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05-20-2012, 11:14 PM - 1 Like   #31
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Just for info, these images give an idea of how useful the full EXIF data can be for identifying what is going on with focus and exposure sometimes, once the data is displayed through PhotoMe:


Name:  d300af_en.png
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Size:  135.8 KB

Name:  k10dae_en.png
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Size:  169.3 KB

[Images from PhotoME - Exif, IPTC & ICC Metadata Editor ]

05-21-2012, 04:44 AM   #32
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QuoteOriginally posted by Ray Pulley Quote
With all due respect, I must take exception to some of what you say above,

The K5 absolutely had a low light/low kelvin AF problem, which was admitted by Pentax and a firmware fix was issued for the problem. I daresay that Pentax knows more about AF than any of us ever will, and based upon my extensive testing with several bodies and many lenses under well-controlled conditions, the problem was fixed on the K5 after this firmware was installed on my two bodies. I shot before and after tests that conclusively prove this, not to mention many real world examples before and after.

As for flash, yes it over-powers ambient light in the foreground in dark scenes, but it has nothing to do with focus. The camera begins focusing as soon as you half-press the shutter release button or press the AF button. It either locks or does not lock, and it does so in the ambient light before the pre-flash goes off and before the shutter is tripped. Unless the flash turns on the AF assist light, it is not involved in focusing in any way.

As for color temp not affecting focus, well this is not the case either. Different wavelengths focus at different planes, and although well-designed optics attempt to correct for this, it is not always possible to completely correct for the problem, especially in relatively simple (cheap?) optics used in AF modules. Pentax designed a color-sensing element into the AF module of the K5 to attempt to correct for this issue, which, in my experience, has been more or less of a problem with every Pentax AF camera i have used.

Looking at the pics posted by the OP, I have a strong suspicion that the ambient light the AF module had to work with was below the lower limit of the PDAF module (-1ev), and if the assist light did not come on, the camera will not focus correctly because it was simply too dark.

There are certainly other possibilities as to the cause of the problem, but if you picture the conditions in most bars at night, it is very likely that it is too dark for proper focusing without the assist light. The OP said he used focus/re-compose (which has it's own problems) so unless he released the AF button between focus and re-compose, had AF-C on, or was using auto-select multi-point AF mode, it is not likely that focusing on a face with just the center point caused a large and consistent front focus due solely to the size of the AF point.

Ray
Im sorry you take exception lol
I was talking in genral though with a possable answer
which the OP may have tried or considered.
Low light autofucus problems does not = front and back focus issues. An awful lot of people are totaly unaware of the size of the focus sensors. It seems the poster in this instance is aware and thats good that rules that out. Im also well aware of the colours focussing on different levels but fail to see the connection at this point. I also appreciate your view of in thinking its its not likely due soley to the size of the focus point because of focus and recompose. If the focus missed its intended target in the first instance I personly cant see the differance it makes. I have also found that the camera can and does indeed use the preflash light to focus when its strugling to lock. Since im not the one looking for answers I would suggest you try and help the OP, since I did not ask for advice.
05-21-2012, 04:53 AM   #33
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QuoteOriginally posted by RKKS08 Quote
@westmill

Of course I was speaking of the IR mark.
When focusing, you are using the light you can see, or the distance scale which is calibrated for roughly the mean of the light frequencies visible for film or sensor. With the extreme low pass filter for IR, you will get ONLY light for which a shifted focus plane is valid. I just do not know - and this was my question - how much of such an effect is still there with well corrected lenses, and whether this could have any visible impact on Pentax AF.
Well please excuse my ignorance lol
Since Ive never seen this as ever been any kind of a probmatic efect in the field or under normal use its something that goes beyond my limited understanding.
It seems the guy that took exception to my trying to help the OP may answer you better, since he seems to have better knowledge of the subject.
05-21-2012, 05:33 AM   #34
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QuoteOriginally posted by westmill Quote
Im sorry you take exception lol
I was talking in genral though with a possable answer
which the OP may have tried or considered.
Low light autofucus problems does not = front and back focus issues. An awful lot of people are totaly unaware of the size of the focus sensors. It seems the poster in this instance is aware and thats good that rules that out. Im also well aware of the colours focussing on different levels but fail to see the connection at this point. I also appreciate your view of in thinking its its not likely due soley to the size of the focus point because of focus and recompose. If the focus missed its intended target in the first instance I personly cant see the differance it makes. I have also found that the camera can and does indeed use the preflash light to focus when its strugling to lock. Since im not the one looking for answers I would suggest you try and help the OP, since I did not ask for advice.
I do not see where I gave you any advice, but mis-information isn't likely to help the OP in any way that I can see.

