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09-11-2012, 02:40 PM   #1
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K-5II autofocus sensors - explanation wanted

I am posting this in hope that someone knowledgeable will enlighten me (and others, I am sure).

Background: The new K-5II has a new autofocusing system that, according to Pentax, includes F/2.8 sensors in addition to the F/5.6 sensors. The sensitivity is said to be -3 -- 18 EV. The old autofocusing sensor was specified at -1 -- 18 EV, I think. In other words, the low-light sensitivity has been increased by 2 EV.

My assumptions: The original K-5 had F/5.6 auto-focus points and it was specified from -1 EV.

My observation: The difference between F/2.8 and F/5.6 is 2 EV -- the same as the improvement in low-light auto-focus sensitivity.

My conclusion: The improvement in auto-focus low-light sensitivity stems fully from the step to F/2.8 focus points.

Questions:
1) Is my assumption that the original K-5's auto-focus points are F/5.6 correct?
2) Is it as simple as I assume: otherwise identical auto-focus points would improve their low-light performance by 2 EV when increasing the size from F/5.6 to F/2.8?
3a) Does this imply that when used with lenses slower than F/2.8, the camera would use the F/5.6 auto-focus sensors only, and then have the same -1 EV low-light sensitivity as the original K-5?
3b) Or, could the F/2.8 sensor aid the F/5.6 sensor for improved low-light performance if a lens-size in-between was used, like for example F/3.5?

Naturally, there are likely also other improvements of the new auto-focusing system. As an example, since the K-30 had improvements in the form of some kind of light diffuser to decrease the risk of the auto-focus color temperature sensor giving false readings, I would assume that such a diffuser is also on place on the new system. New algorithms were also mentioned in the press release (PENTAX - PENTAX Introduces New K-5 II & K-5 IIs DSLR Cameras).

I would be grateful if someone could explain this in more depth to me.

/Jonas

09-11-2012, 03:12 PM   #2
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I think it's to early to say anything since we don't know if the f/2.8 points are dual cross AF points so that they also have f/5.6 sensor on the same point.
09-11-2012, 05:00 PM   #3
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Good questions!
09-11-2012, 05:24 PM   #4
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I suggested in another thread that Pentax should release a technical white paper describing the AF system in the K-5 II:
- what its goals were in moving to the new AF system,
- how the new system works
- some usage scenarios showing advantages of new AF (eg sports, wedding in church, rock concert)
- how to get the best out of the new AF.

Etc. Canon and Nikon do this sometimes, and pros (and geeks) find these papers very useful.

09-11-2012, 09:14 PM - 4 Likes   #5
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QuoteOriginally posted by hjb981 Quote
I am posting this in hope that someone knowledgeable will enlighten me (and others, I am sure).

Background: The new K-5II has a new autofocusing system that, according to Pentax, includes F/2.8 sensors in addition to the F/5.6 sensors. The sensitivity is said to be -3 -- 18 EV. The old autofocusing sensor was specified at -1 -- 18 EV, I think. In other words, the low-light sensitivity has been increased by 2 EV.

My assumptions: The original K-5 had F/5.6 auto-focus points and it was specified from -1 EV.

My observation: The difference between F/2.8 and F/5.6 is 2 EV -- the same as the improvement in low-light auto-focus sensitivity.

My conclusion: The improvement in auto-focus low-light sensitivity stems fully from the step to F/2.8 focus points.

Questions:
1) Is my assumption that the original K-5's auto-focus points are F/5.6 correct?
2) Is it as simple as I assume: otherwise identical auto-focus points would improve their low-light performance by 2 EV when increasing the size from F/5.6 to F/2.8?
3a) Does this imply that when used with lenses slower than F/2.8, the camera would use the F/5.6 auto-focus sensors only, and then have the same -1 EV low-light sensitivity as the original K-5?
3b) Or, could the F/2.8 sensor aid the F/5.6 sensor for improved low-light performance if a lens-size in-between was used, like for example F/3.5?

