Originally posted by ivanvernon Good advice. I had one adapter stuck on so hard it was almost impossible to remove. Also, those 42 to K mount adapters have to fit entirely inside the mount. With an exterior flange that increases the registry distance, you cannot focus to infinity. It is surprising that anyone actually designed and sold such adapters.
In reality that kind of adapters can be incredibly useful.
You just have to take into account that if the guest lens is correctly set for infinity, you won't be able to focus to infinity anymore.
BUT... some lenses offer ample room for helicoid adjusting and some others can be converted to PK mount removing the original bayonet and using the flanged adapter in its place. Of course some lathe/burr/drill work is required...
The flanged adapters are usually 1mm thick, but there is a very thin black version that is about 0.5mm.
One interesting use of the latter would be the conversion of Zenit M39 lenses. Most (all?) of them have an internal washer that has to be sanded down to allow for infinity focusing using an M39>M42 adapter. I resisted the temptation to make the washer thin enough to allow the use of a flanged M42>PK adapter cause I want to preserve the integrity of the original. Removing the washer would allow to focus past infinity, unfortunately it is not an option because other way the diaphragm gets stuck.
There is a possible solution: making a new washer from scratch. There is a material, sold in sheets, that is used to cut gaskets for vintage car/bike engines. It looks like a kind of copper league, and should be thin enough to allow for the use of a flanged adapter, retaining infinity focusing. I'll give it a try as soon as I find a better scalpel and the right material.
Another use of non-infinity adapters is with tubes/bellows. For example I have an Edixa Bellowsmat with swinging front standard. With a flanged adapter it can be quickly mounted/removed from the camera, making its use much more practical.
The same concept applies to M42 lenses that very likely will never be used at infinity, like the Volna-9 or the Industar-61 L/Z.
Never tried myself, some catadioptric lenses focus past infinity because they must be usable under very high temperatures. With a focal of 500mm (or even 1000/1100mm) the variation in register would not be so dramatic, so maybe at normal temperatures the lens could reach infinity anyway.
Screwing long heavy lenses on a camera is a real pain. Anybody who is familiar with the procedure knows what I'm talking about. Having a PK mount hugely improves the ergonomics.