For camera fanatics the acquisition of an old camera is a thrilling event. But if you’re going to collect them, you’d better have some repair skills so that you can also use them. [Fernando's] latest find was this Minox 35mm camera. The aperture needed cleaning, and after reassembling the unit he realized the he had not marked the focus ring when taking it apart. This is not a reflex camera, so you can’t look in the view finder to adjust focus. He came up with his own method to get the focus ring calibrated.
The focal point needs to focus on the film. He simulated this plane using some magic tape, which removes easily without leaving a residue. When the shutter is open, you can see the image projected on this translucent surface. He then set up the camera with the lens 90 cm from a bright light bulb. By adjusting the focus to create a sharp image on the temporary screen, he knows the focus is calibrated, and can reset the focus ring to the 0.9m mark.
Need some help developing that exposed film? You could always give the coffee and vitamin C hack a try.