewhat more consistent. Overall however, it wasn't faster than the K-m.

In normal lighting conditions and when the previous focus position is near the required one (the typical situation), the AF time goes down to a mere 0.3s and feels very quick and snappy.

In available night bar light (~2 LV) I got the impression that both K-7 and Nikon D300 would need about 1s to confirm focus. I didn't take exact measures. The D300 "felt" more reliable though because it didn't noticeably re-adjust focus in the operation.


Conclusion

My conclusion is that the new SAFOX VIII+ as deployed in the K-7 is a significant improvement over the previous installment (the K-m). The 40% faster focus motor and 1 stop extra performance in low light are well balanced with the overall improvement in the K-7 which has a faster shutter and 50% faster burst. The improved accuracy in tungsten light was asked for by many and as it seems, has now been delivered in the K-7.

The K-7 auto focus isn't the revolutionary step some had hopes for. I.e., you will still be able to miss shots ;) But when actually shooting with the K-7, it certainly feels much better than just an evolutionary step. Numbers don't tell the whole story.

K-7 and its continuous autofocus -- PART II

This second part of the article about AF.C in the Pentax K-7 is based on experience in general and from a less controlled test situation.

To speak in general terms, the continous autofocus in the K-7 is significantly improved over, e.g., the K20D. However, so far I know of no autofocus which is fool proof in very difficult situations, like nearby kids running around you wildly. This would require focus latencies well less than 0.1s. I estimate focus latencies for AF.S in the K-7 to be 0.3 to 0.5s, and AF.C may be about 0.2 to 0.3s.

So, I decided to find out just how badly AF.C would fail in such difficult situations. I asked a friend to run "eights" around me, as close and fast as possible. I tried to make as many shots (hi burst) and counted how many good ones and acceptable ones I got. I used the kit lens at 55mm, f/5.6. To tell the truth, I missed to have the subject in center in a couple of shots and I had the AF set to use center point only.

Because of the awkward testing situation, I decided to count as "good" what looked sharp at 1600x1200 resolution. And "acceptable" what looked sharp at 800x600 resolution. "Excellent" would be sharp at full 4650x3100 resolution. The photo at the beginning of the article shows one of the "good" examples from the K-7. I did the experiment with both, a K-7 and a K20D.

Results from random run:

K-7:
35 images in 23 s (1.5 fps)
4 excellent, 5 good, 12 acceptable, 14 fail.

K20D:
24 images in 21 s (1.1 fps)
0 excellent, 2 good, 6 acceptable, 16 fail.

Esp. comparing the excellent/good ones we see that the K-7 could actually capture some of the moments which the K20D couldn't. But it is certainly not fool-proof as well.


Results from straight run:

I made another experiment, with the same setting, where I made the person running straight towards me, as fast as possible, starting from 25m away.

K-7:
17 images
3 excellent, 12 good, 1 acceptable, 1 fail.

K20D:
13 images
6 excellent, 2 good, 2 acceptable, 3 fail.

With ~90% good images, the K-7 looks like a real winner. Of course, I found it a bit cumbersome that only 3 out of 15 good images have been tack sharp (less than from the K20D). Seems like the AF.C in the K-7 has a bit more shutter priority than in the K20D. Nevertheless, with only 60% good images, the K20D was no match for the K-7.

This confirms my general observation with the K-7: in most situations, i.e. in situations where it would be straightforward to keep a moving subject in center will most images turn out sharp. If this is difficult and if one would have to rely on automatic AF field selection then the AF.C in the K-7 will most likely run into trouble too.

Overall, the AF.C in the K-7 is a good match for its improved speed which is a ~70% improvement over the previous K20D.

K-7 and its continuous autofocus -- PART I

Probably the most often criticised feature with a Pentax dSLR is the speed of its SAFOX VIII autofocus system. Interestingly, its accuracy is believed to be very good (which matters most for many situations). But two domains remained to be improved:

• Speed and availability of low light autofocus (AF-S).
• Speed and hit rate of continous autofocus (AF-C) for fast moving subjects (aka action or sports).

In this blog post, I'll study the latter: AF-C. This will be a two part article:

PART I: A car moving towards the photographer

I decided to follow the methodology of a test which was carried out about a year ago by German magazine fotoMAGAZIN (published in issue 5/2008). At the time, I posted a summary about this here:

Magazine Review: K20D/SDM AF.C performance vs. 8 competitors

The method is as follows:

• Continous shots (burst, continous AF aka AF.C) of an approaching white Chrysler LeBaron, tripod mounted and panned centering onto radiator grill. Photograph besides road. Road straight. Test done on a straight highway.
  Original: red Chrysler PT cruiser, closed military airport.
• Controlled speed: 100 km/h (62 mph). Starting photo burst at 100m distance (109 yards, 328 ft). Stopping when passing by. Time to drive through 100m: 3.6s.
• Focal length: 35mm equivalent of 200mm (DA 18-250 at 142.5mm/5.6).
  Original: DA SDM 50-135/2.8 at 135mm/2.8
• Aperture: f/5.6
  Original: f/2.8
• Shutter: 1/180 or shorter
  Original: 1/320 or shorter
• Light: Bavarian white blue sky. Changing conditions.
  Original: Daylight overcast with grayness. Constant conditions.
• Storage: JPEGs onto Sandisk Extreme III.
• Statistics: 6 series, averaged.

