Nothing is better than improvising and experimenting with analogue photography…
…but sometimes some rules can be helpful when your precious camera does not have a metering system (or just doe not work…)
There it is the SUNNY 16 RULE!
The basic rule is:
“On a sunny day set aperture to f/16 and shutter speed to the 1/ISO film speed for a subject in direct sunlight.”
On a sunny day and with ISO 100 film / setting in the camera, one sets the aperture to f/16 and the shutter speed to 1/100 or 1/125 second (on some cameras 1/125 second is the available setting nearest to 1/100 second).
On a sunny day with ISO 200 film / setting and aperture at f/16, set shutter speed to 1/200 or 1/250.
On a sunny day with ISO 400 film / setting and aperture at f/16, set shutter speed to 1/400 or 1/500.
As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. 1/250 second at f/11 gives equivalent exposure to 1/125 second at f/16. More in general, the adjustment is done such that for each step in aperture increase (i.e., decreasing the f-number), the exposure time has to be halved (or equivalently, the shutter speed doubled), and vice versa.
An elaborated form of the Sunny 16 rule is to set shutter speed nearest to the reciprocal of the ISO film speed / setting and f-number according to this table
Aperture Lighting Conditions Shadow Detail f/22 Snow/Sand Dark with sharp edges f/16 Sunny Distinct f/11 Slight Overcast Soft around edges f/8 Overcast Barely visible f/5.6 Heavy Overcast No shadows f/4 Open Shade/Sunset No shadows