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12-05-2013, 02:23 AM   #1
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Changes aperture when manually focusing?

I just noticed that at least with my DA 50 1.8 my K-5 changes the aperture while focusing manually in video mode. When not recording. When recording it works fine. It doesn't happen at 1.8, but above that... I have set the camera to Av priority when shooting video... So the aperture shouldn't change, right?


Last edited by kadajawi; 12-05-2013 at 03:01 AM.
12-05-2013, 04:35 AM   #2
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QuoteOriginally posted by kadajawi Quote
I just noticed that at least with my DA 50 1.8 my K-5 changes the aperture while focusing manually in video mode. When not recording. When recording it works fine. It doesn't happen at 1.8, but above that... I have set the camera to Av priority when shooting video... So the aperture shouldn't change, right?
Ok, since I noticed the problem when I was near my Pentax dealer I paid him a visit. We tried various lens/camera combinations. The only lens that exhibited the problem was the DA 50 1.8. We didn't have any other 50mm versions though. 2 DA's had the problem. Both the K-5 and K-3 are affected (tested 2 K-5). No problem with the K-50, cause it only changes to the selected aperture when you start recording. Otherwise it will go back to something like 5.6. Wtf? Weirdest thing I have ever seen.

Also, the shift in aperture is noticeable, but small. i.e. the aperture changes much more from 2.0 to 2.2 than when you change focus. It also moves quite abrupt, as if you stop down. And once it has stopped down it stays that way. Until you focus back a bit further... Just try it and you will see what I mean.

Last edited by kadajawi; 12-05-2013 at 04:47 AM.
12-05-2013, 07:41 AM   #3
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If the fl changes while focusing it might need to do that, but is it that camera or the lens?

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12-05-2013, 09:36 AM   #4
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QuoteOriginally posted by Adam Quote
If the fl changes while focusing it might need to do that, but is it that camera or the lens?
You mean the focal length? It's the DA 50 1.8, other lenses seem to work just fine. It's an abrupt action, you can hear and feel it. The lens stops down slightly (or opens up). Basically it varies between 2 different apertures. At 1.8 it stays fully open, but from 2.0 to 22 you can observe the effect. We've tested 2 K-5, a K-3, a K-50, 2 DA 50 1.8, DA 18-55 AL, Tokina 80-200, and IIRC the 31mm Ltd and DA 35 2.8. The only lens that had the problem was the DA 50 1.8, both had it. Also, only the K-5 and K-3 (all of them) showed the effect... but only with the 50 1.8. The K-50 as I described earlier has that weird thing where it always goes back to a default aperture when not recording a video... when you start shooting it will go to the selected aperture and start recording. No change of aperture was noticed at the "default" aperture.

I'm curious if this has been noticed before, if it is a bug (maybe Pentax can fix it? Though it is not really a big issue... you may loose a bit of light depending on where you have focused to before starting to record, but it shouldn't be a big issue). Maybe other lens/camera combinations have the same issue?

Reading your comment again... do you mean that since when you focus the focal length does change slightly, it will adjust for it? Sounds possible, but why only on the DA 50? Other primes should have the same issue? And does it really make sense to adjust for the tiny difference in focal length when you may change the FL again once you've started recording (and the camera wisely doesn't touch the aperture at that point)?

12-05-2013, 02:24 PM   #5
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QuoteOriginally posted by kadajawi Quote
You mean the focal length? It's the DA 50 1.8, other lenses seem to work just fine. It's an abrupt action, you can hear and feel it. The lens stops down slightly (or opens up). Basically it varies between 2 different apertures. At 1.8 it stays fully open, but from 2.0 to 22 you can observe the effect. We've tested 2 K-5, a K-3, a K-50, 2 DA 50 1.8, DA 18-55 AL, Tokina 80-200, and IIRC the 31mm Ltd and DA 35 2.8. The only lens that had the problem was the DA 50 1.8, both had it. Also, only the K-5 and K-3 (all of them) showed the effect... but only with the 50 1.8. The K-50 as I described earlier has that weird thing where it always goes back to a default aperture when not recording a video... when you start shooting it will go to the selected aperture and start recording. No change of aperture was noticed at the "default" aperture.

