ISO & exposure comp. -- aren't they redundant?

Discussion in 'Digital Photography' started by Charles Packer, Mar 8, 2009.

  1. My last film camera, a Minolta SLR, beyond shutter speed
    and F-stop, had one other means of controlling how much
    light reached the film: the ASA dial, which was
    customarily set to the same "speed" as the film and left

    My first digital, a Sony Mavica, dispensed with the
    ASA setting but had exposure compensation, called "EV",
    from -2 to +2, easily accessible in two clicks of the
    main button.

    My next digital, an Olympus SP-350, had both the -2/+2
    exposure compensation and "ISO", which seemed synonymous
    with film speed. The former was immediately available
    with up/down buttons; the latter was buried way the
    hell down in the menu, which seemed similar to film

    My current digital, a Canon 20D, reverses the
    accesibility of ISO and exposure compensation. The
    former is a button-press and a wheel; the latter,
    incredibily enough, is a power switch position,
    a half-press of the shutter, a peek in the viewfinder,
    and a wheel.

    Where is the logic in all of this? In the digital world,
    aren't ISO and exposure compensation redundant, anyway?
    Charles Packer, Mar 8, 2009
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  2. Charles Packer

    Marvin Guest

    They are not redundant. If you are taking an action
    picture, as one example, you don't want the shutter to stay
    open long, so you let the camera alter only the f-stop or
    ISO setting. If you want to improve your photo skills, buy
    or borrow a book on photography basics. For a question like
    this, it doesn't matter if the book is about film or digital
    Marvin, Mar 8, 2009
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  3. Which is the right way to do it. You tell your Camera what kind of film
    is inside, so it can calculate the right exposure.

    Right, this way you can under or overexpose a picture when you think
    your camera automatics are not doing it right.
    Thats wrong. the ISO-Setting on a digital camera is really like changing
    the film and telling the camera the ISO-Setting of the new film. So a
    picture taken at ISO400 should look the same as a picture taken at
    ISO100 (except some more noise in an image with ISO400 in some cases)
    40D... the process sounds almost the same but i think it's quite convenient.
    Half-Pressing the shutter button (releasing it, if I want) and turning
    the thumb wheel on the back to change the EV-Correction. Shown on the
    display on the top and in the Viewfinder. A no hassle process.
    Nope, not at all.
    The EV-correction over the ISO setting in earlier days where a way
    around a missing feature. You had to lie to your camera about the film
    speed if you wanted it to behave like you wanted. Now you can tell your
    camera, what you want and it should do it.


    Marco Tedaldi, Mar 8, 2009
  4. Charles Packer

    Dudley Hanks Guest

    I'm not sure how true this is from a technical standpoint, but, the way I've
    always looked at ISO vs. EC is that ISO changes base sensitivity of the
    sensor while EC reduces (or increases) the amount of light admitted through
    the aperture / shutter mechanism.

    By changing the ISO setting, the change adjusts what the camera's circuitry
    does with the data collected -- changing the EC adjusts the actual amount of
    light that strikes the recording medium.

    Hence, quite different results can be obtained.

