Sunny 16 Rule

For much of photography's history, there were no light meters, so basic rules were created to get the right exposure for a picture. If you know the sensitivity of the film you are using, you can get close to the exact settings you need to get a proper exposure.

So the rule is: on a sunny day (bright sunlight), your shutter speed equals the inverse of the ISO of your film at f16.

For example, we use 400 ISO film, so the ideal exposure at f16 would be 1/400. Since cameras don't generally have a 1/400 shutter speed setting, you would use the closest one. In our case, the closest would be 1/500, but slightly overexposing film is better than slightly underexposing, so I would pick 1/250 of a second for the shutter speed in most cases.

If it is not a sunny day, then you would let in more light by decreasing the shutter speed (go to 1/125, 1/60, etc.) or open up the aperture (go to f11, f8, f5.6, etc.).

Here are some links for more information about this rule. If you meter is broken or not functioning well, use this rule.

https://en.m.wikipedia.org/wiki/Sunny_16_rule

http://www.guidetofilmphotography.com/sunny-16-exposure.html

https://www.slrlounge.com/photography-essentials-the-sunny-16-rule/

Remember:
Opening the f stop by one stop (going from f16 to f11) and cutting amount of time the shutter is open by half (going from 1/250 to 1/500), brings in the same amount of light.

Closing the f stop down by one stop (going from f16 to f22) and doubling the amount of time the shutter is open (going from 1/250 to 1/125), brings in the same amount of light.

1/250 of a second at f16 = 1/500 of a second at f11 = 1/125 of a second at f22