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04-01-2014, 04:28 PM   #1
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Pentax FA - electronics and focus brush - what for?

Hey guys,

I have some very basic questions regarding FA lenses.
I once had to partly disassemble a FA 50 1.4 lens; there were quite a few electronic components hidden inside, namely some microchips on a flexible board and a golden "brush" close to the focus scale (see e.g. this thread for many nice pictures of a disassembly made by another forum member: ).

What is this brush being used for? And the microchips?

I am a bit confused as in this disassembly blog post ( vjacesslav: SMC PENTAX FA 43mm 1.9 LIMITED DISASSEMBLE IN 37 PICTURES ) on a FA 43 1.9, I can see neither such a brush, nor microchips.

Do also A-lenses contain electronics other than the conducting pins in the lens mount?

Best regards!

04-01-2014, 05:11 PM   #2
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I'd speculate that the brush is used for relaying to the camera the approx. focusing distance the lens is set to since this information is available in the EXIF data block of a digital image.

The microchip converts various lens information to a digital format that the camera can read (lens identification number, focal length, focusing distance, aperture set, and data about which apertures give the best image quality). Possibly additional information about lens characteristics as well. I have never seen any official information from Pentax about what data is transmitted from lens to camera, and how. It would be cool to get all the detail.

A-lenses do not contain any electronics. Their lens information contacts are either insulated or shorted and require no electronics.
04-02-2014, 06:22 AM   #3
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Original Poster
Dear Ole, thanks for you kind reply.

Does anyone of your then have a clue of how the FA 43 measures the position of the focus ring w/o the brush?

I must admit I naively thought that the focus distande as reported in the EXIF files would originate from the sort-of-triangulation which the phase detection autofocus sensors deliver;

if someone of you has a K or M lens at reach, could you try to focus using the AF sensors, e.g. via trap in focus (to make sure the camera thinks the camera is actually focused on the central focus point) and then check if in the EXIF files there still is an estimated object/focus distance?

04-03-2014, 02:01 AM   #4
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QuoteOriginally posted by Rapfl Quote
Does anyone of your then have a clue of how the FA 43 measures the position of the focus ring w/o the brush?
It doesn't because FA43 also have those brushes.


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