DSLR - film speed equivalency

Discussion in 'Photography Beginners' Forum' started by James79, Jul 21, 2006.

  1. James79

    James79 TPF Noob!

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    Hi,

    I have a couple of questions concerning DSLRs (Nikon D70s in this case, though I guess it would apply to most others) and equivalent film ISO settings. Trying to get to grips with the camera in low light settings, so messing around with setting the ISO value comes into that.

    1. How would the manual ISO settings on the above camera compare with the equivalent in film e.g. would the same aperture / shutter speeds produce a result (including granularity) that is pretty much the same using both systems ?

    2. How exactly does "film speed" work on a CCD?! As far as I understood it, this was related to the grain size of the film ? I've not much experience with film systems (late to the game :( ) so apologies if I'm talking rubbish. Is this in fact just some sort of 'post processing' at work or is it a physical property of the sensor ??

    3. Are Auto-ISO settings recommended in general ? I intend to go manual as much as possible, but while I'm learning ...

    Thanks for any insight !

    Cheers,

    James
     
  2. Digital Matt

    Digital Matt alter ego: Analog Matt

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    I have never actually compared an ISO of a digital SLR to an equivalent ISO film. I know they are close enough though, because I can use my Sekonic handheld meter with my 20D, or any film, and get good exposures. The granularity however, will never be the same. Digital is much cleaner in most cases, and doesn't have "grain" as you would see in a photo. Noise is as close as you can come to seeing grain, but it doesn't have a similar look. It's generally undesireable.

    How does film speed work on a CCD? Well, simply put, a film ISO measures that film's sensitivity to light. It's no different for digital. When you increase the ISO setting for a D70, the CCD becomes more sensitive to light. I believe it does so by just amplifying the signal that each photo site on the sensor receives.

    If you are used to shooting in manual modes with film, you'll want to do the same with digital. As far as ISO, it's pretty much the same. 100-200 is medium speed, for bright daylight, and if you want the least amount of noise. Use ISO 400 for indoors or capturing motion. 800-1600 for lowlight, or sports.
     
  3. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    ISO settings compare generally pretty closely between film and digital. Grain and noise are different though.

    ISO is the light sensitivity of the film or sensor. High ISO film tends to be grainy, and right now high ISO digital tends to be noisy. With film this is because they are using larger light sensitive particles. With digital I think you're just turning up the gain? They are already talking about future DSLRs with a native ISO of 6400 (right now it's about 100), so apparently they must be figuring out how to deal with noise.

    It's hard to learn on auto anything, unless you are paying very close attention to what the camera is doing. Auto ISO or anything is fine as long as you understand what it is doing, and can spot situations where the auto-whatever will have problems.
     
  4. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    I have. Initially it seemed to me that my DSLR was a stop to a stop and a half faster than film at the same ISO. But further experimenting has proven to me that it was sort of an illusion, and that I definately can't trust the LCD to determine exposure.
     
  5. Digital Matt

    Digital Matt alter ego: Analog Matt

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    Yes, the LCDs on DSLRs show the image much brighter. I have the brightness on my 20Ds lcd turned all the way down, and images still appear to be a stop brighter.
     
  6. markc

    markc TPF Noob!

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    The same number should be the same sensitivity. ASA (American Standards Association) and ISO (International Standards Organization) are groups that set standards for use my various industries. Film and digital are different tech, but their sensitivity to light can still be measured using the same standards.
     
  7. Soocom1

    Soocom1 TPF Noob!

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    OK, first a quick review, and introductions to those who do not know.

    The film speed is primarily related to the grain size as you said. The smaller the grain, the less sensitive, thus lower ISO number. there is also other factors to consider in film, and grain when you developer the film. Various developers have different effects on the film because of the caustic nature of the developer. This will result in more material being removed from the film with a stronger mixture. (Developing times, and grainess according to developer type. (I.e. HC111 vs. Microdol-X). Microdol is less caustic than HC111 and therefore is a 'finer grain' developer.

    CCD's and CMOS's work not on the physical changes that light makes on silver halide, but rather on light intensity. The longer and more intense the light striking the sensor, the whiter the final image will become.

    ISO is used to give an equivalent of a sensor's settings to light intensity vs. film reaction, so the photographer will not have to use an entire new set of standards, and because the mathematics and physics behind the science of light is so universally used, why change to a new system?

    So the jist of everything is that ISO settings on film v. digital is pretty close. Funny thing is that with digital, many of the old variables that you got with film is now gone because of the exact and continuousness of electronic signals you get with digital.
    I.e. 2 is always and exactly 2, not 1.000009 or 2.0000001.
     

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