    Take Care,
    Dudley Hanks, Mar 8, 2009
  5. Charles Packer

    Doug Jewell Guest

    Yes. On film, the sensitivity or ISO/ASA is controlled by
    the emulsion on the film and to a lesser extent the
    processing. It was then up to the camera to put the correct
    amount of light onto the film. The ISO setting on the camera
    was selected to match the film so that it exposed correctly.
    The older digital cameras either didn't have ISO controls,
    or only had fully automatic ISO controls. ISO on a digital
    camera is a completely different beast to ISO on a film
    camera. The exposure compensation function is used if you
    think the inbuilt meter is getting it wrong - the inbuilt
    meter will look at the scene, assume it is average grey and
    set the shutter/aperture accordingly. If the scene isn't
    average grey (eg lots of white from snow, or lots of black
    from a black dog), or if you want to deliberately over/under
    expose for mood, then the exposure compensation is used to
    adjust it up/down accordingly.
    Yep - because ISO on a digital is not like ISO on film.
    Remember with film the ISO is fixed by the emulsion. With
    digital though the ISO is dynamic and is determined by the
    amount of amplification done to the signal from the sensor.
    So if you loaded 100ISO film and set your camera to ISO 400,
    it would under-expose because the camera would put 1/4 the
    amount of light onto the film. However with a digital
    camera, the resulting image should be basically the same
    (other than noise). At 100 ISO it lets more light onto the
    sensor and does less amplification. At 400 ISO it lets less
    light onto the sensor, but then amplifies the signal so that
    it gets the same overall brightness in the image.
    Yeah the way the 20-40D do it with that back wheel is pretty
    screwed. Try a Pentax K10 for a far better implementation.
    On film, ISO and EC settings on the camera, were sort of the
    same thing because both controlled whether you put more or
    less light onto the film than what it's ISO said you should.
    Setting the camera to ISO 100 with -2 EC would let the same
    amount of light to the film as ISO 400 would. A lot of
    film cameras didn't have an EC setting, so if you wanted to
    temporarily over/under expose you could temporarily change
    the ISO setting. Alternatively some cameras didn't have ISO
    setting, so if you wanted to expose a roll of film different
    to the standard, you could change the EC setting. Therefore
    on a film camera the 2 settings were kind of redundant - so
    long as you had 1 or the other you could do the same thing.

    On digital however, because the ISO controls the sensor
    amplification, you need both an ISO and an EC function. With
    ISO only, it will always expose at what the meter says is
    correct, so you wouldn't have the ability to force
    over/under exposure. With EC only, you wouldn't have the
    ability to alter the ISO - Lower ISO has the advantage of
    low noise, but requires longer exposures. High ISO has the
    advantage of faster exposures, but at the expense of higher

    Thus on a digital camera, both settings are required.
    Doug Jewell, Mar 8, 2009
  6. Charles Packer

    Pat Guest

    It is interesting but I read your question completely differently than
    all of the previous posters because of 1 very important word in there:

    If you owned a camera with an ASA setting, then you are "older" and it
    is likely that you think in a completely different manner than a all-
    digital person does. The all-digital person might not even comprehend
    the question.

    On film, you are exactly correct. ASA/ISO is exactly the same as
    exposure compensation and they are redundant. No one though of it
    that way "back in the day" but it is true. If you were shooting and
    you thought your auto-exposure was off, say you were shooting
    something bright, you might override the meter by lowering your ASA
    setting. That let in more light and compensated for the shortcoming
    of metering. That's how it was done. It worked because you didn't
    change the film. The film is a constant. You just outsmarted the
    meter. It was exposure compensation before exposure compensation was

    Now just to the digital age. When you change the ISO it isn't the
    same thing because it, in effect, changes the "film" to the new ISO
    number. The constant of the film is gone. So now you pick your ISO
    and use your exposure compensation to give the image more or less
    light than the meter wants.

    So the answer to your question is "yes and no". On film they are the
    same but in digital they are not.

    I hope that clarifies it for you.
    Pat, Mar 9, 2009

  7. This has been very enlightening. First, I never knew that
    film cameras had exposure compensation controls as well
    as the ASA dial. My Minolta wasn't that fancy, I guess,
    and I got into advanced cameras only as the film era
    was pretty much over.'s interesting that the electronic designers
    mapped the analogy of film speed onto sensor amplification
    to the extent that they kept the same numbering scheme
    as film speed. Does this mean that there's an industry
    standard for that? Will a setting of ISO 100 at a given
    shutter speed, F-stop, and default exposure compensation
    give similar results on any brand of camera with a
    similar number of megapixels?
    Charles Packer, Mar 9, 2009
  8. Back in analog days they would have been. You could just lie to the
    camera about the speed of the film to make it under or overexpose. In
    digital days this is not the case anymore :)

    Marco Tedaldi, Mar 9, 2009
  9. Charles Packer

    Pat Guest

    Yeah, ISO is standard and in theory all cameras/films should have
    about the same sensitivity. If they didn't, then light meters, flash
    meters, etc. wouldn't work right. If you're shooting Tri-X 400 and
    someone is standing next to you shooting a digital set to ISO 400;
    then you should have more or less the same exposure. It might be off
    a little but it shouldn't be off too much.

