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Thread: Exposure question - How do you explain this paradox?

  1. #1

    Default Exposure question - How do you explain this paradox?

    I've taken exposure for granted all this while. I have the following question which cannot be found in the books. Hope someone can kindly help me answer. Thanks.

    We all know a camera decides on the correct exposure based on "middle grey". And that it uses reflective metering as opposed to incident metering which should be the case for spot on exposure for all scenarios.

    Now, I'm sure all of us have experienced "hot spots" or "flashing light" on our camera's LCD when we try to take pictures of something white or bright colored in the sun. That means that part of the scene has been overexposed.

    Say you are taking a picture of 2 friends. Friend A wears a dark-colored shirt, while friend B wears a white one. After taking a shot, you review your LCD screen and see that Friend B's shirt has flashing highlights to denote over-exposure. But Friend A's shirt is properly exposed.

    What do you do from here?

    Suppose you only take a picture of Friend A. He looks well-exposed and you can say you've gotten the exposure right. Now let Friend A step out and take only a picture of Friend B. Using the same exposure as that for Friend A, you now find Friend B to be over-exposed.

    Normal layman's logic will be that it's expected. White reflects more light while darker colors absorb more light and hence don't radiate as much light as white. But in photography, as I mentioned earlier, it's not the reflected light that determines what the exposure should be (that could work if what we photograph is close to middle grey) but the light that is INCIDENT on the subject.

    For my example, both Friends A and B are standing under identical lighting. So why should Friend B's white shirt be overexposed while Friend A's darker shirt is properly exposed?

    How do you explain this paradox?

  2. #2
    Moderator Clown's Avatar
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    Default Re: Exposure question - How do you explain this paradox?

    this is to show why you shouldnt rely too much on the camera's computer to find out WHERE to take the meter reading from.

    if A's shirt is overexposed, and you still want the computer to handle your exposure, do a -1 exposure compensation.
    sigh.

  3. #3

    Default Re: Exposure question - How do you explain this paradox?

    hmm.. i re-read your post but can't seemed to find any paradox. I think the part that is new to me is the statement below.

    Quote Originally Posted by Priscilia View Post
    But in photography, as I mentioned earlier, it's not the reflected light that determines what the exposure should be (that could work if what we photograph is close to middle grey) but the light that is INCIDENT on the subject.
    Based on my understanding of your white t-shirt and dark t-shirt scenario; we can use the analogy of dark buildings (black) against bright skies (white) in the same scene?

    When you say using the "same exposure" you are referring to using the same settings?

    If we expose the dark buildings, naturally the skies will be overexposed isn't it?
    Last edited by ahbian; 7th February 2010 at 10:07 PM.

  4. #4
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    Default Re: Exposure question - How do you explain this paradox?

    The answer is simple and self evident.

    In camera metering is based on weighted averages. Also it does not take in to account the dynamic range of the subjects vs sensor or film. That is in the realm of the photographer to correct.
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    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Priscilia View Post
    How do you explain this paradox?
    1. Depends on the metering type that you use... matrix? centre weighted? spot? Thus it is also depends on the so called "identical lighting". Does the lighting tends to be brighter or darker relative to the shirts?

    2. Metering in the camera, as you mentioned, was measured relatively to the 18% of gray. If you want a proper exposure for the shirts and the faces of your subject, by using your camera's metering, you MUST have manual control to the lighting... balance the lighting to their faces, clothing, background, etc.

    3. If you have 2 people who wears different clothing, like wedding couples where the man wears dark suite and the lady wears white, it is wise to do everything in manual, with the help of a light meter in your hand...

    Hope this answers your question...

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  6. #6

    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Priscilia View Post
    I've taken exposure for granted all this while. I have the following question which cannot be found in the books. Hope someone can kindly help me answer. Thanks.

    We all know a camera decides on the correct exposure based on "middle grey". And that it uses reflective metering as opposed to incident metering which should be the case for spot on exposure for all scenarios.

    Now, I'm sure all of us have experienced "hot spots" or "flashing light" on our camera's LCD when we try to take pictures of something white or bright colored in the sun. That means that part of the scene has been overexposed.

    Say you are taking a picture of 2 friends. Friend A wears a dark-colored shirt, while friend B wears a white one. After taking a shot, you review your LCD screen and see that Friend B's shirt has flashing highlights to denote over-exposure. But Friend A's shirt is properly exposed.

    What do you do from here?

    Suppose you only take a picture of Friend A. He looks well-exposed and you can say you've gotten the exposure right. Now let Friend A step out and take only a picture of Friend B. Using the same exposure as that for Friend A, you now find Friend B to be over-exposed.

    Normal layman's logic will be that it's expected. White reflects more light while darker colors absorb more light and hence don't radiate as much light as white. But in photography, as I mentioned earlier, it's not the reflected light that determines what the exposure should be (that could work if what we photograph is close to middle grey) but the light that is INCIDENT on the subject.

    For my example, both Friends A and B are standing under identical lighting. So why should Friend B's white shirt be overexposed while Friend A's darker shirt is properly exposed?

    How do you explain this paradox?
    1. The correct exposure would be the incident light reading, which will make black black and white white.

    2. Your question assumes the camera is dumb. But modern cameras are very smart, esp. the matrix metering type. The camera will pick an exposure which does its best not to blow everything, ie slight overexposure of the white, but enough exposure for the black.

    3. You can test how smart your camera is with an incident light meter. Get a reading, then shoot a black shirt, white shirt and black + white shirt in the scene, and see how often the camera gets it right.

  7. #7
    Senior Member geraldkhoo's Avatar
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    Default Re: Exposure question - How do you explain this paradox?

