LumoLabs: Pentax K-5 low light focus study is published

Started Feb 23, 2011 | Discussions thread
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falconeyes
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LumoLabs: Pentax K-5 low light focus study is published
Feb 23, 2011

LumoLabs: Pentax K-5 low light focus study is published

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. [from the study linked below]

LumoLabs (this is what I call my lab activities) has decided to have a closer look at the K-5 low light front focus issue. After careful evaluation and many hundred test shots I must say that the issue is real.

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 forum article. Fig.1 above may provide a first idea of the work though.

Please, refer to the full blog article and to the complete paper for our findings:

(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.

7. I don't rule out that 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.

Please, refer to the full study and accompanying material before discussion. I am available to answer questions. Enjoy the read

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