Originally posted by falconeye That's not true. It depends but in a complex way.
I did one more test to rule out or confirm another possible explaination. The tests have been around 1EV which is above the threshold AF assist light is triggered. ISO100, 2s, f/1.8 to be precise.
First with tungsten, a bit brighter: focus almost ok.
Next with halogen: focus miss.
So I thought, it may be the flicker from artificial AC light. So, I tried my battery-powered headlight positioned to give indirect light, even a stop less:
So, next with 3LEDs: focus hit!
I thought this is it, it is the flicker the battery doesn't cause. My battery-powered headlight has two more settings: mini tungsten bulb and 1LED. And it rules it out:
Next with battery tungsten: focus miss (luminosity between the 3 and 1 LED cases)
Next with 1LED: focus hit!
At the last position, the AF assist light wanted to come on 3/4 of times. I report the result without it.
The difference between battery powered tungsten and LED is spectrum only, ruling out the difference in luminosity and flicker.
So, daylight and LED are best, tungsten is worse and halogen is worst. A non-monotone dependency on color temperature.
My best guess still is a simple software error where correction factors are running off. Reminds me of the flash situation actually where the distance-dependent correction factor seems to run off too (following some discussion in Germany). I wouldn't be surprised to learn it is the same engineer to blame here...
To Pentax, I advise a code review of the critical sections of the firmware. One of the advantages of open source actually...
And anybody who likes to think it is a hardware bug: Maybe. But the likelyhood of software is much larger comparing the design complexities involved. Always stick to Occam's razor...
Hi Falk,
I did some tests today and the results are interesting and somewhat puzzling at the same time.
I am too lazy to post all the results again here, but you can check out my posts on the K5 low light focus thread at the other forum.
A summary:
I tested a K20D and a K5 and a focus test target (many thanks to Yvon Bourque!) using my FA 50 f1.4.
I shot test shots of the chart illuminated solely by tungsten light.
I measured the incident light at the target position with my Sekonic L-358 meter.
I shot 3 images of my WhiBal cards, one through my stack of ND filters (3 x .9 or 9 stops reduction).
I then focused the camera on the test chart and took a normal image.
I added the ND stack and re-focused and took another image.
The focus shifted to the front significantly.
Using the color dropper tool in ACR, the color temp for the non-ND shots was 2900K. The temp changed just a bit to 2800K with the filters on.
I shot the same test in bright sunlight.
The focus also shifted to FF with the ND stack on.
Color temps were 5000K, 4950K with no filters and 4750K with the ND stack on.
Here is the surprise of the day:
The K20D also shifted to FF under the same conditions, but the K5 shift was bigger.
Here are some quick videos I shot of the shift on both cameras with and without the filter stack:
K5 -
K20 -
Same lens, same lighting, same target, ND on/off.
I cannot say 100% what is going on, but I am more convinced than ever that it is not color temp related, or at least that color temp is not the main problem.
The shift occurs on both cameras in low color temp light and also in full daylight and all that is required to force a shift is a reduction in light level.
Note that at no time did I test below the metering range even though I did find that the K5 will lock well under the camera's ability to meter. It's just that it locks at a FF position.
Ray