Jocko said:
Much good advice already given, but.... The most important initial question is "do you wear glasses"? The dioptre correction on FSU LTM cameras will possibly help if you are mildly shortsighted and happy with removing glasses when photographing. Otherwise the Zorki 3/3m/4 viewfinder is notoriously difficult to use, as it has very little eye relief. Spectacle wearers will probably not be able to see more than 60% of the frame. That is a very serious issue.
The Zorki 5 and 6 have bright, contrasty finders based on that used in the Iskra , but I would generally speak up for the FED 2. It may be dimmer, but it remains entirely practical within the usual limits of the camera's usage - say at a meter reading of 2.8 at 1/30th at 400 ISO. In my experience the high contrast between finder and rangefinder actually makes it easier to focus in low light than supposedly brighter Zorki finders.
All the best, Ian
I strongly second Ian through all his post here repeated, with some additional commentaries:
a) I am not knowledgeable of Soviet LTM cameras beyond some variations of the Fed 2. As for the Fed-2, I own two of them. One has brighter viewfinder and dimmer yellow patch, the other vice-versa. No accident here, both issues are interlinked.
b) Within certain limits, even for very low light, what works best is the dimmer viewfinder with brighter yellow patch.
c) The ultimative solution is to use a separate Soviet finder, which are as bright almost as live viewing.
d) For a standard lens there is no Soviet finder at all, but the Universal Turret Finder which includes the 50mm focal lengtht. But here there is an ergonomic problem when mounting it on a Fed 2, and perhaps other LTM Soviet cameras, as it protrudes backwards, blocking your ability to aproximate your eye to the camera viewfinder. This calls for a Kiev re-thinking. Or buying a pricy Voightlander 50 finder.
e) BTW, just for the protocol, most of old rangefinders, not only FSU, including the amazing Canonets, have viewfinder problems of this or other sort.
f) If you are glass wearer, no doubt Ian's advice is your starting point.
Cheers,
Ruben