ISO, Shutter Speed, Aperture and You.

If you came here, you're looking for information on ISO, Shutter Speed and Aperture and how they relate. Before I make my ham-fisted attempt at explaining it, can I ask you to please check out Bryan Peterson's Understanding Exposure? you bought his book, let me give you a primer on this topic.

ISO, SS, and Aperture all relate to each other to make an exposure on either film or on a digital sensor.

But how do they relate?

If you're outside on a sunny day you can generally expect that your correct exposure would be f/16 @ ISO100 and 1/100th of a second.  But what happens if you want really shallow Depth of field, and you open your aperture to f/2.8?

Well I can tell you that the math isn't that bad. There's 6 stops between f/16 and f/2.8, it's as easy as doubling 1/100th  6 times. (1/200th. 1/400th, 1/800th, 1/1600th, 1/3200th, 1/6400th) So the technically correct exposure at f/2.8 on a bright sunny day, would be ISO100 and 1/6400th of a second.

Now say you're faced with a dark room, and at f/2.8 and ISO 100, your shutter speed was a 1/2 second.  You think you need to get to at least 1/60th of a second and f/2.8 is as open as your aperture gets.  The answer, crank your ISO to 1600 and get the speed you need.


Think of ISO like this.

  • 100 workers will build a nice house in a normal amount of time
  • 200 workers will build the same house twice as fast.
  • 400 workers can double the speed of 200.
  • 800 workers are probably getting to the point of over kill.
  • 1600 workers probably wont build as nice a house as 200 but they're WAY faster!

Now add aperture to that equation: A small number is a BIGGER aperture and a large number is a SMALLER Aperture.

  • Those 100 workers should be given lots of room to work, i.e. f/2.8 and faster to build the house correctly
  • As you make it 200 workers you gotta lock them down a bit to keep them under control. (f/4) to keep it correct
  • At 400 workers to build a nice house, you gotta keep those guys even more controlled. (so maybe choose f/5.6)
  • And so on and so forth.

Now  put shutter speed in that equation:

  • You give your (ISO) 100 workers lots of room (f/2.8) and a pre-determined amount of time (1/100th).
  • Now you have  (ISO)200 workers in the same space (f/2.8), you only give them half the time (1/200th).
  • If you are using (ISO) 200 workers and you have them locked down a bit, (f/4) then give them the original about of time (1/100th)

See how this all works? They're completely interdependent. Just remember that every time you change your f/stop, you need to compensate by either upping or lowering your ISO, and/or doubling or halving your shutter speed.

If you still don't get on the link above to check out Bryan Peterson's book...he explains it better than I can.

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