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The value of a "stop"

Discussion in 'General flash photography, lighting, and technique' started by tbarnette, Jul 20, 2008.

  1. tbarnette

    tbarnette Guest

    I'm wanting to know how much is a "stop". I know what happens when you lower or raise exposure by a stop (or some degree of a stop) and I know how to do it but I don't know the value (you know, the technical stuff). Are there any threads or links that could explain this for me?
  2. The f (aperture) "stops" are 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, and 16.0. Going up in numbers from the small number (f/1.4) each stop passes half the amount of light to the film or sensor as the previous stop. Or, going from the high number (f/16) down, each stop doubles the amount of light going to the film or sensor.
  3. tbarnette

    tbarnette Guest

    Thanks Norm,
    That sounds simple enough. So all the other f-stop settings on my camera are only incremental settings? I wonder why such odd increments?
  4. Not every lens conforms to this standard, but it's a good bet if your lens has an aperture ring, it has the "normal" f Stops with a detent at each stop. It also might have a half stop detent in between, usually not marked, but that is unusual.

    If you have a "G" type lens, without an aperture ring, you read your aperture inside the viewfinder and/or on the camera's LCD screen. In that case, if you have your camera set for 1/3 stop per "click" of the dial, then you'll have two intermediate stops between the "normal" ones, and yes, the numbers seem a little odd.

    If you have the camera set for 1/2 stop per click, then........... well you get the idea.
  5. tbarnette

    tbarnette Guest

    That seems like a fair explanation.
  6. if you want to get really techie about it, the f/ stop is equal to the focal length divided by the aperture diameter- so if you have a 50mm lense, and the aperture measures 25mm, you are at f/2

    correctly, it is the f number we are concerned with- the 'stop' is just the clicky part of the lens that stops the adjustment of the aperture.

    Since the area of the aperture is what lets the light in, Pi x R 2 (pi times radius squared) (sorry, I don't know how to type the notation properly) so that 1/2 the light from f/2 is not f/4, but f/2.8, and f/4 is half that...

    does that help?

  7. tbarnette

    tbarnette Guest

    Yes that helps. Thanks Lawrence... and others. That's the type of technical stuff I was interested in. I can remember things better when I have an explanation.
  8. Ok, so what is the largest possible f?
  9. Some speciaity movie camera lenses go as low as f/.7, but the DOF becomes so shallow as to be almost impossible to film with (I believe Kubrick used these in some films). fastest lenses availble for still cameras (to my knowlegde) fall around f/1.2, though I believe I have heard of f/1 lenses- I *think* cannon made one...

    And just becuase I found it interesting the human eye has an aperture of about f/2 to f/8....

  10. Canon 50mm f/0.95 though quite rare now I believe.
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