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.

Photokina 2012: Nikon spricht über das D800 AF Problem

"I AM here4U" at Photokina 2012

Note: A copy of this blog article will be published in English language too. Please refer to the corresponding version before posting comments. Thank You.

Ich hatte im Vorfeld der Photokina mit Nikon Deutschland vereinbart, ein Gespräch u.a. zum bekannten Problem der äusseren AF Sensoren bei der Nikon D800 Kamera zu suchen.

Am Dienstag hatte ich nun Gelegenheit, die Angelegenheit ausführlich mit Michael Wollburg (Manager Customer Support Nikon GmbH) sowie einem anwesenden Techniker zu besprechen. Das Gespräch war sehr konstruktiv.

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).

Wiesn Girls in dirndl

A girl at Oktoberfest 2011 München (aka Wiesn). Wiesn girl #1

A trademark of Wiesn (the original Oktoberfest) are girls wearing dirndl for showing off their (sometimes remarkable) décolletage also known as cleavage. Dirndl are a traditional dress in Bavaria. It used to be worn by poorer working girls in the past. But more recently, it became rather popular at Oktoberfest because of its figure-hugging qualities. It even appears that the dirndl currently makes new friends all over the world. A fashion trend made in Munich.

So, all of this theoretically provides for ample of photo opportunities like in a giant fashion show. Except that Wiesn is a very crowded party zone and nobody would want to make it easy for you. On the other hand, taking photos is a tolerated sports as long as they are serious and don't show up in the internet without permission (which is why I omitted the face of people I have no permission from). Nevertheless, good décolletage photos are rare (except from prominent people posing to be photographed) and I made a test of what can be achieved.

Equipment wise, I used a Pentax K-5 with the DA* 60-250mm/4 using AF and a Pentax K-x with a DA 15mm/4. I fixed the latter to f/8 and 1.4m manual focus with SR turned off to have sub-second reaction time to take photos in the crowd. No photo bag to keep a low profile. This turned out to be quite a capable combination.

Actually, the girl in the opening image is taken from a video I took with the K-5 at 250mm using manual focus while she walked up the staircase to the Bavaria monument at Wiesn. The manual focus was difficult to maintain (and is not perfect). But I still like the result as a photo I may not have been able to shoot as a still.

Below is this staircase in the background.

Bavaria monument

Below, I'll show impressions from Wiesn 2011. Note that many images are heavily cropped. Also, some post-processing was applied to reduce some hard shadows from the sunny day.

Wiesn girl #2

Wiesn girl #3 (note the cleavage)

Wiesn girls #739.347 to #968.567

Wiesn girl #4 ("Awaiting him while being looked at")

Wiesn girl #5

Wiesn girl #6

Wiesn girl #7

Wiesn girl #8

I'd like to know from any comments if you like the fashion trend at Oktoberfest. It definitely provides for some nice eye catchers. If you have any favourites in this mini series, post a comment. I'd like to know your taste too :)

Thanks for stopping bye and I hope you enjoyed your stay.


Falk

LumoLabs: Pentax K-5 low light focus with firmware upgrade 1.03

Fig.1: "Caribbean Sunset".
Why doing measurements is no substitute to taking photos (taken while I type) ;)

The current story is continued from the blog article Pentax K-5 low light focus after Pentax has published the version 1.03 firmware upgrade officially featuring an improvement of focus operation in low light.

When the 1.03 update was released, many users reported positive findings. So, I was optimistic to find that the low light focus issues are a thing of the past. Unfortunately, this isn't what I eventually found.

Please refer to the updated version of the complete paper for my findings:

• www.falklumo.com/lumolabs/articles/k5focus (HTML version)
• www.falklumo.com/lumolabs/articles/k5focus/K5Focus.pdf
  (PDF version for download and offline reading)

Summary:

Basically, I find no significant difference between firmware versions 1.02 and 1.03 when using my testing scenarios. This means that there is no significant improvement if:

• The background is white
• The target has high contrast
• The AF assist light is disabled

Under the above circumstances, the K-5 auto focus starts to lock focus in a false front focus position at tungsten light levels below 0 EV for a fast lens (like f/1.8) or below about 2-4 EV for slower lenses. This did not change at all with release 1.03. Note that the above light level values would read 2-3 steps higher with a target like a Caucasian skin (18% gray).
Moreover, I found that at low enough light levels, the 1.03 firmware seems to rely more than 1.02 on color information outside the direct selected AF spot. With a uniform color distribution, this can lead to somewhat improved results in the vicinity of the focus shift transition. OTOH, the focus system is more easily fooled by features with a singular color. Overall, the advantages may dominate and the effect is small anyway. Version 1.03 still doesn't seem to make use of white balance information, either manually or automatic.

If I believe that Pentax improved the low light focus situation for many users of the 1.03 firmware, then the progress must be bound to any of the following:

• AF assist light engages more actively (not tested by myself).
• Focus improved in the presence of dark backgrounds (not tested by myself).
• Focus improved with low contrast or dim focus features (not tested by myself).

