[ftpaddict]

Question Question.

Unless they have some sort of electronic doohickey inside that tells the camera its maximum aperture, how does a camera body know when a lens is 1.4 and when another one is 3.5?

[Lowell Goudge]

I think the best resourse is here

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography

In principle, when you look at the camera body contacts, there is one recessed pin. (third pin from the lens locking pin I believe) If this pin is shorted to the lens mount then the camera thinks it is an A lens.

That is the reason this pin is recessed, it requires the presence of a corresponding pin on the lens to activate the apature sensing. Normally it will not contact the lens mount when a non A lens is mounted. The remaining 5 pins on the A mount tell the camera the maximum and minimum apatures

[séamuis]

Im pretty sure this is correct, though maybe not so useful.

with A series lenses six electrical contacts are added to the bayonet ring. One is slightly recessed and allows the lens to indicate whether the aperture ring is set at 'A' or not. If it is, a pin on the lens extends slightly and makes contact, while if the lens is at any other setting the pin is retracted and does not make contact. The other five contacts are used to encode the lens' aperture range. Each contact on the lens is either conducting or non-conducting, providing a binary 1 or 0, respectively. Two contacts encode the lens' minimum aperture—f/16, f/22, f/32 or f/45; although no Pentax K-mount lens has ever had an f/16 minimum aperture. The other three contacts encode the lens' maximum aperture; their meaning is dependent on the minimum aperture indicated by the lens.

yep the link provided by Lowell Goudge tells the same thing but with pictures and diagrams! so yea the link should explain it all. and much much more about pentax lenses.

[ftpaddict]

Thank you, everyone. That pretty much clears everything up.

I was feeling a bit like this guy here > because I used some gear I did not know the exact working principles of.