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07-29-2010, 04:36 PM   #1
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What do the lens contacts do?

I'm curious as to what the lens contacts do on the back of the lens. There are several of them and I'd like to know what function is attributed to each contact. I'd appreciate any help.

If anyone's curious, I've been meaning to convert manual focus lenses to auto focus. I have problems, I know.

07-29-2010, 04:47 PM   #2
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QuoteOriginally posted by hangu Quote
I'm curious as to what the lens contacts do on the back of the lens. There are several of them and I'd like to know what function is attributed to each contact. I'd appreciate any help.

If anyone's curious, I've been meaning to convert manual focus lenses to auto focus. I have problems, I know.
Bojidar Diminitrov has the answer, start from here: K-Mount Evolution, Features and Operation

Six of the contacts (that are there from the A-generation lenses) are like a binary code for the min and max apperture. The 7th contact that came with the autofocus lenses transfer an analog signal that code much more info. It is not hard to change a K mount into an KA mount with the info on min and max apperture. Unfortunately, this is not enough. In the generation shift from the K mount (and the M-generation lenses) to the KA mount (and the A-lenses) Pentax changed the mechanical transfer of apperture from something like logarithmic to linear. So a DSLR (or any camera with the crippled mount) will still be confused when metering (it meters wide open, then try to stop down for the actual shot, but makes the wrong assumtions on how far to move the apperture leveler). Only a few of the late M lenses were made with linear transfer. I hope I got it mostly right.
07-29-2010, 05:17 PM   #3
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Thank you Douglas, that was exactly what I was looking for.

Six of the aperture contacts aside, would it be difficult to replace the elements of an AF lens with the innards of another lens and still retain AF?

I think my plan is to find a cheap scratched up F/FA lens, dig out the innards and replace it with the innards of a high quality manual focus lens. I hope AF will still work even if the original glass is removed.
07-29-2010, 05:25 PM   #4
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QuoteOriginally posted by hangu Quote
Thank you Douglas, that was exactly what I was looking for.

Six of the aperture contacts aside, would it be difficult to replace the elements of an AF lens with the innards of another lens and still retain AF?

I think my plan is to find a cheap scratched up F/FA lens, dig out the innards and replace it with the innards of a high quality manual focus lens. I hope AF will still work even if the original glass is removed.

Not sure how easy that would actually be in practice... although it does sounds similar to a crazy plan I had back in the day when I wanted to swap a manual transmission into a car that originally had an automatic transmission


Last edited by dgaies; 07-29-2010 at 07:08 PM. Reason: typo
07-29-2010, 05:49 PM   #5
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I predict an equally low likelihood of success from either endeavor, but by all means, have a ball!
07-29-2010, 07:05 PM   #6
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QuoteOriginally posted by Marc Sabatella Quote
I predict an equally low likelihood of success from either endeavor, but by all means, have a ball!
Is that a challenge?!
07-30-2010, 12:15 AM   #7
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Take pictures of the progress and keep us updated if you actually do it. If it works, I would be interested in seeing.

QuoteOriginally posted by dgaies Quote
although it does sounds similar to a crazy plan I had back in the day when I wanted to swap a manual transmission into a car that originally had an automatic transmission
Been there, done that, collecting the parts to do it again.
07-30-2010, 01:20 AM   #8
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I think you underestimates the complexity of the mechanics inside a lens. If the lens design is not very similar you will have to replace so many parts that you could just as well build a lens from scratch. What might work is to buy a broken F/FA50/1.4 (damaged glass but the mechanics intact) and replace it with glass from a A50/1.4 or another lens combination where the glass elements should be identical. But if that is a progress could be discussed since the A lenses has much better build quality than the autofocus versions.
What you could do is to start looking for the few M lenses that was built with linear apperture. They should be easy to turn into A lenses.
In any case, good luck, and as rockstarr suggested, post pictures before, during and after the work. That would be nice.

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