Rule of thumb

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Some simple rules

  • The Sunny f/16 rule says:
On a sunny day, with ISO 100, you expose at 1/100s with aperture f/16
  • The minimum exposure time for a lens with focal length N mm is 1/N s. So minimum exposure time for a 50mm lens is 1/50s
  • For every doubling of the aperture, you need to quadruple the exposure time for the same light condition. So if you are exposing 1/200s with f/2.8, you need 1/50s with f/5.6 to get an image that is similarly bright.
  • The minimum focus distance of a regular lens is around 10x its focal length. So a 200mm lens focuses starting at 2meters. Macro lenses are obviously not like that.
  • For a portrait, rule of thumb is to be 20x the focal length away. So 50mm lens 1m, 200mm lens 4m.

These pages are for Amateur Photographers and not really for seasoned photographers and professionals. I have no affiliation or commercial interest with any brand/make. I write from my own experience. I ended up using mainly Nikon, so I am more familiar with this brand than others. See price for notes on pricing as well as photography related links.