Naturally, there are likely also other improvements of the new auto-focusing system. As an example, since the K-30 had improvements in the form of some kind of light diffuser to decrease the risk of the auto-focus color temperature sensor giving false readings, I would assume that such a diffuser is also on place on the new system. New algorithms were also mentioned in the press release (PENTAX - PENTAX Introduces New K-5 II & K-5 IIs DSLR Cameras).

I would be grateful if someone could explain this in more depth to me.

/Jonas
This is a bit hard to explain without a diagram, but I will do my best:

First the K30 improvements -

The AF systems of every Pentax DSLR prior to the K5 suffered from focus shift due to chromatic aberrations in the optics of the AF system. CA is a wavelength issue where the light rays of different colors do not focus all in the same focus plane. CA causes the AF system to think the light rays of certain colors are in focus when they really are not. The K5 introduced a color sensor to try and offset this problem. The k30 uses a diffractive optic design that optically corrects the CA in the AF system. This should be a better solution than the K5 color sensor, IMO.

As for f2.8 sensors -

The f2.8 sensor is not what you think it is. The sensitivity of the AF array is not directly related to the aperture of the AF system.

The AF array has a mask over it that blocks off all but very specific portions of the light coming from the lens. If you think of the AF array as looking back at the lens through these slits, it can only see points located on a very specific part of the lens (there are two matching slits, one on each side).

The points are located on a circle which corresponds to an aperture. In the case of the current AF system, the circle is the size of an f5.6 aperture. If the slots were located further apart they would allow the AF array to "see" a point on a bigger circle on the lens let's say f2.8.

The f5.6 sensors will work on faster lenses because the faster lenses are bigger and will always allow the AF array to "see" the smaller f5.6 circle through the mask.

The f2.8 sensors will not work on the slower lenses, however, as the slower lens isn't big enough to allow the sensors to "see" any lens glass at all. They will simply be blocked as there is no glass out there to "see".

The upside to f5.6 sensors is that all lenses from f1.0 to f5.6 will focus. The downside is that they are closer together, which means they are less accurate, so while the faster glass will focus, it will not be as accurate as it could be.

I hope this helps

Ray
09-12-2012, 02:51 AM   #6
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QuoteOriginally posted by Ray Pulley Quote
This is a bit hard to explain without a diagram, but I will do my best:

First the K30 improvements -

The AF systems of every Pentax DSLR prior to the K5 suffered from focus shift due to chromatic aberrations in the optics of the AF system. CA is a wavelength issue where the light rays of different colors do not focus all in the same focus plane. CA causes the AF system to think the light rays of certain colors are in focus when they really are not. The K5 introduced a color sensor to try and offset this problem. The k30 uses a diffractive optic design that optically corrects the CA in the AF system. This should be a better solution than the K5 color sensor, IMO.

As for f2.8 sensors -

The f2.8 sensor is not what you think it is. The sensitivity of the AF array is not directly related to the aperture of the AF system.

The AF array has a mask over it that blocks off all but very specific portions of the light coming from the lens. If you think of the AF array as looking back at the lens through these slits, it can only see points located on a very specific part of the lens (there are two matching slits, one on each side).

The points are located on a circle which corresponds to an aperture. In the case of the current AF system, the circle is the size of an f5.6 aperture. If the slots were located further apart they would allow the AF array to "see" a point on a bigger circle on the lens let's say f2.8.

The f5.6 sensors will work on faster lenses because the faster lenses are bigger and will always allow the AF array to "see" the smaller f5.6 circle through the mask.

The f2.8 sensors will not work on the slower lenses, however, as the slower lens isn't big enough to allow the sensors to "see" any lens glass at all. They will simply be blocked as there is no glass out there to "see".

The upside to f5.6 sensors is that all lenses from f1.0 to f5.6 will focus. The downside is that they are closer together, which means they are less accurate, so while the faster glass will focus, it will not be as accurate as it could be.

