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01-06-2019, 04:15 AM - 5 Likes   #1
Pentaxian




Join Date: Dec 2011
Posts: 2,066
AF-sensors: powerful little things

I guess most people discussing AF never really appreciate what the little things do and under which circumstances they perform.

Each AF line sensor actually requires two short pixel lines (sometimes double pixels), very much like a short row of single (or two) pixels from the main sensor.

Each of these has to watch the subject through its own optics at the outer part of the image circle of the lens, typically placed where the edge of the aperture blades appear at F5.6.

They only work if they can see the whole framed image in pretty much complete sharpness (maximum DoF), since they do need to compare tiny detail of the frame while the users might see only blur in the viewfinder.

So the image (remember it is not really an image as we know, it is just a single pixel row "crop") is very small and (as I recently read) works as f/30 aperture.

No go combine this with our needs to have fast AF, and let us assume we want the AF to check the image every 1/10th of a second. Obviously this means the "shutter time" plus readout and computation can not be longer than 1/10 of a second.

And now go figure what "ISO" your little sensor row needs to operate at to focus in a scene where the ambient light is only EV 3.
That is roundabout ISO 102.400 at EV 3 already.

With a focus system that works down to EV -3, this requires the AF sensor to work at something like ISO 6,553,600. Obviously in reality the camera prefers to slow down readout times and prolong "shutter time" instead. But still.

Then imagine all the noise existing in that little row of pixels at that ISO, providing false image information.

And still the system needs to compare the two sensor pixel rows to find (and match) a good contrast edge. And we usually do not take images of checkerboards.

I think this is quite an engineering feat.

01-06-2019, 01:21 PM   #2
ne!
Junior Member




Join Date: Dec 2016
Posts: 27
That's a mindblower!

But speaking of pixels, I just bought a Pentax SF1, which I understand is one of the first AF cameras ever made. It operates in one mode-- center spot focusing, but boy oh boy it works incredibly well for 1988, quite shocking really.
01-06-2019, 05:54 PM   #3
Pentaxian




Join Date: Dec 2016
Posts: 1,459
QuoteOriginally posted by ne! Quote
That's a mindblower!

But speaking of pixels, I just bought a Pentax SF1, which I understand is one of the first AF cameras ever made. It operates in one mode-- center spot focusing, but boy oh boy it works incredibly well for 1988, quite shocking really.
The upgrade of that one, the SF-1n was my first Pentax AF SLR body. It came new with the "F" 50mm f/1.7 lens at a very good price. I bought it around 1990 or so.
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