contrast sensitivity, visual acuity, and Photoshop

Discussion in 'Optometry Archives' started by Liz, Nov 7, 2009.

  1. Liz

    Liz Guest

    Argh!

    I've been reading about "contrast sensitivity", "visual acuity", and
    wonder what the difference is. When I try to find this out, I am
    immersed in "cycles per degree". When I try to find out what that is,
    I get math. It's so abstract.
    :-(

    Are these two things different? It seems that both are measured in
    "cycles per degree".

    Is there a way of explaining how CS differs from VA (if it does),
    using terms used in photography?

    If you have poor CS or VA, what do you not see?
    Resolution, shadow detail, sharpness? ...... help!

    thanks,
    Liz
    Indianapolis IN USA


    thanks,
    Liz
     
    Liz, Nov 7, 2009
    #1
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  2. Some of us like that stuff !
    People often use terms imprecisely. I would say "visual
    acuity" presumes some sort of standard contrast, while
    "contrast sensitivity" is somewhat more general.
    Oh, a photog -- look up "Modular Transfer Function" ...
    maybe not if you dislike abstraction more than ignorance.
    "cycles per degree" is similar to "line pairs per mm"


    It is all the same thing! You can see wires against sky
    with very high resolution but cannot see something big
    camouflaged. Contrast is what drives vision, and the
    more of it there is, the smaller details you can see.


    -- Robert
     
    Robert Redelmeier, Nov 7, 2009
    #3
  3. Liz

    Liz Guest

    Sorry Liz. If you are trying to quantify these terms to an objective
    I'm not complaining about the measurement aspect. Of course you have
    to use math to measure it.
    I'm saying that I can't figure out WHAT is being measured. I mean in
    terms of anything concrete that I know what it is.

    Unfortunately I don't know what "spatial frequency", "transfer
    modulation function", are. Sorry! Yes, I looked them up and read
    several times.

    Liz
     
    Liz, Nov 7, 2009
    #4
  4. Liz

    Liz Guest

    Mike, your explanation is SO much better than those articles!
    Degrees as in 0 to 360? The way astronomers say something is "15
    degrees above the horizon"? I think so. In other words, straight
    ahead is 0, straight overhead is 90, and the space in between is
    divided into 90 equal pieces...
    Yes, I see how the distance wouldn't matter.
    OK.
    So a "cycle" is about changes in appearance. From black to white, or
    silver to pink, or any other change that's visibly different from
    what's next to it?
    And the number of "cycles" is the number of pairs of distinguishable
    objects?
    Would black green white pink white grey orange blue be 4 cycles?

    OK. Are they described using cycles too?
    OK.

    Try this:
    You have a white piece of paper with horizontal black bars on it.
    Someone walks further and further away with this piece of paper.
    Finally they are so far away that you cannot see the bars; you only
    see the whole paper as grey.
    That is an example of a limit of your visual acuity.
    Right?
    (In Photoshop I would call this "resolution".)

    Suppose instead of black and white, you have black and dark grey. You
    can tell them apart when the paper is near you.
    The person walks away.
    They haven't walked very far - nowhere near as far as they went with
    the black and white paper - when you can no longer see any barring.
    The whole paper looks one dark color.
    This is a failure in contrast sensitivity?
    (In Photoshop I would call this "tonal contrast".)

    But..... hmmmm.

    I feel as though there is a lot more but I have to think of how the
    situations differ.
    I think there must be interactions between being able to discern a
    sharp edge and being able to see a tonal difference.

    Liz
     
    Liz, Nov 7, 2009
    #5
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