M42 lenses will need to be manual stopped down to meter with the camera. There is no linkage to utilize the ‘auto aperture’ feature of *some* m42 lenses – I say this because not all m42 lenses are ‘auto’.
Many are ‘auto’. If they are ‘auto’ they need a manual override switch – if you wish to shoot anything other than wide open.
Does that make sense?
On my K10D the procedure worked this way:
1) The camera would need to be set to “allow use of aperture ring”
2) Physically open the aperture with the aperture ring – for focusing purposes.
3) Physically close the aperture (stopping down) with the aperture ring for the desired reason (e.g. depth of field).
4) Hit ‘green button’ to meter.
5) Hit shutter button to take picture.
This process needs to repeated each time in taking a picture – unless you’re shooting wide open, then you can neglect using the aperture ring all together and just hit the ‘green button’ to meter between shots.
Or you can also set the camera on ‘aperture priority’ and the camera will continuously meter without hitting the ‘green button’ – although it seems to be less accurate way of metering – that’s the way I did it.
I tired it. I used an M42 lens for months. It works, and you can get used to it very quickly – but I got tired of my own focus errors and the cumbersome metering process.
Then there are M series Pentax lenses. Everything above is the same, except the camera will stop down the lens for you when you hit the ‘green button’ to meter, and it will stop down the lens when you hit the shutter button – no physically turning the aperture ring to meter.
Then there are the A series Pentax lenses. Everything is the same above except you don’t need to meter at all, as the camera is aware of the aperture setting. . . or you can control the aperture on the camera.
If you really want to get into old manual lenses, buy the cheapest m42 lens you can find and an appropriate M42 adapter. . (the flush mount type), and go out and learn how to use it.
Then break down, save your pennies, and buy a FA 50mm.
Honestly the old screw mount lenses are great, but they are way overpriced. An old M42 Takumar f/1.4 50mm will cost as much as 70 USD, an M series will be 100 USD, and an A series will be 150 USD.
A brand new FA 50mm will be 200 USD. So why (M42) screw with it?
Tho, if you’re really up to try it out, look for an old Yashica M42 55mm (5.5cm) f/1.8 on Ebay for like 12 dollars. It’s *extremely* sharp. Cut your teeth on something cheap like that before getting into the expensive Takumar world.
Just MHO.