As I don't have a version 1.02 K-5 anymore, I cannot find it out. But any progress brought by the 1.03 firmware upgrade is limited rather than universal.

As much as I applaud Pentax to having addressed the problem, as much I am a bit disappointed they didn't dig deep enough to address the root problem: that the K-5 AF subsystem locks focus even in a situation where the colorimetric sensor(s) fails to determine the light color. It simply shouldn't lock focus at all then. Or ask for user assistance like a priming shot. The light-sensitive AF system of the K-5 has too strong a color dependency to autofocus if the color of the focus feature remains unknown.

A better workaround than 1.03 in firmware is feasible and should stay on Pentax' agenda.

Please, read the full paper linked above if you have further questions and come back here to leave comments or questions. General comments about the issue should still go to the general (earlier) article while firmware release 1.03 comments should go here. Thank You.

LumoLabs: Pentax K-5 low light focus

Note: I updated the article to version 1.3.
2011, March 27.

Note: I updated the article to version 1.1.

I think, the feedback and experiments done after version 1.0 helped to understand the observed focus behaviour rather well and version 1.1 takes this into account.

An important (new) aspect is a description how low key studio photography, esp. when combined with an f/4 lens (or slower), can lead to inaccurate focus.

2011, March 07.


Fig.1: Accuracy of the Pentax K-5 phase detect AF vs. luminosity in EV. The above chart includes all measurements, i.e. various lenses, light colors, distances and apertures. The accuracy is measured as deviation of the focal plane from the sensor plane, in µm.

In modern times, each new release of a digital SLR camera seems to be accompanied by teething troubles. This applies to all makes across the board. Sometimes, they are fixed quickly by the vendor, like the “Green Line Syndrome” video issue in the Pentax K-7, the “Hot Pixel” video issue in the Nikon D7000 or the “String of Pearls” stain issue in the K-5. Sometimes, they aren't like the shutter-induced blur issue with the K-7 which LumoLabs succeeded to document in this blog.

Currently, there are two remaining teething troubles for the K-5 which are widely reported: wrong PTTL exposure with external flashes in some situations (confirmed in writing by 3rd party flash makers, even for Pentax' own flashes). And a systematic wrong lock of autofocus in low (tungsten) light.

Therefore, LumoLabs has decided to have a closer look at the issue. After careful evaluation and many hundred test shots we found the issue to be real. Pentax has unofficially reported to work on the issue. The pressure is on them to address the issue and the author hopes that our findings may contribute to their efforts. Buyers of the K-5 must be able to be confident that the issue is fixed sooner than later.

A preliminary copy of the paper was provided to Pentax earlier this week and the head of Pentax Europe officially receives a printed copy today. I have been told that Pentax engineering will receive a copy too.


The study

The results are too complex to be presented in the scope of a blog. Fig.1 above provides a first idea of our work.

Please refer to the complete paper for our findings:

• www.falklumo.com/lumolabs/articles/k5focus (HTML version)
• www.falklumo.com/lumolabs/articles/k5focus/K5Focus.pdf
  (PDF version for download and offline reading)

You'll gain a deeper understanding of Fig.1 too ;)

In a nutshell, this is what we find:

1. The K-5 as it presently ships indeed has a flaw in its phase detect autofocus module or software which causes it to front focus in low light below a lens-dependent threshold in EV.
2. If it does, it seems to consistently focus ≈ 255 µm behind the sensor plane (although with a significant ± 75 µm scatter of results which is about twice as large as the normal scatter of result).
3. Faster lenses seem to keep working in lower light but of course, are prone to more blur when the front focus does eventually happen. Slower lenses can already start to front focus at light levels metering as 4 EV or 6 EV even. A fast lens may work down to 0 EV in white light.
4. Light sources other than daylight emphasize this problem as they simply appear darker to the AF module. Moreover, it seems to be moderately color blind for red which further emphasizes the effect in deep tungsten light.
5. The effect is real and can negatively impact the daily work of a photographer.  On the other hand, it is possible to run into a low light tungsten situation without the problem.
6. The paper clarifies conditions to hit or avoid the issue. White light (halogen is not white enough though) and a wide lens stopped down help to work around the problem. AF assist light typically doesn't help though. But an LED flash light does. ;)

It may be interesting to note what we did not find: There seems to be no strong dependency on the light color. Except that colored light causes the transition to happen earlier as it means less usable light for the AF module. It seems to have a somewhat low sensitivity esp. for red light. Moreover, there seems to be no dependency on actual aperture or distance.

Move focal plane by 0xFF µm? Yes, do it so! :)

This may describe what's going on behind the curtain: a µm-valued variable becomes 0xFF (255) and causes a false shift of the focus plane by 255 µm. I call it the +0xFFµm hypothesis. :)
- Is it likely? No.
- Is it possible? Yes.

Please, read the full paper linked above if you have further questions and come back here to leave comments or questions. Thank You.

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.