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03-07-2012, 08:16 PM   #1
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Autofocus/metering contrast/comparison between pentax, nikon, and canon systems?

So I'm sure this has been discussed to death, but hey I'm new. I'm curious how the pentax phase detection system works exactly. I'm guessing that reads a beam of light from the pentaprism...or does it pass through the mirror? I'm correct in assuming it is black and white? What is the fstop sensitivity and does it have an fstop equivalent of its own? I understand how phase detection works, I'm just curious to understand how pentax has implemented it. Forgive me if any of these are stupid questions. I'm trying to understand how the metering and autofocus systems really work so I can understand their differences between implementations.

03-07-2012, 10:12 PM   #2
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You could start here, and then search the forums and Google a bit to update what's there.

Also paragraphs are your friend

EDIT: of course one should refer to some of Falconeye's work when discussing Pentax AF, like his paper on the K-5 AF, which has lots of information on the K-5 phase-detection AF system.

Last edited by rawr; 03-07-2012 at 10:21 PM.
03-08-2012, 05:11 AM   #3
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Thank you. I will start there. Also sorry about the lack of paragraphs. Its hard enough just typing this out on my phone.

I can do paragraphs though. Sometimes I forget how long a block of text is as I ramble on.
03-08-2012, 06:25 AM   #4
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Real quick - the AF module is in the bottom of the camera; light passes thru a half-silvered part of the main mirror to a sub-mirror behind it and is reflected down into the AF module's optical system which splits the light into sets of 2 images and projects them into different AF detection areas. I.E. the images appear on pairs of line sensor arrays on the AF CCD. These are recently arranged in a 1-3x3-1 grid (the 3x3 are cross-type sensors, the 2 end ones are vertical line-type).

The position of the images along the array(s) are compared to come up with a defocus amount(s). This amount is used to know how much to spin the AF motor, which returns encoder pulses back to the CPU. Various algorithms and monitoring devices are used to determine which sensor area(s) to use, which line sensor in that area, whether there is enough light/contrast, whether the defocus amount will be enough to get lock, etc.

The multizone photo metereing IC is indeed above with the pentaprism, near the eyepiece.

Start with the lumo link as mentioned. Then go to Google Patents. Look up "Pentax AF".

I recommend US Patent 7496290 B2
& US 2006/0104621 A1.


Last edited by jmg257; 03-08-2012 at 07:03 AM.
03-08-2012, 03:50 PM   #5
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Wow. I read the k5 af study. Mind blown. It almost makes me want to pass on the k-5. I love my k-7s low light focusing ability. It does fail at a point, but usually when the viewfinder is very dim. It looks like the k-5 is far more erratic in low tungsten light. I'm pretty ok waiting for the k-3 or whatever replaces the k-5. My k-7 is serving my exceptionally well so far and just keeps on taking pictures. Pictures that I rather like.

Edit: I must add that its pretty clear why remving all the complex parts of a slr would be a huge cost savings measure. After seeing how complex the af system actually is, the economies of scale involved with bringing such tech down to say $1000 is mind blowing.
03-08-2012, 04:12 PM   #6
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Last edited by beholder3; 08-11-2013 at 06:59 AM. Reason: [deleted]
03-09-2012, 12:50 AM   #7
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QuoteOriginally posted by zosxavius Quote
It almost makes me want to pass on the k-5.
If you are happy with the K-7, and you don't do a lot of low-light/high-ISO, then sure, there may be little reason to think about the K-5 anyhow.

But having used the K-5 for a while now, I don't understand all this talk of low-light focus problems. Whether it be tungsten, dim rainforest floor, or LED stage lighting, K-5 AF works fine for me. The firmware updates seem to be doing good things too in this department. My technique and skills usually fail me well before the camera technology lets me down.
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