Click images to expand to full size in a separate window.Many classic cameras haven't been used in over two decades.You'll need to know the exact distance to the focal plane (Leica techs focus at 1m, 10m, and infinity), but it sounds like a great idea since the dot is small and coherent.It won't work as well with infinity focusing unless you have a big laser and a very distant target (in which case, the local police or FBI might become interested in you as well).But if you find a nice camera that's worth 0 and needs 0 in repairs and you can buy it for 0, then go for it.Before reading this page, checking out a used classic camera fully before buying it is a better preventative solution.
Dave Passmore (Dave PDawg) has sent me some photos of a focusing aid that uses the focusing screen off an old Nikon camera mounted on a cheap 20x magnifying loupe (linen checker). As a focusing aid, Dave also suggests pointing a laser pointer onto a wall and focusing against it.
I do not recommend trying to replace the mirror box foam in your SLR until you have considerable experience, it's much harder to get right and much easier to get goo on your mirror, which would be a very Bad Thing™.
On most rangefinders, adjusting the vertical or horizontal rangefinder patch is as hard or easy as finding the adjustment screws. I have instructions on each camera page on how to RF align the Canon P rangefinder, FED 3, Kiev 4, Zorki 4K, Leica M7, and just got tips on how to align my Kodak Retina IIa.
My own technique is to always use a lens hood and to only clean the lens when it is absolutely filthy.
I clean the front and rear elements of my cameras on an "as-needed" basis.
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There are three type of simple repair jobs that I do myself: 1) cleaning the lens; 2) replacing the foam; 3) calibrating the rangefinder; and, 4) regular exercise. I have a list of recommended repair shops at the end of this page.