f stops

Discussion in 'Canon EOS' started by arunarchi, Sep 9, 2005.

  1. Basics - when I change my f stop, I do it to either gain depth of
    field (f22) or blur the background (f4)- and I leave the shutter
    speed to the camera to get a decent exposure.

    So, lets say I shoot at f16 and the shutter speed turns out to be
    1/90 sec. If I were to change the f to 19 and leave the shutter
    speed at 1/90 sec. in the manual mode, would that be considered
    stopping down?

    or

    when I change to f19, would it be considered stopping down if I were
    to leave the shutter speed to the camera?

    thanks

    Arun
     
  2. 'Stopping down' is simply a phrase which indicates that you have reduced the size of the aperture, in this case from f16 to f19. Shutter speed is irrelevant to this meaning.

    Obviously if you stop down to f19 and leave the shutter speed at 1/90th sec then the photograph would be underexposed by about 1/2 a stop. You should therefore either leave the camera to decide with aperture priority mode, or change the shutter speed accordingly (in this case to 1/60th sec).

    Regards,
    Matt
     
  3. yea as the previous post stated, stopping down is reducing the aperture size (making the f/stop a bigger number), and to keep the same exposure you must compensate with speed/time by increasing it.

    -- another note for 'stop down' I don't know if it was a universal thing, it is something that I actually learned from my dad, but I call the DOF preview button the 'stop down button' because it essentially closes the aperture down to the point it will be in the photograph so that you can preview the result (this can be almost useless at smaller apertures)

    as for your question, both would be stopping down, because stopping down has nothing to do with the time of exposure (except the effect of having to increase it.. it is like a side effect).
     
  4. IMO, the terminology is slightly ambiguous here. As the previous posts have said, "stop-down" usually applies only to aperture. But there is another usage where it can be applied to the exposure where one can meter a scene and then decide to stop down or open up a stop. In that usage you would be stopping down half a stop in terms of EVs.

    -Anupam
     
  5. The multiple uses of the term "stop" in photography can be confusing at first, but I have only noticed the term "stopping down" applied to changes in aperture from a lower number to a higher number.
    When referring to changes in shutter speed, one increases or decreases exposure by "x" number of stops, but I can't say I've ever noticed anyone referring to "stopping down the shutter speed".
     
  6. 'Stopping down' usually refers to the iris becoming smaller (less exposure) although numerically the 'f' number becomes larger. This is because in basic lens design the 'f number' is the focal length divided by the diameter of the lens (the iris is regarded as the diameter) e.g. a lens of 100mm focal length with an iris of 10mm diameter has an f stop of f10. One 'stop' changes the exposure by X2 and this changes the area of the iris by X2, that is why the f stops change in what appears to be strange steps. Stopping the shutter by one stop can be confusing but to a photographer has just the same effect as stopping down the iris i.e. halving the amount of light by doubling the shutter speed.
     
  7. So, lets say I shoot at f16 and the shutter speed turns out to be 1/90 sec. If I were to change the f to 19 and leave the shutter speed at 1/90 sec. in the manual mode, would that be considered stopping down?
    Correct me if I am wrong but this should be illustrated by three scenarios here:
    1. Metered/Proper exposure: f16 @ 1/90s
    2. Stopping down the aperture 1 stop: f/22 @ 1/90s
    3. "Underexposing" by 1 stop: f/22 @ 1/90s OR f/16 @ 1/180s
    The difference would be you would "stop-down" the aperture and still be able to keep your original EV by increasing shutter speed - other wise you are underexposing by one "stop" (or "losing a stop").
    - Harman
     
  8. Hi everyone
    Thanks for ur inputs - it is clear to me now the difference between stopping down and under exposing. But, one thing that is not clear to me is:

    going back to the same example- lets assume f16 @ 1/90 gives proper exposure. For that setting stopping down would be f19 @ whatever. If u were to start at f19, and the camera gives a 1/60 for shutter speed, stopping down for that setting would be f22 at whatever! the second situation is a two stops down from f16! I'm not trying to confuse myself here, but only keen on understanding the meaning of stopping down and its significance when one talks about it.

    Hope I made myself clear - am open to any explanation.

    Arun
     
  9. "lets assume f16 @ 1/90 gives proper exposure. For that setting stopping down would be f19 @ whatever. "
    I do not think that to be the case. A one stop down from f/16 is f/22. However going from f/16 to f/19 is a fraction of one stop (it is half a stop less). You may want to look at this:
    http://en.wikipedia.org/wiki/F-stop
    and
    http://www.gbbc.org.uk/crh/fstops.htm
    and if you can take in the technical aspect, this thread could be worth looking into:
    http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00CRUg
    Hope that helps.
    - Harman
     

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