When you buy an used camera, especially a very old one, chances are that the focus will be out of whack. Sometime it will be off just a bit, other times a lot.
The procedure to adjust it varies from camera to camera, but in every case you will have to find a suitable target – i.e. a subject truly at infinity – to achieve a correct focus.
This, especially if you live as I do in a city surrounded by mountains, can be a bit difficult. One of the best way is focusing on the full moon; unless you are into astronophotography you will not find on Earth – literally – a subject more at infinity than the moon. Unfortunately to do so you will have to wait for a night of full moon, hoping that will not be cloudy and that nobody will take you for a crazy person 😉
Luckily there is a much simpler, if less known way: using a collimator. “What? And I should buy an expensive piece of equipment just to check my infinity focus? No way!” – I can hear you all saying. What you don’t know is that, if you happen to have a camera and a tele lens, you already own a collimator…
Here you can see a focus check performed on a nice Rolleicord – the poor man version of the Rolleiflex TLR cameras. All you have to do is:
1) tape a piece of ground glass* with a couple of lines marked on – a pencil will suffice – to the film plane of the camera whose focus you want to check (with a TLR like the Rolleicord you will have to check, obviously, also the viewing lens). This is critical: remember to tape the ground glass with the matt** side against the film plane.
2) point a flashlight – preferably a LED one because are way brighter – or a table lamp straight into the ground glass; the lens has to be at full aperture and the shutter locked with a remote release in B or T
3) point your other camera with a tele lens set to its infinity stop and at full aperture mounted on straight into the lens of the camera you want to adjust. Focus the lens on the camera you want to check to infinity. When / if the lines on the ground glass will appear sharp well, congratulations, you have lab-perfect infinity focus. Otherwise you will have to adjust your camera – not the “collimator” camera! -, following the specific instruction for its model, until you can see the lines are sharp.
To adjust the viewing lens of a TLR you follow the same procedure, except that this time the flashlight goes straight on the ground glass in the waist level finder.
For this job a Sony Nex is perfect because it lets you zoom up to 14x, so you can actually see the surface grain of the ground glass; with such magnification spotting even minor faults in the focus becomes trivial. I tend to use mine with a cheap but really good Minolta MD 135mm f/3,5, that on the Nex is equivalent to a 200mm on a full frame.
Now the Xenar of my two old Rolleicord are sharp as a tack! Talk about old lenses…
*I’ve seen pretty much everything used for this purpose on the net, from scratched cd jewel cases to masking tape. Please, do yourself a favor and by a piece of real ground glass for a couple of euro! The plane of focus has to be perfectly flat, so jewel cases or – worse – tape do not work… After all you haven’t shell out all that money for a high quality camera to take unsharp pictures!
**Troubles identifying the matt side? Orient the glass to catch the reflection of an open window: if you can clearly see the window image in it this is the glossy side, if you can just see a “blob” of light then that is the matt side