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06-07-2012, 05:42 PM   #1
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K5 backfocus in poor light

Like some suggestions and experience that will help me with a backfocus problem with the K5

I don't seem to have any backfocus issues when using my pentax 60- 250 in good light, However this in part may be due to having the aperture lower. In poor light I have a really bad backfocus issues. I am using the one central focus point to focus.

My focus adjustment with my Sigma 500mm is backfocus on about 8. I have the latest firmware installed on the K5.

How do people deal with back focus issues, adjust the camera focus setting in poor light only and then turn it back when the light is good?

Thanks

06-08-2012, 04:28 AM   #2
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In my personal experience majority of misfocused images are due to "user-error" (i.e. my error) rather than the camera's. I try to be very very carefull when focusing on complicated subjects especially in low light. I keep in mind the actual size of the AF spots and try to do a good job selecting the right aperture/ISO for situation and this helps a lot.

If I remember correctly AF-sensors require certain minimal amount of light to function, so there might be just not enough light period.

Part of photographers task IMHO is waiting for/ensuring good light and I try to follow that. I just don't shoot in poor light. Quite often even if the focus is spot on, poorly lit scenes are pointless.

Manual focus does help. I guess your lens does have the "quick shift" option. I would use that. I do have the Pentax viewfinder magnifier O-ME53 and can not imagine living without it.

Focusing using liveview may be helpful as well.

This would be my troubleshooting procedure if I had too much misfocusing:
- check for user-error. be extremely careful/aware about every detail (size of AF point, the scene has enough contrast etc, do not shoot white walls or transparent fences/or do it smartly)
- think again if it makes sense at all to shoot under such conditions
- use manual focus/ quick shift or live-view
- "do the math" - in case you are shooting static objects, do pre-focus, go for hyperfocal ... (pre-set manual focus basically)
- I do not fine-tune AF specifically for low light, though it is definitely a valid option if you know your misfocusing is consistent during your photo-shoot.

Hope that helps.

P.S.: Btw, there is a bunch of threads on this forum on K-5 AF issues under certain conditions, do the search if you are interested.
06-08-2012, 05:23 AM   #3
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Thanks vanyagor
06-08-2012, 06:19 AM   #4
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Curious - how "poor" is the poor light? I always read about such issues as "lowlight" AF, but can't qualify it.

Does it happen only if/when the AF assist light doesn't fire?


I have taken numerous fireside & night shots in like -2.9 EV that focus just fine, as long as the AF assist comes on of course.


Last edited by jmg257; 06-08-2012 at 06:55 AM.
06-08-2012, 08:24 AM   #5
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If I remember correctly K-5 AF has been reported to malfunction in tungsten with low light levels, so fireside & night shots should not be affected indeed.

Here is the most extensive study on the subject by a member of the community (who as a theoretical physicist should be qualified enough): Falk Lumo: LumoLabs: Pentax K-5 low light focus

Falk Lumo's study contains quantitative details and qualitative analysis too so I refer you to that if you are interested.

If I understand correctly the worst scenario happens when you shoot something reddish (such as a baby face for instance) in tungsten low light, though I have not read the study carefully.

Falk did his study with an older firmware and some have reported the newer firmware updates have fixed the issue if I remember correctly, though I personally have not found those reports to be very reliable. I did not look carefully into it as I don't find the whole issue limiting me enough to bother.
06-08-2012, 10:58 AM - 1 Like   #6
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if you know the exact AF correction needed for your lens...might as well save a user function...change the AF fine adjustment then by then using that setting would give you the correction you need during a specific situation...thus you can go back to original settings by just going out of that specific user setting...just a quick work around though...i guess..
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