ISO setting on a dslr?

Discussion in 'Digital SLR' started by shipping, Dec 27, 2005.

  1. shipping

    shipping Guest

    Why is there an ISO setting for a dslr? I always set my ISO setting for tha
    appropriate film I used, but without film, why bother with ISO?

    Thanks in advance.
    shipping, Dec 27, 2005
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    Jack Dale Guest

    Increasing the ISO will permit you to use a faster shutter speed with
    the same aperture or to use a smaller aperture with the same shutter
    speed. This is useful in situations where flash is not permitted, for
    example, churches or museums.

    Increasing the ISO may increase the noise in the photograph.

    Jack Dale, Dec 27, 2005
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    Prometheus Guest

    It defines the sensitivity. More importantly: so that you can have the
    shutter (motion freezing or blurring) and depth of field you require
    (differential focus) without blowing highlights or loosing too much in
    the shadows; unlike film where you have to decide before you load-up and
    go out, with dSLR you can vary it from frame-to-frame. I do wish my dSLR
    would let me select and adjust ISO setting in the same way I change
    shutter and aperture.
    Prometheus, Dec 27, 2005
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    shipping Guest

    Thank you Jack. Is there a standard ISO setting for a dslr?
    shipping, Dec 27, 2005
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    Jack Dale Guest

    Not really. I keep my ISO as low as possible.

    Jack Dale, Dec 27, 2005
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    C J Southern Guest

    In theory, 100 gives less noise than 200, but in practice it's very hard to
    tell the difference. On Canon 350D, 20D etc 400 is also VERY good - 800
    starts to show a bit of "grain" (but it really depends on how big the photo
    is printed/displayed) - and 1600 / 3200 (20D) in my opinion look pretty
    rough, but you can have a good degree of success cleaning up in photoshop
    using noise filters.
    C J Southern, Dec 27, 2005
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    JPS Guest

    In message <gXisf.10850$>,
    It's not just the ISO per se that determines noise; it also depends
    highly on the relative exposure level at that ISO. An ISO 1600 shot of
    a white object with grey lettering on it, exposed well, will have less
    noise than an ISO 200 shot of the same scene that is average-metered on
    a DSLR with relatively clean amplification.

    Looking at it another way, the absolute exposure determines the original
    signal-to-noise ratio in the sensor, and the higher the ISO, the more
    numbers are used to represent luminance in any given shadow range, and
    the cleaner those shadows will be. Low ISOs are never a good
    alternative when they result in under-exposure.

    Some people have accused me of looking at things backwards, but really,
    this is how things are really happening in the camera. An
    ISO-independent exposure is taken in the sensor, based soley on subject
    lighting, f-stop, and shutter speed. At this point, ISO only has one,
    single and indirect effect - it affects metering. ISO does not play a
    role again until the data is read out and digitized; at ISO 1600, about
    1/16th the range of sensor voltages becomes the RAW numbers up to 4095
    (through higher amplification); at ISO 100, roughly all of them. So, if
    your sensor exposure only uses half the range, shooting at ISO 100 would
    be image-destructive, as compared to shooting at ISO 100. If only
    1/16th the range is used, ISO 1600 will give the highest quality
    results. Once you understand this, you can make more optimal
    exposure/ISO decisions.
    JPS, Dec 28, 2005
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    Sionnach Guest

    For the same reason that you chose the ISO of your film.

    As anyone using an film camera ought to know, higher ISO numbers mean
    greater sensitivity to light, but also usually mean some loss of sharpness.
    So you choose a particular ISO of film to best suit the lighting conditions
    under which you'll be shooting.
    The ISO setting on a film camera is simply to tell the camera the ISO
    rating of the film you're using, which allows the camera to adjust automatic
    settings to the film. And quite a few SLRs don't actually have an ISO
    setting; they automatically read the film cartridge, and adjust themselves.

    The ISO setting on a DSLR doesn't serve the same purpose; instead, it's
    the digital equivalent of choosing the ISO of your film.
    Same as with film, setting a higher ISO gives higher sensitivity to light,
    but you'll lose some sharpness.
    IOW, in circumstances where I would load my film camera with 200-speed
    film, I'll set the DSLR's ISO to 200. If I find that doesn't give me a fast
    enough shutter speed (I'm usually shooting high-speed dog sports, and flash
    is prohibited), I'll up the ISO, knowing my pictures will be a bit grainer
    but I won't get motion blur.

    The beauty of the DSLR, in that respect, is that you can change ISO from
    frame to frame as shooting conditions warrant, whereas with a film camera
    you have to change the entire roll of film.
    Sionnach, Dec 28, 2005
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    JPS Guest

    In message <>,
    Whoops! That last 100 should have been 200.
    JPS, Dec 28, 2005
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    C J Southern Guest

    I (think I) understand what you're saying, but ...

    .... let's see if I've got this right ...

    "If I take 6 shots - one at each available ISO on my 20D - and each are
    correctly exposed so that the full tonal range is used - then the lower ISO
    will always have the lower noise".

    However ...

    "If I'm struggling to get a shot in poor light - in terms of noise - I'm
    always better off to use a a higher ISO than to have an image more than a
    stop under exposed at 1/2 that ISO"?