    There is some differences in color though. Just like Kodacrome and
    Ektracrome had very different looks to them, different manufactures
    also have different looks to them. It isn't as much as the different
    films, but there's a difference. So if you own 2 cameras and shoot
    them both at the same time/event, it's probably a good idea to have
    them the same brand (if not model).

    Good luck, old guy ;-)
    Pat, Mar 9, 2009
  10. Charles Packer

    Dave Cohen Guest

    That's a lot of learning. Perhaps one should start by learning how to
    make a meaningful post.
    Dave Cohen
    Dave Cohen, Mar 9, 2009
  11. Sure. But that is not, what the OP asked about. We have three variables.
    Sensitivity (iso)
    Exposure time

    If we change one we have to change (at least) another one too the get
    the same result. Thats nothing new.

    The thing is, that if I change the ISO-setting on a digital camera, it
    change two things in fact. I change the "sensitivity" of the sensor and
    tell this to the camera so it's taken into account. So in the end
    effect, nothing but noise-level should change.

    If I change the setting on an analog camera (without changing the film),
    the exposure is changed.

    IF I set my camera to "manual" no automatics apply at all, so you have
    to care for the exposure for yourself. With manual setting you don't
    need EV-correction.

    Marco Tedaldi, Mar 9, 2009
  12. There used to be also DIN (Deutsche Industrie Norm-german industrial
    standard);I remember in the 80's film marked ASA 100, 21 DIN. There used
    also to be Gost, the soviet standard, which of course became obsolete after
    the disbanding of the Soviet Union.
    Tzortzakakis Dimitrios, Mar 9, 2009
  13. Charles Packer

    dj_nme Guest

    That's if you have the camera set to auto expose.
    Then it will compensate for you changing the ISO sensitivity by altering
    the shutter speed or aperture or both to get the same mid-tone exposure.
    The same happens with film cameras with full automatic control of both
    aperture and shutter speed.
    Most of the early AE film cameras only have "aperture priority" AE which
    alters the shutter speed to meter for an ISO/ASA change, simply because
    the camera body had no way of altering the aperture.
    That's only if your film camera isn't set to (or doesn't have an) AE mode.
    The same thing will happen with most digital cameras set to manual exposure.
    That's because there's nothing the camera can do to change the exposure
    settings without being in one of it's AE modes.
    dj_nme, Mar 9, 2009
  14. Charles Packer

    Pat Guest

    The K1000 was fully manual and therefore did not have/need exposure
    Pat, Mar 10, 2009
  15. Charles Packer

    Bob Larter Guest

    They mostly don't - at least none of the basic 35mm film SLRs I've ever
    used have had EC, so we used to either shoot with the meter off the zero
    position, or change the ASA setting to give the same effect.
    Ditto. I learnt on very basic SLRs in the late 70's, & switched to
    digital when the EOS 10D came out.
    Well, it would've been silly of them to create yet another new
    sensitivity rating.
    Yes, thank god. There was DIN & ASA, then ASA became ISO.

    Will a setting of ISO 100 at a given
    It should do, roughly. (The number of megapixels is irrelevant, though.)
    Bob Larter, Mar 12, 2009
  16. Charles Packer

    Doug Jewell Guest

    I had an old ricoh that had EC. It was an extra dial
    attached to the ISO dial, so internally it did exactly the
    same thing to the meter as changing ISO.
    Doug Jewell, Mar 15, 2009
  17. Charles Packer

    JoelH Guest

    Where is the logic in all of this? In the digital world,
    Not at all.

    You have three ways of controlling the exposure of an image: ISO, F-
    stop, and shutter speed. The right combination of those three will
    result in an identically-exposed image. For example F/16, 1/125 and
    ISO 100 is the same as F/16 1/500 and ISO 400 (or F/8, 1/500, ISO 100)

    The two examples I just gave heppen to be the right exposure on a
    sunny day, and most good metering systems will give you these
    settings. By contrast, most metering systems will give you
    underexposed scenes of snow. So if you're taking pictures of snow,
    you will want to use exposure compensation to add more light.

    I will say that most dSLRs offer convenient access to f-stop and
    shutter speed, but not ISO. That's purely a remnant of how older SLRs

    JoelH, Mar 27, 2009
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