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12-13-2010, 09:05 AM   #16
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Falk, Interesting theory about the AF aperture, but it wouldn't explain why it would change from bright to dim conditions. LV would focus accurately actually, but would not be useful trying to focus on dancing people. Also I always though the camera focuses with aperture wide open, at least that is what it seems to be doing. When I press the DOF preview, the viewfinder always gets darker unless I am trying to shoot wide open.

In very dim conditions (0 to 3EV), the camera behaves like, it is "lazy"; it focuses, locks and "says: good enough" while being not focused accurately, while the K10d and K7 would fine tune the focus a little. I test in conditions when I get reasonable exposures in M with f/1.4 ISO6400 and 1/20 to 1/30 second.

Having a white LED light helps quite a bit for stationary shots and I have been looking for the one I've seen posted on this forum couple of months ago that screws in the tripod socket under the camera.

It seems like an electronic problem, but I do not know if it is related to my camera only or all K-5 cameras.

I am curious how the 35mm f/2.4 performs in these conditions, maybe the DOF is just too narrow for this camera.

12-13-2010, 10:49 AM   #17
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QuoteOriginally posted by falconeye Quote
Eric.

I cannot be sure but I have a theory:

Many large aperture lenses have spherical aberration. This means that the focus at f/5.6 (where phase AF takes its measure) and at f/1.4 (where the exposure is taken) do differ.

The Pentax firmware has correction data to compensate for this. But it may only include Pentax made lenses. Or it's in the lens rom and Sigma doesn't entirely support the lens rom features.

At normal lighting it would then work because you wouldn't use the lens wide open.

Hi Falk,

i think it's all about then lens rom. The lens rom has some correction data for spherical aberration.
It's possible that with LV this correction is not applied, maybe due to false implementation from sigma or not using the data with the current camera firmware.


For more information see lens rom update for DA21

http://***.digitalfotonetz.de/viewtopic.php?t=79794

I speculated some years ago that my sigma 135-400 APO has the same problem.

http://***.digitalfotonetz.de/viewtopic.php?t=17380&highlight=henning+rom


http://***.digitalfotonetz.de/viewtopic.php?t=42498&highlight=henning+rom


It's still focusing correct with F8 or higher, with values smaller than F8 the lens has BF. Focusing with F8 and then reopening to F5.6 without touching the AF button is OK, the image will be perfectly sharp.


"***" = "forum"


networker

Last edited by networker; 12-13-2010 at 11:32 AM.
12-13-2010, 12:39 PM   #18
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I've also noticed the k-5 seems to front focus a lot in low/tungsten light, and I can't remind the k-7 did. Even with Pentax lenses and Limiteds. I'll have to experiment a bit more to see how bad it really is, but I've certainly noticed it.
12-13-2010, 01:29 PM   #19
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QuoteOriginally posted by Eric Seavey Quote
Falk, Interesting theory about the AF aperture, but it wouldn't explain why it would change from bright to dim conditions. LV would focus accurately actually, but would not be useful trying to focus on dancing people. Also I always though the camera focuses with aperture wide open, at least that is what it seems to be doing. When I press the DOF preview, the viewfinder always gets darker unless I am trying to shoot wide open.
I think that's all still in line. In dim conditions, you shoot at wider apertures and phase AF has its own effective aperture which is about f/7 or so, allowing a f/5.6 lens to focus at the near distance.

QuoteOriginally posted by Eric Seavey Quote
In very dim conditions (0 to 3EV), the camera behaves like, it is "lazy"; it focuses, locks and "says: good enough" while being not focused accurately, while the K10d and K7 would fine tune the focus a little.
That sounds odd. So I did some more tests myself, using an FA*31/1.8 which is Pentax made.

I can confirm your findings. At the edge where the AF assist light would come on (sometimes it did, sometimes not) and where it didn't, focus was missed. With LV and AF assist light, it was much better (a difference like: can read the text easily to cannot even say it is text!).

