Auto focus with m42 lenses and an F series 1.7x AF adapter.

Here is an M42 mount Vivitar 170mm auto focus lens on a K1. It works perfectly.




The SMC Pentax-F 1.7X AF adapter gets mixed reviews. The general tenor being that it is useful but of limited application. They can also be very expensive.
If you want to extend its usefulness you can convert many M42 lenses (and others) to AF by making the same modification as that needed to make the Jessop A series extension tubes work properly - you scrape off some of the black paint so that the rear flange of the lens shorts out two of the contacts on the adapter.




The problem is that the lens clearly rotates when it is screwed into the adapter, so how can we ensure the paint is removed from the right place?
Cut a couple of bits of sticky paper from a self adhesive printer label. Stick one on the side of the AF adapter and mark on it the location of the two pins nearest the red dot. Screw your chosen lens on firmly. Stick the second tab on the side of the lens and transfer the two index marks across. Now you know exactly where to remove the paint from.




The paint is normally hard and stubborn and can be quite difficult to scrape off. I use a small flat screwdriver. Depending on where the rear element is, be very careful in case your blade jumps!




Not all M42s are amenable to this treatment - you need one with a flat metal rear flange. You also need to focus with the iris wide open and hold the half press while you stop down to your taking aperture. If you let the shutter button go and then stop down it will try to re-focus when you press the shutter button again and it might fail because the adapter needs at least f4 (sometimes f5.6) to operate successfully.

I hope that is useful because there are a lot of good M42 lenses out there that don't get used simply because they are manual focus.

Thank you for the tutorial. I wonder whether copper tape may be used to provide the same benefit without permanent modification to the lenses.

I'll try it out if I get the time.

If you know of a printer in your area doing "diecutting", they use very thin and narrow metallic tape for the "make ready" of the form they're working on. This tape is outstandingly sticky, and conduct electricity with very low resistance. You could ask them for a few centimeters...

That's a good idea. Never knew it existed! ...... just ordered some off EBay. It's being marketed as "copper slug tape" because it's good at repelling slugs and snails !

Not all M42s are amenable to this treatment - you need one with a flat metal rear flange. You also need to focus with the iris wide open and hold the half press while you stop down to your taking aperture. If you let the shutter button go and then stop down it will try to re-focus when you press the shutter button again and it might fail because the adapter needs at least f4 (sometimes f5.6) to operate successfully.Not all M42s are amenable to this treatment - you need one with a flat metal rear flange. You also need to focus with the iris wide open and hold the half press while you stop down to your taking aperture. If you let the shutter button go and then stop down it will try to re-focus when you press the shutter button again and it might fail because the adapter needs at least f4 (sometimes f5.6) to operate successfully.

If you set focus to the back button it stops the refocus problem, at least it does on my K50