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07-15-2008, 12:07 PM   #1
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Pentax documentation about AF sensor positions

Dear Pentaxians,

we have heard many comments about wrong focusing because the AF sensors appear to be larger than one would naively think.

The following is an official Pentax drawing about their size and positioning for the 11 sensor Safox system. Please, use to your advantage.

Pentax Safox sensor layout

Source: I accidentally found this info in the German Pentax *ist film camera booklet on their German website (PDF):
http://www.pentax.de/downloads/photo/de/PENTAX%20%20_star_ist/Slim-Jim-ist-GER.pdf.
It officially applies to the Pentax *ist film camera only. But I guess, it applies to all current Pentax DSLRs as well.

This is the text which goes with the drawing: "Wie aus der Skizze hervorgeht, sind die CCD-Sensoren in unterschiedlicher Form angeordnet." (As shown by the drawing, the CCD sensors are layed out in varying shape.)

07-15-2008, 01:34 PM   #2
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Thanks very much. Really useful info here...
07-15-2008, 03:54 PM   #3
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interesting, in my head I was picturing more of a dot than a dash ( a smaller spot area for each of the sensors, not a lengthy line)

So the sensor picks up from anywhere along one of those black lines? I think I was expecting or wishing for more of a precise, smaller dot area. like the red box in the viewfinder.

Are the cross/intersections more sensitive than along the straight portion?
07-15-2008, 05:22 PM   #4
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QuoteOriginally posted by falconeye Quote
Dear Pentaxians,

we have heard many comments about wrong focusing because the AF sensors appear to be larger than one would naively think.

The following is an official Pentax drawing about their size and positioning for the 11 sensor Safox system. Please, use to your advantage.

Pentax Safox sensor layout

Source: I accidentally found this info in the German Pentax *ist film camera booklet on their German website (PDF):
http://www.pentax.de/downloads/photo/de/PENTAX%20%20_star_ist/Slim-Jim-ist-GER.pdf.
It officially applies to the Pentax *ist film camera only. But I guess, it applies to all current Pentax DSLRs as well.

This is the text which goes with the drawing: "Wie aus der Skizze hervorgeht, sind die CCD-Sensoren in unterschiedlicher Form angeordnet." (As shown by the drawing, the CCD sensors are layed out in varying shape.)
Useful reference information. Good Thread.

07-15-2008, 05:57 PM   #5
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QuoteOriginally posted by augustmoon Quote
So the sensor picks up from anywhere along one of those black lines? I think I was expecting or wishing for more of a precise, smaller dot area. like the red box in the viewfinder. Are the cross/intersections more sensitive than along the straight portion?
I think your's is the common belief. And why I tought I better post this Pentax drawing

Each long black line is a sensor, each working pretty much like the split screen indicator in some focus screens. The camera computes the correlation between luminosities along either side of a linear sensor and the shifting distance at which this correlation reaches its maximum. So, the feature with the biggest size and strongest contrast along each linear sensor will determine its signal. Therefore, all points along a black linear sensor line have equal sensitivity.

The cross/intersections are just this: cross/intersections. Those points have no special meaning.

For each cross sensor, there are signals from both the horizontal and the vertical sensor. The one with the strongest signal will win.


All this isn't Pentax-specific. All SLRs work like this. Only the very exact AF sensor layout is Pentax specific.


Addendum:
It has been reported that the K20D focuses better onto small forground objects than the K10D. This can be achieved in the firmware by weighting the linear sensor shifting correlation function with some function of the shifting distance (weighting foreground contrast stronger). It does not mean that the sensors must be smaller.

Last edited by falconeye; 07-15-2008 at 06:04 PM.
07-15-2008, 08:21 PM   #6
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QuoteOriginally posted by falconeye Quote
I think your's is the common belief. And why I tought I better post this Pentax drawing

Each long black line is a sensor,..... So, the feature with the biggest size and strongest contrast along each linear sensor will determine its signal. Therefore, all points along a black linear sensor line have equal sensitivity.

