Iso/

Discussion in 'Digital Photography' started by DELINDA, Jan 17, 2009.

  1. DELINDA macrumors regular

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    Jun 13, 2008
    #1
    I am interested to learn how ISO /shutter speed & aperture relate to each other . I use a point and shoot (g10) and would like to know how and why you use these settings . Thanks
     
  2. Cliff3 macrumors 65816

    Cliff3

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    #2
    This wiki article has a thorough discussion on the subject of ISO: http://en.wikipedia.org/wiki/Film_speed This article on exposure would be worth reading too: http://en.wikipedia.org/wiki/Exposure_(photography)

    In a nutshell, ISO is analogous to film speed, and represents the sensing medium's sensitivity to light. The higher the number, the greater the sensitivity. The trade-off is that the greater the sensitivity, the greater the noise (or graininess, if the sensing medium is film).

    Understanding Exposure is frequently recommended here and everywhere and would be well worth your time to read.
     
  3. termina3 macrumors 65816

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    #3
    They're intricately related.

    A lower aperture number ("wider") means more light to the sensor. This is at the cost of a slew of things that I don't want to go into. If you can drop the aperture in low light, drop it.

    A slower shutter speed means more light to the sensor. This is at the cost of potential subject or camera movement (and therefore blur in the photo).

    A higher ISO value means less light is necessary for the sensor to capture an image. This is at the cost of noise.
     
  4. swiftaw macrumors 603

    swiftaw

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    #4
    Definitely pick up Understanding Exposure by Bryan Peterson
     
  5. leighonigar macrumors 6502a

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    May 5, 2007
    #5
    Wah! It's late and I've just typed a mini-ramble - it goes off topic a bit at the end but hopefully this will help a bit:-

    To get the right exposure for a given speed of sensor (ISO) you need a given shutter speed.

    If the shutter speed is halved (i.e. twice as fast, 250th instead of 125th) you'll need to use an ISO value twice as high to get an exposure that looks about the same (e.g. ISO 400 instead of ISO 200).

    Likewise, if the ISO value is decreased then you would need a faster shutter speed, in the same ratio, to get the same exposure.

    Aperture is the same kind of idea. The aperture gives you an idea of the amount of light which is getting through the lens, to the medium. The ratio of the lens' focal length to the diameter of its aperture, or f number, is used as a measure of aperture. In the same way as in the above examples, if the shutter duration or sensitivity (ISO) are halved, then the area of the aperture must be doubled, e.g. from f/4 to f/2.8 to achieve the same exposure.

    A doubling in the amount of light which reaches the sensor, or in the sensitivity of the sensor to light is one stop. f/4 to f/2.8 is an increase in aperture of one stop (the hole has twice the cross-sectional area - twice as many photons get through), ISO 200 to ISO 400 is a doubling in sensitivity - one stop (half as many photons are required to give the 'same' image), and the difference between a shutter speed of, say, a 125th and a 60th is also one stop (twice the duration of opening - twice the amount of light which reaches the film).

    Getting the right exposure is therefore an exercise in ensuring that the aperture, ISO and shutter speed match the scene and the result you require. Generally speaking I would set the ISO first (as I am a luddite and think like I'm still using film), to the lowest value I think will be acceptable - say, ISO 200 for a normal walkabout, then I normally work in aperture priority. The reason for this is that I am usually more interested in the range of objects, from near to far that appear 'in focus'. Smaller apertures (BIGGER f/ numbers) give more apparent focus, and therefore scene sharpness. If I want to have a face against a blurred background, a big aperture (small f/ number) would be uses to give a shallow depth of field. Conversely, if I was taking a still-life or macro and wanted everything to be as in focus as possible I would use a smaller aperture (BIGGER f/number).

    If I was taking photos of moving objects I might use shutter priority I suppose, but in practice I never ever do as generally the range of apertures available and good is lower than the range of fast shutter speeds. I'd rather pick an aperture to get a desired shutter speed and know the effect.
     
  6. ThunderRobot macrumors regular

    ThunderRobot

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    #6
  7. JNB macrumors 604

    JNB

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    #7
    Very concise explanation.

    Only mistake I found (and I'm really just nit-picking here). You're correct that it's twice as much light gathering, but the aperture opening isn't twice as wide, in the case of f/4 to 2.8, it would be 1.4 times as large.

    Twice as wide would be 4 times the light gathering (pi x R squared) ;)
     
  8. leighonigar macrumors 6502a

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    May 5, 2007
    #8
    That is an error on my part, I meant the area! Apologies!

    I've fixed that incidence. It will have been puzzling as I alternated between area and width doubling in there somewhere. Thanks for the observation.
     
  9. JNB macrumors 604

    JNB

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    #9
    Well, I typo'd and had to correct my own. Need a second cuppa coffee. :p
     
  10. gamera~ macrumors member

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    Apr 28, 2005
    Location:
    Boston MA
    #10
    The latest episode of "Tacksharp" (a photography podcast) might be helpful. Link http://tacksharp.tv/2009/01/episode-6-camera-modes.html. It's about camera modes (e.g., Portrait, Landscape, Aperture Priority, Manual, etc...) If you want to change all or some of the 3 basic variables of the camera — aperture, shutterspeed, ISO — changing modes is how you go about doing that is on modern digitals.
     

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