Originally posted by Damian
2mm roughly. It varies from side to side by up to .09mm. 2-2.02mm on one side to 2.09-2.11mm on the other. Basically, it's not entirely flat. Chinese quality control.
I've only messed with the 58 1.8 other then the 58 1.4. I took the optics out of the 58 1.4 for cleaning, but did not mess with the focusing helical. It worked fine, so there was no need to fix it. I can't remember if there are any shims or rings that can be removed. I'll have to go back and look at the pictures from the cleaning to see if there is anything visible. That might be an option though. My goal for this project was to make no modifications to the lens, but simply swap mounts. Totally reversible modification in case I don't like the lens, I can revert it and sell it like it was. Typically they are worth more as collectors items then as "modified for pentax".
Ok.
The shims in my case were 2 very very thin rings that might not be anough.
I did however make a similar thing with a 58/1.8.
I made it non destructable but one part that made me earn some space was to remove the half circular plate that that sits between the mount and the aperture ring.
I have unfortunatly sold that lens but I found some images of it.
First image shows that half circular plate. Remove it and save about a mm.
I did remove the whole original mount and replaced it with a pentax mount from a scrap lens.
I didnt just place the new mount over the old one.
Its still fully reversable as long as you remember were you put the parts
Second image shows the focus stop screw.
Its easy to unscrew it and then find the real infinity, make a little mark and drill a small new hole for the screw.
When you reverse it just move the screw back to its original hole.
using this method you dont have to do anything to the mount. The 1.5 mm you need is just a cm or so that the focus stop screw needs to move.
You should have more then that to play with but if you do this just make sure the Aperture lever works zoomed fully in and out.
Last image shows the inside and at the bottom you can see a dark area. In the right side of that dark area is a "square" hole. Thats were the aperture lever goes down. On my 35/2.8 Topcor RE I replaced the mount with an old Vivitar mount and managed to get full aperture control just like any Pentax-M lens. Green button stop down works perfect. It did take some tinkering to get it right though so not as easy to do as it might sound.