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Is it safe to view ISO as global brightness?

This is how I look at ISO. Is this right?


  • TrevTrev Moderator
    edited July 2016
    Well I suppose you could look at it like that in so much that opening your Aperture lets in more light, and slowing the shutter speed will do the same.

    The more light that strikes the sensor, the more exposure to that image is given.

    ISO is the modern terminology like the old 'film speed', of ASA was, the faster the film speed, the better the light hitting the emulsion on that film.

    Negative Film used to be called 'ASA' (American Standards Association) but unlike ISO (International Standards Organisation) you had to 'buy/purchase' different rolls of film in different ASA , eg: ASA - 25/50/100/200/400/800/1600/3200 were the specific ones to buy and once a roll was put into the camera, you had a dial you choose the ASA, like choosing ISO these days, but unlike ISO in digital cameras, you were stuck with that unless you knew what you were doing and developed your own negatives/film and either 'pushed or pulled' the timing through the enlarger to allow the correct amount of light in for good exposure onto the photographic paper.

    Exactly like today with ISO, with one huge benefit, you can 'dial' in any given ISO on the same 'roll of film = CF/SD Card' but even though you entered a 'correct ISO' value, your Aperture/Shutter must correlate correctly for good exposure and like processing negs through an enlarger, you can change the exposure in post with Photoshop, Capture, Lightroom, etc.

    So on that note, it's not so much a global brightness and is part of the 3 settings you need to obtain correct exposure.

    Of course, just like Film ASA, the higher the film speed, the grainier the image become, and with ISO, the higher the ISO, the noisier (like grain) the image will become because of the different sensors out there.

    And just like Film ASA, Digital ISO also is improved on quality with large sensors to capture more light, as opposed the the larger negative formats (like Medium Format) in film giving much much more information. The larger the format/sensor, the less compression is required to capture the image, and so a large format camera using 10x8 inch plates, well you can imagine the quality those produced even waaay back in the 1800's. I've seen images produced from those plates and the details are astonishing.

    ISO is basically the last factor you determine to get correct exposure, because generally you would select a shutter speed you want or aperture, but then adjust ISO accordingly for correct exposure. Of course if you are working in dark conditions you may know immediately you will need say 1600+ ISO and have a test, but generally the ISO is the last number in the equation.

    And just to mix it up a bit, film speed used to be called 'din' which was a value of speed  in 'degrees'.

    ASA 100 film is equivalent to 21 degrees, and you used to see ASA 100/21degrees (with the o degree symbol).

    That expression of film speed was way back from the 1850's when a German astronomer calculated the 'film speed' for astronomical photography on the plates from memory, or it was Dutch, forget, but something like that, and so many degrees equated to how much sensitivity to get an image on that plate. (you can look it up I am sure).

    There ends my small amount of knowledge on that which I picked up a long time back.

    Phew! Sorry my little fingers got carried away there keano. :)

  • rs_eosrs_eos Member
    For a strictly ambient-light situation, ISO is indeed global as it's one of the three variables in the exposure triangle (along with shutter speed and aperture).

    For a setup with both ambient and flash, ISO is still global, and along with aperture and shutter, still drive the ambient.  For the parts of the image being illuminated by flash, ISO and aperture are taken into account, but shutter doesn't affect exposure*.

    * Well, it's a bit more complicated as shutter will determine if you end up using high-speed-sync and depending upon the situation, you may no longer have enough flash output, so one could argue it does kinda affect exposure in an indirect way.
  • Neil vNNeil vN Administrator
    Yes, you could view ISO as global brightness .. especially if you have to stay with a certain aperture range for Depth-of-Field, and a certain shutter speed for movement. (Whether to have subject movement, or whether to have sharp images.)  So that forces you to get to correct exposure via ISO.
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