Calculating exposure

Discussion in 'Exposure Discussion' started by hoffy, Jul 10, 2009.

  1. hoffy

    hoffy Member

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    Howdy,

    I would like to start using my analogue gear in the studio. I have a small setup, using 2 300W (Guide Number 55) monoblock strobes with various brollies and soft boxes. Up until now, I have been using my digital equipment with it.

    My issue is that I don't have a light meter readily available. Easy, I thought, just use the digicam to work out the exposure and all is well.

    BUT, what if I want to use ISO50 film? Like most digicams, mine bottoms out at ISO100 (& even then, it appears that this is an afterthought, as apparently ISO200 is the optimum).

    So, still using the digicam, what would be the best way to work out the exposure for ISO50? Should I set the lights with the digital at ISO100 and then adjust exposure on the analogue to suit? How would I then calculate the exposure for the analogue?

    Yes, I know, a light meter is going to be the best solution, but at the moment I don't have one.

    Any thoughts appreciated.

    Cheers
     
  2. Mick Fagan

    Mick Fagan Subscriber

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    Work out the f/stop at the optimum, which is 200.

    For example, your system tells you that at 200 you require f/11 for correct exposure.

    You are using film that is two stops slower, so you open the lens two stops to f/5.6.

    It really is that simple, but ensure that the distances between your subject and your flash units remain constant. If you move them, take another reading.

    Ideally you should run a test strip of film before doing anything important.

    Mick.
     
  3. hoffy

    hoffy Member

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    Howdy Mick,

    So, you are suggesting ISO200 (not 1/200?!). OK, its that simple. I just didn't want to make sure I am missing anything.
     
  4. bdial

    bdial Subscriber

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    ISO 50 film needs double the light of ISO 100. That means you need one stop more exposure than the digi would indicate.
    "light meter" is a good use for a digi, no worries there.
     
  5. Mick Fagan

    Mick Fagan Subscriber

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    Yes, ISO 200 not 1/200th of a second.

    Mick.
     
  6. alanrockwood

    alanrockwood Member

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    What about setting the digital camera at 100 iso and shooting it through a 2X neutral density filter?
     
  7. Nikanon

    Nikanon Member

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    Setting the camera at 100 and shooting through a 2x gives equivalent resulting exposure of that of iso 50, this is because the factor 2x is plugged into the equation of 2 to the power of C equals F, where F equals factor and C equals change in stops, so 2 to the power of 1 equals 2, therefore you change your ISO by one stop at 100, to equal 50, or shutter speed, or aperture. Although when metering with a digital camera, do not rely on the exposure given, only rely on what the meter says, if you make an exposure at an average (matrix metering) metering and see that it looks good and then use that on your film you will not obtain the same densities, the characteristic curve of films vary from film to film and change based on many factors whereas digital is always a constant. Although light meters do not always agree it is a safe bet relying on your digital meter will you get roughly accurate exposures, of course that is based on how it is used!
     
  8. rst

    rst Member

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    For "studio" work at home with my little flashes I also use a digital SLR as a light meter. That works out pretty good and as has been already said, I decide on the aperture for the analog and then stop down the digital by as many stops needed to match my film speed with the speed of the the DSLR. The reading I take is only based on the histogram, knowing that my film is able to capture more tonality than my "light meter". It is nice to check for the shadows and lights on the display if you have to work without a modeling light.

    Cheers
    Ruediger
     
  9. alanrockwood

    alanrockwood Member

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    I'm speculating a bit here, but I'll bet it would be possible to use a digital camera to help balance light filtration as well... probably better than a light temperature meter and free of charge for anyone who already has a digital camera.