I'm curious if this has been noticed before, if it is a bug (maybe Pentax can fix it? Though it is not really a big issue... you may loose a bit of light depending on where you have focused to before starting to record, but it shouldn't be a big issue). Maybe other lens/camera combinations have the same issue?

Reading your comment again... do you mean that since when you focus the focal length does change slightly, it will adjust for it? Sounds possible, but why only on the DA 50? Other primes should have the same issue? And does it really make sense to adjust for the tiny difference in focal length when you may change the FL again once you've started recording (and the camera wisely doesn't touch the aperture at that point)?
Does the aperture change at the moment of capture when using different focus settings? You could take a couple of long-exposure shots to verify. If so, then take a few more normal exposures and see if the exposure changes at all. If not, then the behavior is probably be design.

I've seen zooms stop themselves down at the short end of the zoom range so that the aperture can remain constant, but I don't think I've ever experienced what you're describing. It definitely sounds like something that camera is doing on purpose, though...

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12-05-2013, 06:28 PM   #6
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QuoteOriginally posted by Adam Quote
Does the aperture change at the moment of capture when using different focus settings? You could take a couple of long-exposure shots to verify. If so, then take a few more normal exposures and see if the exposure changes at all. If not, then the behavior is probably be design.

I've seen zooms stop themselves down at the short end of the zoom range so that the aperture can remain constant, but I don't think I've ever experienced what you're describing. It definitely sounds like something that camera is doing on purpose, though...
No, it ONLY (as far as I have observed it) happens in video mode, and ONLY when you are not recording something. LiveView in any still photography mode is not affected. I'd have to record the AV output to demonstrate... maybe I'll do a few videos to clarify things. The aperture will stay whatever it is the moment you start capturing.
12-05-2013, 06:33 PM   #7
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It will stop down automatically to a middle aperture in live view in bright light -- to protect the sensor I guess. I don't know about video mode.
12-05-2013, 10:33 PM   #8
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QuoteOriginally posted by vonBaloney Quote
It will stop down automatically to a middle aperture in live view in bright light -- to protect the sensor I guess. I don't know about video mode.
The K-50? It wasn't particularly bright though... We could easily shoot at 1.8 at that place without overexposing.

And it doesn't explain the K-5 stopping down at f22... Depending on where you focus to.

12-05-2013, 11:28 PM   #9
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QuoteOriginally posted by kadajawi Quote
The K-50? It wasn't particularly bright though... We could easily shoot at 1.8 at that place without overexposing.

And it doesn't explain the K-5 stopping down at f22... Depending on where you focus to.
The K-5 does it in live view, the K-01 does it in live view. I never use video mode so I can't comment on that. So what I mean is if you are just using live view, pointing the camera at things, etc, if the light is bright it will stop down to f/5.6 - f/8 ish, otherwise it will remain wide open. (The LCD compensates for the brightness change, but naturally it does affect the depth-of-field.) This can be a pain when you are trying to manually focus a fast lens. However, if you use the manual focus assist to zoom in on the subject, it forces it to open up. And then when you take the shot it closes to the selected aperture as it should.
01-31-2014, 06:56 AM   #10
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Ok, I've decided to record a video of the bug. I have discovered that sometimes this bug also appears while recording a video. Couldn't reproduce it though... But I do have videos where you can see the exposure changing due to focusing.

This is how the lens looks like when the bug occurs.


And this is how video recorded by the K-5 looks like when it occurs.

Last edited by kadajawi; 01-31-2014 at 10:35 AM.
06-27-2014, 10:58 AM   #11
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Bump. Nothing? Am I the only one who is even bothered by this? It is very noticeable in my videos, especially cause it happens at a distance where you'd regularly focus back and forth past this point.


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