I have a Zenit 122 camera, 3rd version (shown here)
Can I put on it an external flash for digital Nikon reflex, for instance? If not, which kind of flash should I buy for it? Is there a flash compatible both with digital Nikon reflex and Zenit 122?
With few limitations, you can use almost any arbitrary combination of cameras and flashes. It is no problem to use a Nikon flash on the Zenit 122. The flash shoe and the functionality of the middle contact pin is standardized and supported by most camera and flash models. This pin is simply used by the camera to tell the flash when to trigger.
What will not work is any furth 'cooperation' between the camera and the flash like TTL metering or automatic transfer of film speed and aperture settings from the camera to the flash. I do however believe that all Nikon flashes have an A mode (automatic), in which the flash takes over the light metering without any support from the camera. This will work with the Zenit camera as well. You will have to set the film speed and aperture setting you are using on the camera manualy on the flash unit for this to work. You can of course also use the flash in manual mode, but that usually requires some quite tedious calculations.
“An external flash for digital Nikon” means it would be a TTL or iTTL flash.
A TTL Auto flash will not work well because it will not be able to communicate with the camera. This usually means it will only fire at full power in TTL mode.
If the TTL flash also has a manual mode, it could work, but you will need to do lots of calculations using the flash power guide number and subject distance to get the the right aperture for the correct exposure.
The best flash to use is an older film era flash with an Auto Thyristor sensor at the front of the flash which will automatically vary flash power to match the aperture and ISO selected on the camera.
Some older film era Nikon flash units also used Auto Thyristor. Newer Nikon’s like the SB-800 and SB-900 also have an Auto Thyristor option.
A good example of an older Auto Thyristor flash is the Vivitar 283.