Ray

05-21-2012, 05:40 AM   #35
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QuoteOriginally posted by westmill Quote
Well please excuse my ignorance lol
Since Ive never seen this as ever been any kind of a probmatic efect in the field or under normal use its something that goes beyond my limited understanding.
It seems the guy that took exception to my trying to help the OP may answer you better, since he seems to have better knowledge of the subject.
FYI,

Lenses have long been marked with a red line (not so much any longer) that indicated the approximate focus offset for use in shooting IR images. The poster was simply using this as an example of how different wavelengths focus at different focal planes, which you said above you already knew.

Many have theorized that the FF issue is related to IR focus shift in the PDAF system. I am not a believer in this theory after working with low light FF in all of the Pentax DSLRs I have owned. This has been discussed and tested on the other forum by adding a hot mirror IR filter over the lens, which did not solve the problem.

Ray
05-21-2012, 05:49 AM   #36
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I did not give miss information !!!!! Neither do i need or care if you take exception to my trying to help someone. If you dissagree or have a solution or other ideas, thats good.
Way to go !!!!! I consider it you are trying to correct and give me advice when you are Quoting and responding to me directly. If you can help the OP then please do so !
05-21-2012, 08:55 AM   #37
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I think lens also affect the low light focus ability
my kit lens focus really poor in very low light situation. a lot of photo like OP
but my 16-50 is a lot accurate.
some people say SDM lens focus more accurate than screw drive lens.
05-21-2012, 09:21 AM   #38
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QuoteOriginally posted by sanjeevdas Quote
For small targets I would agree. But in a lot of my shots the subject is fairly larger than the semi circle surrounding the focus point.
I have had the exact same problem on the odd occasion.
With me its normaly in the dark interior of a church.
I just put it down to the camera reaching its low light limits. I also, like you, have and use nikon. I use the D300. I also know the Nikon would not have struggled in the same conditions. When faced with this, the only way around it, that Ive found so far is to use flash with the red eye function set. Its a long burn which allows the camera to see and focus clearly. Exposures are also superb. Im reading this thread too, for the same reason as you. Other than maybe a faster lens, which would reasonbly allow more light through for the sensors to work, the preflash option seems to be the only solution so far

05-21-2012, 10:24 AM   #39
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IMHO one shouldn't be having to resort to manual focusing on latest generation lenses and dslr whatever the excuses used.

As was mentioned earlier, Nikon's AF is three generations ahead of Pentax and I dare say the flash exposure on average is far more accurate.

@ OP - you have your portability in the K5 but have traded for an inferior AF experience - now you will have to work at it where the Nikon d7000 simply would have done it in its sleep.
05-21-2012, 05:59 PM   #40
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QuoteOriginally posted by westmill Quote
I did not give miss information !!!!! Neither do i need or care if you take exception to my trying to help someone. If you dissagree or have a solution or other ideas, thats good.
Way to go !!!!! I consider it you are trying to correct and give me advice when you are Quoting and responding to me directly. If you can help the OP then please do so !
I quoted and responded to you, listing the specific things that you had said that I believe to be incorrect. Correcting inaccuracies or incorrect information is one way to help the OP.

That you see this as somehow giving you advice is odd, but the world is full of odd things.

You keep claiming that using red-eye pre-pre-flash will solve exposure and focusing problems. I find this improbable at best, but perhaps you could post some examples with and without so we could all learn something and the OP's problem would be solved?

I presume you know that you will have to make sure that the AF assist light on the flash or body is not coming on to make sure that the red-eye pre-pre-flash is working as you say? A bit of black tape over the assist light areas will suffice.

The only use I have for red-eye is in eliminating pre-flash blinkies on people with fast blink responses. I have not seen any circumstance where the pre-pre-flash improved focusing or exposure, but I would be happy to learn otherwise.

