How do you explain exposure?

Discussion in 'Exposure Discussion' started by JBrunner, Oct 29, 2010.

  1. JBrunner

    JBrunner Moderator Staff Member Moderator

    Messages:
    7,075
    Joined:
    Dec 14, 2005
    Location:
    Basin and Range Province
    Shooter:
    8x10 Format
    Tried to explain exposure reciprocity to someone today, and didn't feel like I made much headway. For instance, they had trouble understanding the difference between film speed and shutter speed, and the whole half and double relationship between shutter speed, film speed and aperture.

    What is your best method for explaining exposure? Water, funnel, and a bucket?
     
  2. MattKing

    MattKing Subscriber

    Messages:
    16,817
    Joined:
    Apr 24, 2005
    Location:
    Delta, BC, Canada
    Shooter:
    Multi Format
    For f/stop, I like using a door.

    If a door is open all the way, it lets in X amount of light

    If the door is open 1/2 way, it lets in less light

    If a door is open 1/4 way, it lets in even less light

    If a door is open 1/8 way, it lets in even less light.

    The ratios are related to the area that is open.
     
  3. 2F/2F

    2F/2F Member

    Messages:
    8,003
    Joined:
    Apr 29, 2008
    Location:
    Los Angeles,
    Shooter:
    Multi Format
    If all else fails, I do it with Polaroids! :D
     
  4. Q.G.

    Q.G. Inactive

    Messages:
    5,682
    Joined:
    Jul 23, 2007
    Location:
    Netherlands
    Shooter:
    Medium Format
    The "classic" way is using the water tap and bucket analogy.

    Good exposure is the bucket filled to the rim.
    Underexposure a not yet full bucket.
    Overexposure a bucket overflowing.

    Film speed is the size of the bucket. A faster film is a smaller bucket, a slower film a larger one.

    Shutterspeed the time it takes to fill the bucket.

    Aperture how far the tap is opened, i.e. the rate of flow.

    Turn the tap open more, and it will take less time to fill the bucket.
    Close the tap a bit, and it will take longer.
    Change the size of the bucket leaving the tap as it is, and it will take longer or shorter, depending on whether the new bucket is larger or smaller.

    Something everyone will understand, even 3 year olds.
    :wink:
     
  5. AgX

    AgX Member

    Messages:
    11,928
    Joined:
    Apr 5, 2007
    Location:
    Germany
    Shooter:
    Multi Format
    film speed and shutter speed

    I assume one problem lies in designating two different things with the same term "speed" in common English.

    In German the terms "sensitivity" and "time" are used respectively.
     
  6. RalphLambrecht

    RalphLambrecht Subscriber

    Messages:
    8,213
    Joined:
    Sep 19, 2003
    Location:
    Florida
    Shooter:
    Multi Format
    I agree. This is the most popular way to explain it, and it is used in several text books. Just google for 'exposure bucket' and you'll find many examples. Here is one:

    http://www.howitookit.com/wp-content/uploads/2009/06/water_faucet1.jpg
     
  7. Q.G.

    Q.G. Inactive

    Messages:
    5,682
    Joined:
    Jul 23, 2007
    Location:
    Netherlands
    Shooter:
    Medium Format
    Yet, using the bucket and tap analogy, the word "speed" doesn't create confusion: no matter the 'speed', the rate of the flow of water, it takes less time (i.s, is speedier) to fill a small bucket than it takes a large one.
     
  8. ann

    ann Subscriber

    Messages:
    2,919
    Joined:
    Sep 10, 2002
    Shooter:
    35mm
    when it comes to dealing with halfing and doubling i use the relationship to 1inch to 2 inch which seems to help.

    I can still remember many years ago (and i mean many as i have been doing at this for over 60 years) that relationship made no sense to me; and all of a sudden one day the light bulb went on and in fact i have seen a diagram using lightbulbs .
     
  9. hpulley

    hpulley Member

    Messages:
    2,214
    Joined:
    Oct 6, 2010
    Location:
    Guelph, Onta
    Shooter:
    Multi Format
    Aperture is simply the size of the hole that lets the light into the camera, onto the film. Aperture is a ratio of lens diameter divided by lens focal length, always reducing the diameter to 1 for easier comparison. Since 1/2 is bigger than 1/4 that explains why 1/2 lets more light in. Understanding that the area of 1/2 is 4 times that of 1/4 tells you why it isn't 2x the light. Area is always squared (volume is worse, no one gets volume being cubed).

    Shutter speed is a poor term, if it actually WAS a shutter speed number it would be alright but it is not. It is shutter duration. Again it is a fraction so 1/2 is longer than 1/4 second. Long shutter means blur, short shutter means frozen action. Shutter speeds are not areas so they are not squared, 1/2 is twice as long as 1/4 so it lets in twice as much light.

    Film sensitivity isn't actually included in the exposure but we still need to consider it to choose an appropriate exposure. Film sensitivity is a much better term. More sensitive to light means you can use a shorter shutter duration to get enough light or a smaller aperture. 200 is twice as sensitive as 100 which is twice as sensitive as 50 so 200 is 4x as sensitive as 50.

    Reciprocity failure is explained by a few chemical properties. Activation energies mean that at very low light levels there may not be enough energy to trigger the chemical reaction caused by normal levels of light. Over time reacting there is less chemical left to react is another factor. Some very sensitive films for short exposures are very poor in long exposures; some fairly slow films are actually better at long exposures. Only the data sheet can tell you about the failure of one particular film.
     
  10. ic-racer

    ic-racer Member

    Messages:
    7,473
    Joined:
    Feb 25, 2007
    Location:
    Midwest USA
    Shooter:
    Multi Format
    With pre-teenage children in the darkroom I totally avoid the need for any understanding of reciprocity. All the negatives are printed for 60 seconds and we lith print. The kids can easily understand the longer the print is in the developer, the darker it gets; and exposure is out of the discussion.