For each cross sensor, there are signals from both the horizontal and the vertical sensor. The one with the strongest signal will win.
Thank you for clarifying this, so in using the center "point" AF, I've actually been using a much more broader area where a stronger contrast element not necessarily where I've been aiming the small red square could have been triggering the AF. I can see how this helps the camera more easily find a ready and suitable element to lock onto, but not necessarily what I myself would have intended it to.

I'll keep this in mind when I am using AF, and use as a good reminder to use manual focus in critical or confusing times.
07-15-2008, 11:01 PM   #7
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This might explain how it can mis focus when the center point is on a foreground object! Not sure it actually helps in practical use... if you've got that central point on something you tend to expect it will use that area to focus... how can you force it to use the very centre?
07-16-2008, 03:30 AM   #8
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Well..

I had seen this diagram long ago. IIRC, it was Roland Mabo who pointed us to a link for the diagram years ago.

However, from what I measured (also long ago) for my K100D, the actual sensitive area is yet smaller:-

RiceHigh's Pentax K100D Full Review

If it is really as large as shown, it is really worrisome. It is just because the user has no way to pinpoint focus plus the resolution of the AF sensors will not be really high (coz it is magnified which just means more errors).

QuoteOriginally posted by falconeye Quote
Dear Pentaxians,

we have heard many comments about wrong focusing because the AF sensors appear to be larger than one would naively think.

The following is an official Pentax drawing about their size and positioning for the 11 sensor Safox system. Please, use to your advantage.

Pentax Safox sensor layout

Source: I accidentally found this info in the German Pentax *ist film camera booklet on their German website (PDF):
http://www.pentax.de/downloads/photo/de/PENTAX%20%20_star_ist/Slim-Jim-ist-GER.pdf.
It officially applies to the Pentax *ist film camera only. But I guess, it applies to all current Pentax DSLRs as well.

This is the text which goes with the drawing: "Wie aus der Skizze hervorgeht, sind die CCD-Sensoren in unterschiedlicher Form angeordnet." (As shown by the drawing, the CCD sensors are layed out in varying shape.)


07-16-2008, 04:01 AM   #9
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I sensed this was the case.

I only use the centre point, and found it was bigger than originally thought.
07-16-2008, 05:21 AM   #10
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QuoteOriginally posted by RiceHigh Quote
However, from what I measured (also long ago) for my K100D, the actual sensitive area is yet smaller: [...] the resolution of the AF sensors will not be really high
To really know, one would have to dissassemble the AF system. Or find a picture of it.

You may be right that it is only about half as large. Or not.

If I do a test with both, a *istDS and a K20D, focussing on a single vertical line in AUTO focus point mode, I get 2 focus points to confirm simultaneously if I put the line in between them. So, both see the line at the same time! The gap in between them must be marginal. If I single select the focus points, the center sensor stops right before the "(" mark. And the sensor left to it takes over right after this very mark!

The sensitive area of the 1 inner focus point is restricted to the square as indicated by the "( )" marks.
The sensitive areas of the 8 outer focus points are restricted to the rectangle as indicated by the "[ ]", excluding the inner square above.

Therefore, I agree that the inner lines in the drawing should be slightly shorter and the outer lines slightly larger. But still w/o much of a gap.

The drawing I provided seems to show the engineers intention as well: That the AF sensors cover the entire middle 50% surface. Otherwise, AUTO focus point selection would make no sense.

Maybe, the drawing was provided before. But in the last 6 months or so nobody ever made reference to it in the many threads discussing the AF sensitive areas. I discovered this info from a forgotten PDF and the drawing was extracted from the PDF by myself...


With regard to your resolution concern: Do you actually know the pixel resolution of the Safox AF CCD sensors?

@RH: Please delete the image in your quote as it makes the thread less readable. Thank You.
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