Obviously, the K-5 firmware uses an a priory estimate of what minimal contrast(*) should be to have aquired acceptable focus which takes the amount of availabe light into account.

And obviously, this estimate is wrong. As seen by not even activating the AF assist light. The AF module "thinks" to see well while it is almost blind.

So, I withdraw my spherical aberration theory.


My preliminary assessment: A firmware bug with wrong a priori thresholds or a failure to correct for lens aperture when assessing scene luminance. So, the AF module uses a too short AF module exposure time and disables AF assist light.


___
(*) phase AF uses the correlation function between two halfs of the linear AF sensor data. This is a one dimensional contrast measure. The phase is the shifting distance between both halfs where this contrast is maximized. In low light, this contrast remains small if AF exposure time isn't increased (the AF "thinking" time).

12-13-2010, 02:01 PM   #20
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On the other forum there's a thread about a response from Pentax's distributor in Spain re: the sensor stains.

Tacked at the bottom was a note about a new firmware version coming soon that will fix/improve the K-5's AF performance in low tungsten light.

The thread: http://forums.*************/forums/readflat.asp?forum=1036&thread=37177457

Adam
12-13-2010, 02:10 PM   #21
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Thanks Falk for confirming. I actually used AF-C because that is what I need to use when taking dance photos, and cannot rely on the AF light.

Wow! I am a bit surprised by all of this.... It sounds like there will be an update for the flash problem and AF.... and perhaps a list of other stuff... This is the first time I ever purchased a camera so soon after being released, so I am used to the camera performing quite well out of the box.

I did contact Sigma and explained my problem. Their response is to send the lens in to have it checked by a technician. At this point I think I will hold off until the firmware update.

Also at this point I do not see the need to purchase the 35mm f/2.4, since it will probably not perform better.

Adam, the link is not working again. Can you copy and paste the link perhaps without the https://www.

Thanks
12-13-2010, 03:23 PM   #22
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QuoteOriginally posted by Eric Seavey Quote
Adam, the link is not working again.
You need to talk to the other Adam (the one who runs the forum) about this. It is an intentional blocking of DPR.

Just click on the link and, after the browser has resolved the URL to the final one, replace all the stars with "dpreview. com" (without the space).

BTW, there is a "https://www.pentaxforums.com/forums/pentax-k-5-forum/125751-updated-sensor-stain-report.html" thread related to this topic on this forum.

Last edited by Class A; 12-17-2010 at 02:56 AM.
12-13-2010, 05:58 PM   #23
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Thanks indeed there is a problem with tungsten light in very dim light. I just did a test but used white LED as light source and the camera "tries" harder to focus, hunts a bit even, but eventually focuses accurately.

12-13-2010, 09:58 PM   #24
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QuoteOriginally posted by falconeye Quote
Eric.

I cannot be sure but I have a theory:

Many large aperture lenses have spherical aberration. This means that the focus at f/5.6 (where phase AF takes its measure) and at f/1.4 (where the exposure is taken) do differ.

The Pentax firmware has correction data to compensate for this. But it may only include Pentax made lenses. Or it's in the lens rom and Sigma doesn't entirely support the lens rom features.

At normal lighting it would then work because you wouldn't use the lens wide open.


You may be able to test for this by doing a manual focussing at f/5.6 (use the DOF preview to enforce it and try to use a good focus target like a zone plate chart). And then shoot at f/1.4. I occasionally observed sup-par focus using this method with fast some lenses. Even a careful manual focus in 10x LV can be off.


