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.