March 27, 2011

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

Caribbean sunset. © 2011 Falk Lumo
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:


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.


  1. Are you saying that "locking focus" when it shouldn't is the *only* problem?

    Or is it fair to say that the K-5 loses the ability to focus correctly too early?

  2. @Anonymous, yes, that the autofocus can lock a false focus is the main concern. With 1.02 and 1.03. It is a concern because it reduces trust in a photographer's tool and requires some experience to be reestablished.

    Under certain circumstances (dark background, slow lens) this could happen too early too. I didn't see it in my tests but it remains a possibility that the latter behaviour improved with 1.03.

  3. I'm almost sure, that there were two separate focus issues with K-5 AF system. The first one - one that I was talking about before and which was correctable by custom tungsten microadjustment (well, at least in my case) engaged in USER mode whenever I was in tungsten light situation, was fixed by firmware 1.03 absolutely and completely.
    I don't have that problem anymore and daylight calibration settings work perfectly in (almost) all tungsten light situations that where problematic before.
    The second problem is the one that you described and it is the problem with serious FF shifting with some shooting situations around 1.5-3EV. In case of my camera, I don't see any problems with white or mixed background whatever color the target is, but there definitely is the problem with a dark background and small, relatively bright and contrasty target - AF locks in a front of it constantly if the light level falls low enough.
    Strange thing is, that is has no problems to lock properly on dark elements on that background itself and it does it constantly with perfect consistency if pointed to (still no AF assist light engaged). System starts to lock on lighter target properly if that target is slightly bigger in the relation to dark background area (metered light level at a target surface is still the same) or if I put some (slightly) brighter elements in the background that change the light-dark areas ratio. If I can understand that focus shift is cured by changing the scene light reflectance (changing the target size relation to the background or percentage of dark areas in the frame), I don't really get how can it lock properly on much darker and less contrasty element in the background with no problems at all, if the scene is the same with exactly the same reflectance. I think that there must be indeed some kind of a software bug that just turns off the proper defocus setting in some scenarios.
    And in the end my K-5 can really focus good enough at -1EV in most situations:

  4. IMHO, Safox9 sux. Too many problematic K5 and Kr to search another issue. I owned a Kr and I'm sure that all Kr's have problem. The difference is made by the owners: some of them are bothered by the ff issue, others don't.
    I had all the disponibility to become a pentaxian, but with such of toy cameras, I can't. I bought a Canon 550D and the focus is where must be. For me Pentax is a closed chapter. Closed by them not by me.

    PS: I was banned on the Pentaxforums just because I criticized Pentax. Shame on the moderation team!

  5. @G.D. Vlad - IMHO, you have no idea what you're talking about. Period.

  6. What about general focus confirmation? My understanding is that this would also be affected - is this correct?

  7. @Anonymous (anonymous posters please leave a nickname...): I am sorry but I don't understand your question. In very dark territory where K-5 misfocusses, the focus confirmation with manual focussing (or a focus trap) would be off by the same amount too. You can even focus manually and correctly (e.g. using LV) and read the "computed defocus" from the phase AF subsystem in the manufacturer part of the EXIF data.

  8. could be interesting to repeat some of the testing with a newer sample (serial 40...). I can't get rid of my suspicion that Pentax had more than just a firmware bug when they started to sell.

  9. Wow, I'm so relieved and disappointed at the same time. I bought a K5 because of the focus issues I had with the K10D in hopes that sticking with Pentax was a good decision with the reviews that came with it. Although jaad75 has had no issues with white backgrounds I've done two shoots over the last two weekends where virtually none of the images have any value except for the web and 8x10's. White seamless with white tile boards. Subject is just super soft. Metered with a 758 and background lights flagged with 8 ft insulation boards to make sure no spill was affecting the image due to the first shoots issues. Same result. I also shoot with their 16-50 2.8 and 50-135 2.8 almost exclusively.

    Seems to me the most important function of any camera is to focus properly. Only analogy I could come up with is a smartphone is great and does lots of stuff but if I can't make a phone call reliably what good is it.

    Falk - You hit it on the head, you must trust your equipment is doing what is says it is doing.

    So since it is clearly a problem is there a work around with live view or something else to make sure you are getting the most out of the camera in every situation.

  10. Hi Falk,

    I'd first like to thank you for all of your hard work on this. You've done a great job of confirming what a lot of others are thinking.

    Like another commenter I have also observed that lenses with SDM, HSM, and DC motors don't have the issue while screw-drive AF lenses do. Any idea why that would happen? Have you experienced this at all? While shooting at weddings and the NYC car show my FA50 and DA40 both experience the FF issue while the DA18-135 and Sigma 17-50 do not.

    Also, have you heard anything about a new version of the firmware being developed to further address this issue?


  11. @Sean, I cannot confirm your observation about SDM vs. screw drive. Both types are in my study and both behave the same. So, my guess is it doesn't happen.

    When it happened to you at weddings and a car show, I doubt your problem was the low light front focus issue at all. Larger apertures have thinner DoF and that may have been your problem. Or your primes need AF calibration. I don't know.

    I didn't hear anything about yet another firmware release to address this -- except a rumor that some people within Pentax think it may happen.

  12. Valuable information and excellent design you got here! I would like to thank you for sharing your thoughts and time into the stuff you post!! Thumbs up


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