Why Expose Differently in Sunlight an Overcast.

Discussion in '35mm Cameras' started by Denny B, Jun 8, 2005.

  1. Denny B

    Denny B Guest

    I read in this forum that one exposes slightly
    differently on a sunny day as opposed to an overcast day.
    It appears to me that on an overcast day you give
    about 1/2 f stop more exposure.
    If my observation is correct why is this so ?

    If you are using a 1degree spotmeter to measure
    the subject do you have to apply this extra exposure
    on overcast days ?

    Thanks in advance
    Denny B
     
    Denny B, Jun 8, 2005
    #1
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  2. Denny B

    Matt Clara Guest

    No, the one degree spotmeter, or any meter, if used correctly, will give you
    the proper exposure. However, there's a rule for exposing without a meter
    called the sunny f16 rule, which indicates that in bright sunlight with100
    ISO film, a shutter speed of 1/100 (1/125, really) and aperture of f16 will
    give you a correct exposure for objects lit by the sun. Overcast days get a
    slightly different treatment, and different film speeds will indicate
    different exposures as well. Here, read all about it:

    http://www.apogeephoto.com/oct2000/sunny_f16_rule.shtml
    http://www.bradmitchellphoto.com/tips/Sunny16.htm
    http://www.kbcamera.com/exposureguide.htm
     
    Matt Clara, Jun 8, 2005
    #2
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  3. Denny B

    Peter Irwin Guest

    I think this was with respect to colour negative film,
    and especially the lowish contrast 160 films being discussed.
    Negative films generally have rather a lot of overexposure
    latitude. There are some fairly subtle changes that you get
    when you increase exposure.

    If you compare a C41 film shot at "correct" exposure to
    one with an extra stop of exposure, the one with the
    extra exposure will show:

    - slightly finer (or less obvious) grain. This can be quite
    obvious with the faster films, though you are generally better
    off with a slower film than overexposing fast film. Traditional
    B&W films show slightly worse grain when exposure is increased,
    though a slightly generous exposure combined with reduced development
    is a good recipe for fine grain.

    - slightly higher contrast. The "toe" of most films has low
    contrast; increasing exposure puts more of the scene on the
    straight-line portion of the film curve. This is the effect
    which I think Peter (Bandicoot) was thinking of when he
    suggested a generous exposure for Portra 160NC on an overcast
    day. On a sunny day, the lower contrast of the film shot at
    rated speed could be very useful. I shoot more B&W negative
    than colour negative and I find this somewhat counter-intuitive
    because with B&W I would give a generous exposure on a sunny
    day (to get detail in the shadows) and then make the scene fit
    onto the paper by some combination of printing at low contrast
    and dodging or burning.

    - slightly reduced sharpness and resolution. This will be so subtle
    with a one stop increase that it won't be noticed unless you are
    shooting test charts. Increased exposure will give an increase
    in the effect of light scatter in the emulsion. If you try a four
    or five stop increase in exposure this will get more obvious.
    The aim in this case is to move the darker parts of the
    scene off the toe and into the straight-line portion
    of the film curve.

    Peter.
     
    Peter Irwin, Jun 8, 2005
    #3
  4. Overcast days are really really difficult to expose for, in my experience.
    In fact, I don't think I've ever got it right. Here are what I _think_ are
    the problems with ..

    Overcast days:
    1. Bright back-lit sky, everything else in shadow, high contrast of scene.
    2. Diffuse lighting increases shadow brightness, reducing contrast of
    subject.

    On a bright day (with blue sky):
    1. Sky is medium-light tone (apart from sun, of course), scene is well-lit -
    lower contrast tbetween sky and subject than in overcast conditions (someone
    may want to correct me here)
    2. Unidirectional lighting increases contrast of subject (which can be too
    much).


    So what effect does this have on the photo?

    Overcast days:
    1. Either a blown-out sky which is totally white, or underexposed subject,
    which is grainy and ill-defined and lacking contrast on film. Or both.

    Bright days:
    2. Often sky and subject has similar lightness values. Sometimes problems
    with large contrast between shadows and areas in light, with blocked-out
    shadows.

    And the solution?

    Overcast days:
    1. Wait for better light!!!
    2. Leave the sky out of compositions
    3. Meter off the subject (spot/centre) and recompose. This sacrifices the
    sky, blowing it out often.
    4. Use ND grads to reduce the contrast difference and stop the sky blowing
    out.
    5. Select subjects which have a high reflective contrast (e.g. a skunk/a
    zebra - especially useful for this weeks SI), rather than textures or 3d
    subjects which require unidirectional lighting for there to be any contrast.

    Bright days:
    1. Often no problem
    2. If contrast too high, use fill flash
    3. Be very precise with the exposure.
    4. Sacrifice shadows.
    I think a spot meter should give the correct reading as long as you select
    the right tone and exposure compensation (ir required). There's nothing you
    can do about the low subject contrast, though.

    Duncan.
     
    Duncan J Murray, Jun 8, 2005
    #4

  5. While I don't disagree with anything anyone has written so far, I will
    make one humble suggestion.

    The answer to all such questions is results not procedure. If it works
    for you and you get the results you want, it is the proper way of
    calculating exposure. If not, it is not proper. There will be situations
    where what works for me does not work for you.

    If it works use it, along the same idea my cat's have taught me, if it
    feels good do it.
     
    Joseph Meehan, Jun 8, 2005
    #5
  6. Denny B

    Denny B Guest

    Matt Clara,
    Thank you for your reply. I take it then that exposing
    slightly more, applies to this sunny f16 rule only.
    And it does not apply if you are using a Spotmeter.

    thanks
    Denny B
     
    Denny B, Jun 8, 2005
    #6
  7. Denny B

    Matt Clara Guest

    That is correct. However, keep in mind that any meter is trying to read
    everything as grey (or is that "gray"...I can never remember), so a
    spotmeter on a white cloud will give you an exposure setting that will
    render that cloud darker (grayer) than it really is, in which case some
    exposure compensation is required--unless you _want_ it darker than it
    really is, in which case, it's not required. Hope that makes sense.
     
    Matt Clara, Jun 8, 2005
    #7
  8. Denny B

    Alan Browne Guest

    A good read of
    http://www.fredparker.com/ultexp1.htm

    and a review of the tables inside will help you imensely, in addition to
    the links provided by Matt.

    Cheers,
    Alan
     
    Alan Browne, Jun 8, 2005
    #8
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