    C J Southern, Dec 28, 2005
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    Jeremy Nixon Guest


    To put it another way, underexposing at low ISO in an attempt to avoid "high
    ISO noise" is not a good idea, and will result in more noise than using a
    higher ISO.
    Jeremy Nixon, Dec 28, 2005
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    JPS Guest

    In message <HZnsf.10884$>,
    Exactly. A lot of very poor exposures have resulted because people
    don't understand this, and try to stick to low ISOs unless it results in
    obviously dark images.
    JPS, Dec 28, 2005
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    JPS Guest

    In message <HZnsf.10884$>,
    One common question I see at DPReview is "why is the sky so noisy in my
    ISO 100 picture?". The sky is noisy because it dominated the camera's
    metering, and wound up at a dark midtone range of exposure at ISO 200,
    and the red channel. being much weaker than the blue or green, is even
    more poorly exposed, and is posterized. If the sky is the brightest
    thing, and composes most of the frame, then +2 or more EC was called
    for, and while the best alternative is to do +2 EC at ISO 100, if it is
    not possible, +2 at ISO 400 is still better than 0EC at ISO 100.

    Another way to look at it is that high-key exposures don't vary much in
    quality between ISOs; not the way the midtones and brighter shadows do.
    Can you tell if the top or bottom is ISO 1600, vs 100, without reading
    the caption?:
    JPS, Dec 28, 2005
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    Paul Furman Guest

    If there aren't any bright whites to be blown out, you should overexpose
    to get the image as light as possible, even if that means pushing the
    ISO. Then tone it down in post processing.
    Paul Furman, Dec 28, 2005
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    C J Southern Guest

    How much "headroom" is there between what Canon call "a standard exposure"
    and a blown highlight?

    I always use the lowest ISO that will give me the minimum acceptable shutter
    speed - although it's "a given" that the metering/exposure will be set for a
    standard exposure.
    C J Southern, Dec 29, 2005
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    JPS Guest

    In message <jJIsf.11026$>,
    If you use the camera's metering on a grey card, then a 90% white will
    be about 1 stop below clipping in the green RAW channel, 1.5 stops below
    clipping in the blue, and 1.9 stops below clipping in the red, with the
    20D, in daylight with daylight white balance. Some of the cameras will
    have a little less headroom in the blue channel, like the 10D/300D,
    which will have only about 1.1 stops.

    With high-key (mostly white), or low-contrast scenes (or any scene where
    the brightest areas dominate the frame), using the matrix metering, it
    is safe to go to +2 EC or sometimes even higher (in manual mode).
    The cameras still meter much like they were doing so for film; film
    records best in the middle zones, generally, and slide film is meant to
    be projected without processing, so camera paradigms and common wisdom
    often forget that digital is best just short of clipping, generally
    speaking, in both the sensor, and in the RAW data.

    Low ISOs are great - if you can do them without under-exposing,
    blurring, or losing too much DOF or sacrificing lens quality to max
    apertures. The photon budget is often restricted, however. I'd shoot
    everything at ISO 200 on my 20D if I could, but I can't. You have to
    learn by experience what the trade-offs are between aperture, shutter
    speed, and ISO, but one thing is clear; you get the highest quality
    recording at any given combination of shutter speed and aperture when
    you use the highest ISO that doesn't clip desired highlights. You can
    always do better than an ISO 100 with a histogram that is empty at the
    right, at a higher ISO.

    Of course, the histograms on our cameras are for JPEGs, and not for the
    RAW data, so they aren't always clear. I keep the contrast set to -2 on
    my 20D, and then any white highlight (with the correct WB setting) will
    clip the histogram and image just before it clips the green channel of
    the RAW data. For colored highlights, it gets very tricky.

    JPS, Dec 29, 2005
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    C J Southern Guest

    Thanks John - you make my head hurt, but I think it's all for a good cause!


    C J Southern, Dec 29, 2005
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    JPS Guest

    In message <AzZsf.11235$>,
    No pain; no gain.

    "Simple & easy to swallow" is usually wrong.
    JPS, Dec 30, 2005
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    C J Southern Guest

    I know, but with me it's usually a case of "getting to grips with it -
    having lunch - then having to start all over again"!

    Seriously, it does help - at least now I'm still slow, but I'm starting to
    make better choices in the field, and definately getting a higher % of
    keepers because of it. As far as ISO selections go - for the most part, it's
    not difficult in that in most cases it's dictated by needing a certain
    aperture for DOF control, and a minimum shutter speed to avoid shake (even
    with an IS lens).

    I do find the grain of higher ISOs disappointing, but recently discovered
    that I can run noise reduction filters over it then sharpen the shit out of
    it, and still end up with something that looks quite reasonable up to about
    8 x 12".

    I still need to learn more about exposure adjustments (I mostly use partial
    metering, with the subject in centre frame).


    C J Southern, Dec 30, 2005
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    JPS Guest

    In message <ck8tf.11356$>,
    To each his own - I generally don't do any noise reduction, because even
    the best software removes detail as well. Noise is most visible in
    large prints, and 100% computer views (or downsizes with the brutal
    "nearest neighbor" algorithm, which actually increase visible noise).
    When I use auto-exposure modes, I set EC based on contrast (I use
    evaluative metering). Small white objects might make me drop the EC
    down to -1/3, like egrets or gulls that are small in a scene (or the
    sunlit parts of them are small), or sparse white clothing. Same for
    dappled light, sun and mostly shade, about +1/3 to +1 for medium
    contrast scenes, and up to +2 or even higher (in manual) for
    low-contrast scenes. If I elect to shoot at ISO 800 and +2 EC in heavy
    fog, I will wind up with a much higher quality recording which can have
    its contrast boosted more in post processing than if I shot at ISO 200
    with 0 EC.

    Of course, I also shoot a lot in conditions where + EC is impossible;
    ISO 1600 grossly under-exposed. Colors are not perfect; lots of noise;
    but these shots are only possible in this kind of light. Almost three
    stops under-exposed at ISO 1600 (and already with a compromised shutter

    JPS, Dec 30, 2005
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