[epicurus]

You could try light metering the way I learned. Full Sun is f16 at 1 over whatever filmspeed your using. ISO100 would be 1/100 second. Add to or remove from the exposure as the brightness changes. Try to learn the difference in light level for 1 stop of exposure - or buy a light meter. I too had a Zenit but the model without a lightmeter. I still use my Weston V meter even though my camera has an excellent built in meter.
Although every camera has a built-in meter it's a case of if they'll let you see what they've measured. Lots keep it from you so you can't trouble your pretty little mind with stuff the maker thinks shouldn't concern you!