So, some conditions had to be changed. Not necessarily in favour of the new setup (assuming that the DA SDM 50-135/2.8 is superior to the DA 18-250).

At f/5.6, depth of field (DoF) is larger than at f/2.8. E.g. at 50m distance, it is 15.7m for f/2.8 and 33.9m for f/5.6. This may look like a big difference. However, the AP-C circle of confusion (20µm) is 4 pixels large and watching for 2px wide sharp edges means that we can compensate for this. Let's just require that 100% crops look pretty sharp.

To have a point of calibration, I decided to test both, a K20D and a K-7.

Disclaimer: I later discovered that I made the following mistakes in running the test. It doesn't ruin the results but should be taken into account: Fine sharpness set to 3 (too high); ISO in some runs set to 1600 (too high), others to 200 (too low); JPG noise reduction not turned off; JPG quality set to 4 stars (too high); driver not driving exactly constant speed.

The problem with 4 star JPG quality is that not 38 (K20D) or 40 (K-7) images are shot in hi-burst but only about 17. The K-7 can (theoretically) shoot 19 images within the time a car needs to travel 100m (3.6s). More if one launches too early or the car is too slow.

Here are my results:

K20D results:

5 runs, per run (total=++/+/o/-):

1. 13=8/3/2/0 too slow
2. 12=6/4/1/1 too far (150m), speed ok)
3. 13=7/0/6/0 too slow
4. 12=9/2/1/0 little too slow and little too far
5. 14=7/5/1/1 too slow and little too far

Fig.1: The 100% crop images for run #2 are all shown in the compound image above and these are the ratings: ++/+/++/++/++/+/++/++/+/+/o/- (last one was too near to have any chance, and last one is not a crop).

The magazine's original result was like

• 7.8=5.8/2 (meaning 5 or 6 good ones and always 2 bad ones; 25.6% bad; rank 9/9)

Taking run #2, discarding the first 4 images (farer than 100m) and counting 2/4/1/1 as 6/2 we would roughly get at the same result. So, I decide to count the first two qualities ++/+ as "good".

In an attempt to simplify my analysis, I then get my new result:

• 12.8=10.2/2.6 (20.3% bad; rank 8/9)

K-7 results:

Due to its higher speed, the K-7 fires away 14 to 20 shots. It slows down towards the end. Maybe because of AF.C needing more time, maybe because the buffer fills up (after ~17 shots in 4 star quality). In another test I could verify that a busy AF.C can indeed slow down the burst speed. Fortunately, I did run #5 over 50m/2sec and the K-7 still took 7 images :)

7 runs, per run (total=++/+/o/-/(subject out of frame):

1. 20=13/3/3/0/(1) too slow
2. 16=11/3/1/1 little too slow and little to far
3. 14=13/0/0/1 correct speed and distance!
4. 17=15/1/0/1 little too fast and too far
5. 7=5/0/1/0/(1) correct speed and 50m (run #5 is not counted!)
6. 20=19/0/0/1 correct speed and too far
7. 16=15/0/1/0 too slow and too short

Run #3 are 7% bad images only. And to quote a comparison: 13 good images is what the D300 delivered in the original test (rank 6/9)!

Fig.2: The 100% crop images for run #2 are all shown in the compound image above and these are the ratings: +/++(8x)/+/++(3x)/o/+/- (last one was too near to have any chance, and last one is not a crop).

In the same attempt to simplify my analysis, I get this result (counting out of frame as bad):

• 17.2=15.5/1.7 (9.9% bad; rank 4/9 -- about same as D300)

Summary and verdict:

The K-7 makes about 50% more good images and about 50% less bad images than the K20D in this test. This looks like a lot. But believe me: when I first saw the images, I thought ... "where the hell is the difference"? Both cameras for sure can capture driving cars except when they are very nearby. The difference is subtle more than evident. But on the other hand, it is significant enough to score side by side with a D300. Seems others also put their pants on one leg at a time. In practical terms, the AF.C will still fail in many situations when one would wish otherwise...

Additionally, the systematic errors in my test are large. So, let's me say it this way then:

The AF.C on the K-7, compared to the K20D, is an improvement making it significantly coming closer to the performance of a D300. How close exactly it comes, I don't know. Could be very close, or not.

This concludes PART I. The next part will study, in a just a bit less formal way, what happens with people running randomly around you ...

-> Continue to Part II