Digital ISO Question

Discussion in 'Digital Photography' started by Designer Dale, Jun 28, 2009.

  1. Designer Dale macrumors 68040

    Designer Dale

    Mar 25, 2009
    Folding space

    I am an old film photographer and I understand the chemical goings-on in silver salts that give batches of film different ISO ratings. I am a bit perplexed as to how this works with digital. I have researched the topic but have not found anything that satisfies my curiosity.

    A memory card is a data recorder. It only records fast or slow and has not much to do with the ISO rating. Right/Wrong?

    The camera sensor is the source of ISO in the digital world and physically larger sensors can capture more data. This I understand, but how does it vary something like sensitivity? Is the sensor on a dSLR manufactured with a set maximum ISO and adjusted down to other speeds within a set of parameters in-camera via menus?

    Thanks in advance. I enjoy viewing the wonderful pics here every day.

  2. iBookG4user macrumors 604


    Jun 27, 2006
    Seattle, WA
    The memory card has nothing to do with ISO, you are correct. The sensor is the source of ISO. The sensor captures the image by being sensitive to light, it needs a certain amount of light to write the picture. The ISO is how sensitive the sensor is set to be to the light and thus the higher the ISO the less light is needed to write the picture.

    Noise is basically the result of the sensor being sensitive to heat. The higher the ISO rating, the more noise is introduced into the picture because of this. The maker of the camera set the maximum ISO by how much noise they believe is acceptable in the picture.
  3. toxic macrumors 68000

    Nov 9, 2008
    i don't know the gritty details of how a sensor makes itself more sensitive, but its more or less simply amplifying the signal.

    a sensor is not manufactured at max ISO and adjusted down. rather, it's manufactured with set sensitivities corresponding to the ISO scale. any ISOs not corresponding to those sensitivites that are still available in-camera are produced by software.

    as an example, all Canon cameras, other than the 1-series, have native sensitives of 100, 200, 400, 800, and 1600 (some also have 3200 and 6400). however, in-camera there are intermediate ISOs (e.g. 125, 250) and expanded ISOs (L, H). all of these are software-produced - H (or H1) is the camera's highest native ISO pushed one stop. ISO 125 is ISO 100 pushed one-third of a stop. all non-expanded ISOs in 1-series cameras are native.
  4. taracat macrumors newbie

    Jan 17, 2008
    sydney, australia
    Yeah this is exactly what iv'e been wondering too..

    I work in Audio and the higher the signal to noise ratio the better - so if the lighting/lens can handle it ISO 100 would presumably always be the best setting I gather ? ISO seems to simply amplify the sensor, making the highest quality to shoot with the lowest setting.

    Please correct me if i'm off base here :p
  5. Abstract macrumors Penryn


    Dec 27, 2002
    Location Location Location
    The signal from the sensor is amplified. ;)

    You get more noise, but you'll also get more signal. However, the reason the signal-to-noise would probably get worse when increasing ISO on camera is mostly because the electronics through which the sensor signal must go through also adds a bit of noise. Longer signal pathways should result in more noise as well.

    There's also a difference with regards to how this is handled by a CCD sensor, and a CMOS sensor. Technically, or if we were more advanced, you'd probably get cleaner images from a CCD sensor, but.....well....that's not reality at the moment.
  6. Designer Dale thread starter macrumors 68040

    Designer Dale

    Mar 25, 2009
    Folding space
    Thanks. That is just the answer I was looking for. It would be very difficult to vary the sensitivity of the sensor itself. It's just a receiver. Amplifying the signal sent from the sensor I understand. Rather like pushing film in the darkroom.

  7. ChrisA macrumors G4

    Jan 5, 2006
    Redondo Beach, California
    The sensor, by the "photo eletric effect" converts light photons to electrons. The more light that falls on a pixel the more electrons are generated., the charge of a pixel raises with exposure to light. Theses charges are shift out of the sensor and amplified and then the voltage from the amplifier to converted to digital.

    The ISO adjustment controls the gain of the amplifier.

    Noise is from two sources. (1) thermal electrons. These are random electrons generated by heat rather then photons (or rather more like self illumination of the sensor) this is most seen in long exposures. and (2) "shot noise" this is the "normal" noise we see and is do entirely because photons are random events. Light itself is not continuous so there is a random fluctuation in the number of photons that will hit a given pixel. the noise we see is because when the light is low (and we need high ISO) we are dealing with statistically small numbers of photons. It the "statistics of small number" problem. With more light we get many, many electrons in each pixel so the pixel to pixel variation is small.

    This explains why larger sensors have less noise, more total electrons per pixel.

    The thermal noise is mostly a non-issue if the exposures is in the normal range of hand held photography

    This is not correct. It says that noise is an engineering problem. That we just can't build good enough low noise amplifiers. Well OK there is a third source of noise. It's called "read out noise" and this is added by the D/A converter and amp. But it's very minor.

    The bigist nose of noise that we see in normal pictures is cause by the physics of light. If the subject is so dim that only 12 photons of light stick a pixel on a 1/100th second exposure then we can expect the some pixels will get 13 and others 11, just be chance. Even if the subject is truely evenly lit.

    Technology can address thermal noise (with cryogenic cooling) and readout noise can be addressed with more sophicticated sampling techniques and more bit depth. but shot noise is here to stay. Only large diameter optics and large area sensors can help.

    This explains it better
  8. marioman38 macrumors 6502a


    Aug 8, 2006
    Elk Grove, CA
    So would it better to not use the "expanded" ISO in camera and just use the max sensor ISO, and push in post? I shoot in RAW.
  9. TK2K macrumors 6502


    Jun 4, 2006
    the maximum highest ISO setting will give crazy noise, but minimum
  10. toxic macrumors 68000

    Nov 9, 2008
    if you need to see the histogram or how the photo turns out, it's useful. otherwise, controlling how much it's pushed is better left to you.

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