Sunny f/16 Rule

A more easy to remember way for manual exposures is the sunny 16 rule.

It states that in a sunny day, set aperture to f/16 and the shutter speed should be 1/ISO number (or the nearest value).

For example if your ISO is set 100 then the shutter speed of 1/100 sec will work well with f/16 in sunny days (when shooting in direct sun of course and not in the shade).

Cloudy days or shooting in the shades will get things down between 1 and 3 stops as following:

• 1 stop down when slightly overcast: Can be recognized by slightly softness around shadows.
  
• 2 stops down when overcast: You can hardly recognize shadows.
  
• 3 stops down when heavily overcast: No shadows at all.
  

Of course you are free how to handle the lesser stops. You can go down from f/16 to f/11, f/8, and f/5.6 or to retain deep DOF you can instead move to higher ISO or longer exposure (you may need tripod for exposures longer than 1/30 sec.