    Go get Michael Freeman's book Perfect Exposure.

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    Default Re: Exposure question - How do you explain this paradox?

    are you shooting on aperture priority or manual?

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    Default Re: Exposure question - How do you explain this paradox?

    One must factor in the physical limitations of the sensor's dynamic range as well. If contrast is too high, you still have to have sacrifice either the extreme ends of the highlights or shadows even if you meter the subject perfectly (unless you bracket-and-merge or hdr)...

  10. #10
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    Default Re: Exposure question - How do you explain this paradox?

    Can I ask where is the sun?
    In outdoor shots, u need to judge where is the light source and the ambience lighting. Mostly I fix it with just soft diffused fill flash.

  11. #11

    Default Re: Exposure question - How do you explain this paradox?

    You'll still have to consider reflected light as well, after all its reflected light that enables us to see subjects in the first place.

  12. #12

    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Clown View Post
    this is to show why you shouldnt rely too much on the camera's computer to find out WHERE to take the meter reading from.

    if A's shirt is overexposed, and you still want the computer to handle your exposure, do a -1 exposure compensation.
    I'm not so much referring to what I should do about the situation but WHY it happens based on what I've described.

    I have to say it's quite an academic question... In practice, photographers with some experience will know what to do with it.

  13. #13

    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by ahbian View Post
    hmm.. i re-read your post but can't seemed to find any paradox. I think the part that is new to me is the statement below.



    Based on my understanding of your white t-shirt and dark t-shirt scenario; we can use the analogy of dark buildings (black) against bright skies (white) in the same scene?

    When you say using the "same exposure" you are referring to using the same settings?

    If we expose the dark buildings, naturally the skies will be overexposed isn't it?
    The paradox is: if it is the incident light that rightly determines exposure, why should how "white" or how "dark" a shirt is forces you to change the exposure accordingly?

    Your dark building against bright skies is a different example. That's because the amount of light falling on them is different, so naturally their exposures will be different also. In my case, the two people are standing side by side, SAME amount of light falling on them, just that one wears a white shirt, the other a dark one. On a bright sunny day, it is possible to see the white one over-exposed even if the dark shirt is properly exposed.

  14. #14

    Default Re: Exposure question - How do you explain this paradox?

    trying to understand why it is a paradox.

    do a spot metering on the faces (unless you are telling me one is black the other is white).

  15. #15

    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Ian View Post
    The answer is simple and self evident.

    In camera metering is based on weighted averages. Also it does not take in to account the dynamic range of the subjects vs sensor or film. That is in the realm of the photographer to correct.
    I don't think the camera's metering has anything to do with the issue? I'm referring to a given ISO, aperture and shutter speed set by the photographer for a fixed situation involving 2 shirts of different colors.

    I've also thought about the dynamic range issue. Good point. However, upon deeper thought, I think it's not that also. A camera's dynamic range refers to how much "tonal information" it can hold altogether in a scene, where some subjects are dark and bright because DIFFERENT amounts of light are falling onto them.

    On the contrary, in my earlier example, the SAME amount of light is falling on the subjects. What is it about whites or light colored subjects that the camera cannot hold details as well as darker colored ones? So can I say exposure does not only involve considering incident light but the reflective component as well? That's where I find the paradox coming in....

  16. #16

    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by theveed View Post
    One must factor in the physical limitations of the sensor's dynamic range as well. If contrast is too high, you still have to have sacrifice either the extreme ends of the highlights or shadows even if you meter the subject perfectly (unless you bracket-and-merge or hdr)...
    See my earlier argument on why it is not likely to be an explanation of dynamic range. Please keep the discussion open. I'm not saying I'm right, just presenting my point. So feel free to agree or disagree amicably.

  17. #17

    Default Re: Exposure question - How do you explain this paradox?

    If you think about it, its reflected light from subjects going into the lens that produces the image, incident light going in would just be unwanted flare..

  18. #18

    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by wootsk View Post
    Can I ask where is the sun?
    In outdoor shots, u need to judge where is the light source and the ambience lighting. Mostly I fix it with just soft diffused fill flash.
    Ermmm, out of topic wootsk... I'm searching for the WHY, not how to handle the situation.

    Pardon me guys, it's so philosophical and academic....

  19. #19

    Default Re: Exposure question - How do you explain this paradox?

    Priscilla,

    Academic or not, this is a very interesting topic.

    I'll wait to see what others say on the subject first. Got to dig rusty brains, and even then, dunno if brain will fall apart.
    Last edited by Dream Merchant; 8th February 2010 at 12:45 AM.

  20. #20

    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by J-Chan View Post
    You'll still have to consider reflected light as well, after all its reflected light that enables us to see subjects in the first place.
    Exactly...that's the so called "layman's every day common sense". White reflects more light than darker colors so white should be brighter. But in photography, this is not quite what we have been "taught".

    In photography 101, we learn that reflective metering is just a "convenient" way the camera decides on how to expose subjects. It works for general scenes with good mix of tonalities. But once it differs from middle grey, we have to do exposure compensation.

    And thus, to ensure exposure is done correctly, the best way is to make use of incident metering, which is what a light meter does.

    So back to my same scenario (sorry for sounding like a broken record!!!): Suppose you take an incident meter using a light meter. It gives you what must be a good exposure setting to use. Back to our old 2 friends, one wearing dark and the other light shirt. Bear in mind, BOTH of them are under the SAME kind of lighting. When you take the shot, the white shirt is over-exposed while other parts of the scene is ok.

    I'm just trying to see how I can reconcile the part about incident light and not reflective lighting determining exposure. That's what all the books will tell you. But I find this to be an insufficient condition?

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