As I said, just a theory. But maybe worth a test.
it is a long known thing (several years already) - for example my Pentax FA50/1.4 focuses differently @ the same f1.4 aperture on the same camera (tested on 4 cameras *istDL, K10D, K20D, Kx) under the tungsten light vs under the day light... the test is done by shooting the target from tripod under the day light, then waiting for night time and shooting the same under the tungsten light - neither camera & tripod, nor target were moved.
12-14-2010, 02:31 AM   #25
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QuoteOriginally posted by PhilippeG Quote
I've also noticed the k-5 seems to front focus a lot in low/tungsten light
A few days ago a wrote this in another topic. I also noticed that under tungsten light my K-5 front focuses with any lens. My K-m with the same lenses, under exactly the same lightning (in my living room) never did this! I even tested it with both camera side by side.
The interesting thing is: in theory the K-m should front focus under tungsten and the K-5 (due to the dedicated sensor detecting tungsten) shouldn't. In practice it's the opposite. Seems as if the K-5 applied focus correction needlessly and in the wrong direction, so instead of correcting the alleged front focus under tungsten it just creates front focus. This is just speculation from me, so the real reason can be something totally different. Anyways it's annoying.
12-14-2010, 03:39 AM   #26
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QuoteOriginally posted by simico Quote
A few days ago a wrote this in another topic. I also noticed that under tungsten light my K-5 front focuses with any lens. My K-m with the same lenses, under exactly the same lightning (in my living room) never did this! I even tested it with both camera side by side.
The interesting thing is: in theory the K-m should front focus under tungsten and the K-5 (due to the dedicated sensor detecting tungsten) shouldn't. In practice it's the opposite. Seems as if the K-5 applied focus correction needlessly and in the wrong direction, so instead of correcting the alleged front focus under tungsten it just creates front focus. This is just speculation from me, so the real reason can be something totally different. Anyways it's annoying.
I can only add that I have the same experience, but with the K10 instead of k-m. The K10 that is supposed to front focus works pretty good, the new improved + version in the k-5 front focuses more than can be compensated for...
12-14-2010, 01:23 PM   #27
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This seems to confirm my suspicion that there is definitely something going on under tungsten low light conditions with front focus.

In my experience it's worse with non-Pentax glass - with a K5, my Sigma 17-70 HSM returns such poor focus indoors that it's unusable at apertures wider than f.5.6.
12-16-2010, 04:27 AM   #28
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QuoteOriginally posted by amvic Quote
This seems to confirm my suspicion that there is definitely something going on under tungsten low light conditions with front focus.

In my experience it's worse with non-Pentax glass - with a K5, my Sigma 17-70 HSM returns such poor focus indoors that it's unusable at apertures wider than f.5.6.
Well, it's not just tungsten, but also candle light, as I showed in these samples I posted on the Spanish forum:

http://www.pentaxeros.com/forum/index.php?topic=39024.0

The unifying factors seem to be dimness and orange/yellow hue.

As I explained later in that thread, right after the K5 tests I slapped the DA70 on my Kx and it nailed the focus in almost every shot (out of 40-50 shots) under the same lighting conditions. I'd be curious to know what Falk and everyone else think about this test.
12-16-2010, 05:56 AM   #29
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I haven't tested my K-x, but I tested my K20D and K100D Super and reported the results in this old thread. The lenses tested were FA 50mm, D FA 100mm, DA 16-45 and Cosina A 28mm:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/42773-front-focusing-done-death.html

After reading this thread, I spent a couple of hours with the DIY Nikon focus chart. I noted no significant issues with front or rear focus in daylight across four lenses and two cameras. I found definite front-focussing issues under tungsten light. Conclusions:

1. I did my tests under two separate tungsten lamps, one a trilight. I concluded that front focussing is related to lighting wavelength, obviously, but not to lighting level. I did not test with fluorescent light.

2. Front focussing was present equally whether I used MF or AF.

3. Front focussing is directly related to aperture settings. I had no problem with any lens on any camera under any light at F4. At F2.8, I had slight front-focussing under tungsten light. At F1.4, I experienced severe front focussing.
12-16-2010, 07:29 AM   #30
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It sounds like this tungsten (or whatever light wavelength caused front focusing) focusing inaccuracy can be remedied through firmware.

Otherwise, can the AF assist lamp be forced on when conditions are suspect?
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