If I have 400 speed film in my camera but I set my ISO to 200 what am I doing to my film and by how much?

Discussion in 'Film and Processing' started by camilla_toft, Nov 17, 2009.

  1. I'm in a beginning photo course and was asked this question and didnt know how to answer it, if you could help me out I'd appreciate it!
     
  2. You are overexposing by one stop. If you are shooting color print (AKA negative) film, this isn't a big deal. If B&W film, you can "pull process" it - meaning you give it less developing time. See this site for a lot of combinations of film, developer and ISO setting.
     
  3. I'll give you some clues so you have a chance to figure it out yourself:
    If you kept the film speed in the camera the same, but adjusted the aperture, by one notch either way, what would you be doing?
    How is adjusting the film speed similar to that, and how is it different?
    Write back and we'll coach you closer to the answer. As soon as you begin to solve this for yourself, and realize what it means, then you'll start to have more control over what you're doing. There are subtle changes that can be made to the image by understanding this type of question and its answer.
     
  4. Hey, that's cheating ; )
    Check out the learning section of this site for lots of great information.
     
  5. thanks for the help guys :)
     
  6. Doubling or halving either shutter speed or ISO, equals 1 stop plus/minus. So ISO 200 film is 1 stop slower than ISO 400 film, and 800 ISO is 1 stop faster than 400 film. And 1/250th of a second shutter speed, is twice the light (1 stop more than) 1/500th of a second, and half as much light as ISO 125.
     
  7. Ooh, sorry for ruining the surprise. Anyway I'd recommend reading about exposure - for example:
    http://www.photo.net/learn/making-photographs/exposure
    http://www.photo.net/learn/basic-photo-tips/aperture-shutterspeed-iso/
    http://www.photo.net/learn/basic-photo-tips/correct-exposure/
    Actually, there's a huge amount of useful material in the whole http://www.photo.net/learn/ section.
    If you understand the relationship between ISO, shutter, aperture and the level of light available you'll quickly see what changing the ISO does to the exposure.
     
  8. James G. Dainis

    James G. Dainis Moderator

    As has been said you are overexposing the film by one stop. No big deal, many people routinely set the ISO to one stop faster than the box speed. I always set Tri-X 320 ISO sheet film to 160 ISO to get a denser negative to work with.

    If you are in aperture priority and have the aperture set at f/11, with 400 ISO the camera meter may set the shutter speed to 1/500 sec. If you change the ISO to 200, the meter will select 1/250 shutter speed for that same f/11 aperture. The 400 ISO film will be one stop overexposed..
     
  9. Short answer:
    Pulling by one stop.
     
  10. James G. Dainis

    James G. Dainis Moderator

    Pulling or pushing is done when you develop the film not when you expose it.
     
  11. James is correct but he used more words in his answer then I did.
     
  12. "James is correct but he used more words in his answer then I did."
    If you reduce the number of words in his answer by half, you will pull this thread by one stop.
    If you double the number of words in his answer, then double it again, you will push this thread by two stops. And so forth.....but be careful of reciprocity failure, it might get you.
     
  13. Ahahah Ernest, your answer made my day, thank you!
    ...but be careful of reciprocity failure, it might get you.
    OMG, I'm still holding my belly from laughing!!!
     
  14. You are simply over-exposing it by 1 stop. I always shoot TX @ ASA 200 and hold back on development time. It produces tighter grain, greater contrast and better sharpness I have found. If it is color negative film you will probably still be ok. The highlights are going to be blocked somewhat but it will still be very printable, unless you overexpose it further when you take the photo. If you are shooting transparency film, you are in trouble. Slide film has very narrow latitude compared to negative film. Back when all color I shot was Kodachromes, you had to be on the money +/- a half stop or printing them could be a bear.
     

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