I hope this helps

Ray
Thanks, that clarifies things quite a bit. A conclusion would then be that the F/5.6 and F/2.8 points could very well have the same low-light sensitivity -- good news, since it implies that the increased low-light sensitivity would be beneficial both for fast and slow lenses.
09-12-2012, 09:45 AM   #7
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But the question is if they would have F/5.6 and F/2.8 points on the same point...
I wonder if someone here can shine some light on that?
09-12-2012, 09:53 AM   #8
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QuoteOriginally posted by Anvh Quote
I wonder if someone here can shine some light on that?
Very punny

09-12-2012, 10:39 AM   #9
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09-12-2012, 03:52 PM   #10
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Last edited by beholder3; 08-11-2013 at 07:19 AM. Reason: [deleted]
09-12-2012, 09:45 PM   #11
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QuoteOriginally posted by Anvh Quote
But the question is if they would have F/5.6 and F/2.8 points on the same point...
I wonder if someone here can shine some light on that?
The AF array is just a small sensor, and the points are just different parts of the sensor illuminated by the slits in the mask. It seems that all points would have to be the same, so my guess is that they are equal in sensitivity.

Of course, if you use a f5.6 lens instead of a f2.8 lens, the sensors get a corresponding lower amount of light, so your low light focusing range is reduced by the slower lens, but would still be lower than the current K5 using the same lens.

Ray
09-13-2012, 09:57 AM   #12
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QuoteOriginally posted by Ray Pulley Quote
The AF array is just a small sensor, and the points are just different parts of the sensor illuminated by the slits in the mask. It seems that all points would have to be the same, so my guess is that they are equal in sensitivity.

Of course, if you use a f5.6 lens instead of a f2.8 lens, the sensors get a corresponding lower amount of light, so your low light focusing range is reduced by the slower lens, but would still be lower than the current K5 using the same lens.

Ray
incorrect.
Canon EOS DSLR Autofocus Explained


QuoteQuote:
A dual-cross point is an even more complex AF sensor element. The cross-type sensors described above are a vertical line and a horizontal line, i.e.in the shape of a '+'. Dual cross-type points add another cross-type point in a diagonal orientation, i.e. an 'x' that is superimposed onto that '+'. All of the implementations of dual-cross points to date combine an f/2.8-sensitive diagonal 'x' with an f/5.6-sensitive orthogonal '+'. So, with a lens slower than f/2.8 you get a standard f/5.6 cross point, and with an f/2.8 or faster lens, you get the increased accuracy of an f/2.8 baseline, and with the ability to detect lines in multiple orientations.
09-13-2012, 10:15 AM   #13
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When I looked at this a while ago, I also found it very informative.

Canon EOS-1D X AF Settings Guidebook

or direct PDF link (11MB)

It would be good for Pentax to produce a similar document about the new K-5II AF system.
09-13-2012, 10:16 AM   #14
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QuoteOriginally posted by Anvh Quote
Thanks for this!!
09-13-2012, 06:16 PM   #15
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QuoteOriginally posted by Anvh Quote
Sorry. Read it again.

What you posted has nothing to do with the light sensitivity of the sensors. Baseline is EXACTLY what I was describing earlier and just like whether a point is cross or not, has nothing to do with the sensitivity of the AF sensor to light or the amount of light reaching the sensor. Every point is actually a pair looking at rays split into two. How far apart they are sets the baseline, f2.8 point pairs being further apart than f5.6 pairs. This is why the f2.8 pairs cannot see anything with a f5.6 lens attached as I described earlier. The further apart the points are, the more accurate the focus calculation.

If the array is not a hybrid of completely different sensors with different efficiencies and pixel well depths, the pixels of the array will all be equally sensitive to light.

Also, putting on a lens with a maximum aperture of f5.6 will result in exactly 25% as much light getting to the AF sensors than a f2.8 lens. This means the AF array will not be able to focus in light as low as a faster lens. After all, the AF array is fixed and has a set sensitivity and efficiency, which sets the absolute level where focus can be achieved. Note that Pentax specifies the AF level using a FA 50 F1.4 lens.


Ray

Last edited by Ray Pulley; 09-13-2012 at 07:27 PM. Reason: My bad - f.28 - f5.6 = 25% not 50% duh.
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