Exposure Latutude vs Zones

Discussion in 'Beyond the Basics' started by satinder_2000, Dec 31, 2005.

  1. satinder_2000

    satinder_2000 TPF Noob!

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    I am a novice to Photography (actually picked a Camera after 5 years when earlier I used to Photograph with the f-16 rule :mrgreen: ). But now I am getting confused between the Concept of Exposure Latitude of the film and tendency of the Camera meter to read 18% gray.

    E.g. if I have 'complex' subject, where the three areas of the Photograph have readings of 1/125 & f8 (normal gray), 1/125 & f16 (snow) and 1/125 & f4 (a black door). Obviously I'll shoot at 1/125 & f8. Now the film specification tells me that it has an Exposure Latitude of 2 stops. So a stop of overexposure and underexposure will not make any difference.

    If I try to underexpose by 1 stop (from f8 to f11), I should see some gray tone in Snow (36% gray) and on 1 stop overexposure (say f5.6) I should be see some gray tone on black door. The Exposure Latitude statement claims there won't be any difference. :er:

    Is there something i am missing?Why am I confused?
     
  2. Hertz van Rental

    Hertz van Rental TPF Noob!

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    You are confusing 'exposure' with 'latitude' and they are not quite the same thing.
    Exposure is to do with the amount of light the film receives.
    Latitude is to do with how much over-exposure you can get away with and still produce useable results.
    In order to really understand the difference you need to know how film works.
    If you gave a film a range of exposures and measured the density produced on the neg, plotting the results on a graph would give you something like this:
    [​IMG]
    Density up the vertical axis, exposure along the bottom.
    As you go up the plotted graph from bottom left to top right you are increasing exposure and therefore density.
    The least amount of exposure (giving the lowest density or Dmin) is at point -1.
    The maximum amount of exposure (giving the greatest density Dmax) is just above point 10.
    As you go up the slope the density increases because the developed silver grains get larger. This means that as density increases, so does grain.
    For a number of reasons, the difference between black and white on a neg is 7 stops (under normal circumstances).
    To get the maximum out of the film, that is minimum grain, you keep the exposure as far down the graph as possible.
    If you peg Black as being at point -1 (Dmin) then White will be around 6 or 7.
    That would put mid-tone Grey at point 4.
    Grain would be at a minimum.
    Under-exposing will move the White, Black and Grey points down the graph. If any point goes below Dmin then nothing is recorded and information is lost. You lose shadow detail.
    Over-exposing will move B, G and W up the graph.
    Density will increase over the whole neg, printing times will increase and so will grain. But the results will be acceptable.
    If any point goes above Dmax then the tonal range gets compressed towards the highlights and information is lost.
    The number of stops difference between this happening and the optimum exposure is the film latitude. In the example shown it is approximately 3 stops.

    Does that make it clearer?

    Any questions?
     
  3. satinder_2000

    satinder_2000 TPF Noob!

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    Thanks for that. That is really a valuable information. I agree it is important to understand the film(s). But on the basis of above description, what scares me now is a thought of a Slide Film. If the film have a latitude of 1/2 stop, does it mean I have to be very very accurate?

    I think I must read some book on that and practice some snaps. The film will be a Slide one though.

    Any comments?
     
  4. Hertz van Rental

    Hertz van Rental TPF Noob!

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    It does, indeed.
    Slide film can appear very forgiving as it will cope with quite a wide exposure range in the one image due to it being viewed by transmitted light. It will scan quite well too.
    You run into problems when you want to print it, though, the range being far too large for most positive methods to cope with.
    Shouldn't pose too much of a problem if you put it on a computer and print with an ink jet.
    The danger with slide is loss of detail in the highlights. The best method is to take a reading off the brightest thing you want to hold detail and then open up 3 stops. A white card is very useful for this.
     

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