Ray
05-21-2012, 09:15 PM   #41
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Doesn't Work

QuoteOriginally posted by westmill Quote
I have had the exact same problem on the odd occasion.
With me its normaly in the dark interior of a church.
I just put it down to the camera reaching its low light limits. I also, like you, have and use nikon. I use the D300. I also know the Nikon would not have struggled in the same conditions. When faced with this, the only way around it, that Ive found so far is to use flash with the red eye function set. Its a long burn which allows the camera to see and focus clearly. Exposures are also superb. Im reading this thread too, for the same reason as you. Other than maybe a faster lens, which would reasonbly allow more light through for the sensors to work, the preflash option seems to be the only solution so far
I thought that the claim about using red-eye pre-pre-flash to obtain low light focus was interesting enough to test once it got dark enough tonight.

I setup my K5 on a tripod with the AF540 mounted and then taped over all AF assist lights on the flash and body. I measured the ev level with my Sekonic hand-held meter and it was -.6 ev. The lens was the DA 16-50 f2.8, so this light level is well under the low limit for the camera/lens combo and it would not lock on the subject (as expected).

Of course, since it would not lock focus and the menu setting was set to focus priority, you cannot even trip the shutter and therefore the flash will not do anything, much less provide focus assist via the red-eye setting.

I changed the focus priority to release, meaning the camera will fire even if focus lock is not achieved, and then set the flash for pre-pre-flash red-eye and took an image. Result: subject completely out of focus.

I then changed to AF-C, and set the camera to release priority and took another image. Result: subject completely out of focus.

Unless I am missing something, the red-eye pre-pre-flash has no effect on achieving proper focus when the light level is below the lower limit specified for the camera/lens combination (+2ev in this case).

Ray
05-22-2012, 03:10 AM   #42
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QuoteOriginally posted by Ray Pulley Quote
I thought that the claim about using red-eye pre-pre-flash to obtain low light focus was interesting enough to test once it got dark enough tonight.

I setup my K5 on a tripod with the AF540 mounted and then taped over all AF assist lights on the flash and body. I measured the ev level with my Sekonic hand-held meter and it was -.6 ev. The lens was the DA 16-50 f2.8, so this light level is well under the low limit for the camera/lens combo and it would not lock on the subject (as expected).

Of course, since it would not lock focus and the menu setting was set to focus priority, you cannot even trip the shutter and therefore the flash will not do anything, much less provide focus assist via the red-eye setting.

I changed the focus priority to release, meaning the camera will fire even if focus lock is not achieved, and then set the flash for pre-pre-flash red-eye and took an image. Result: subject completely out of focus.

I then changed to AF-C, and set the camera to release priority and took another image. Result: subject completely out of focus.

Unless I am missing something, the red-eye pre-pre-flash has no effect on achieving proper focus when the light level is below the lower limit specified for the camera/lens combination (+2ev in this case).

Ray
Your obviously not as clever as you think you are then are you because it does indeed work lol
Iv just turned out the lights in the studio, I had to open the door a tiny amount, because total blackness, I took a pic of one of the studio lights no problem. I could barley see it. Focus good, exposure good !
Get a life !
05-22-2012, 04:40 AM   #43
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QuoteOriginally posted by westmill Quote
Your obviously not as clever as you think you are then are you because it does indeed work lol
Iv just turned out the lights in the studio, I had to open the door a tiny amount, because total blackness, I took a pic of one of the studio lights no problem. I could barley see it. Focus good, exposure good !
Get a life !
Right, and you made sure to block off the AF assist light, and you also metered the scene to be sure that the light you let in was below the limit where the camera will lock focus, right?

This does not work, period, but go ahead and stay in your fantasy world and keep spreading BS if you like.

Ray
05-22-2012, 05:05 AM   #44
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QuoteOriginally posted by Ray Pulley Quote
Right, and you made sure to block off the AF assist light, and you also metered the scene to be sure that the light you let in was below the limit where the camera will lock focus, right?

This does not work, period, but go ahead and stay in your fantasy world and keep spreading BS if you like.

Ray
What an idiotic arrogant prat you are !!!!
You presume wayyyyyyy too much.
Im not intrested in your thoughts or opinions or what and why you do or anything else.
05-22-2012, 05:10 AM   #45
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QuoteOriginally posted by sanjeevdas Quote
For small targets I would agree. But in a lot of my shots the subject is fairly larger than the semi circle surrounding the focus point.
Sorry about the rude interuption lol
Just see if it solves the problem
good luck !
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