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

  1. #41

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

    You've repeated your qn many times.... Maybe it's not that ppl don't understand the qn, but that you don't understand the answer?

    A little understanding of incident light metering and zone system will put you right. Just google or wiki.

    In brief,

    1. The incident light reading is the correct one to use. But there can be more than one incident reading to be made in a scene.
    2. The fact that your highlights can still blow doesn't mean the reading is wrong; it's due to the dynamic range of your sensor. That's why in the film days, we said expose for the shadows and develop for the highlights. For slides and digital, we have to expose for the highlights, because the limitation on the medium is the highlights. That's why when shooting slides/digital you have to take an incident reading from the highlights while shooting negs one takes the incident reading in the shadows.
    3. For a backlit subject, if we want the shadow detail, we can expose for it using the inicdent reading for the shadows, this will overexpose the background and blow it out. Again, this doesn't mean your reading is wrong, as above. However, the usual remedy in such situations is to fill flash so that you can bring the shadow exposure more in line with the highlight exposure, and hopefully reach an exposure where both do not blow out.


    Quote Originally Posted by Priscilia View Post
    Ha... phew, can I assume you do get the point I'm trying to raise?

    To put in another way.... if reflected light does have a part to play in determining exposure, then I feel it must be "incorporated" into the exposure "equation".

    1. Common sense and pure physics tell us a white surface reflects more light than a black one. So if you use this explanation, then perhaps we can explain the situation I brought up -- the white shirt reflects more light into the camera's sensor than the darker one, so for a given exposure, the white shirt is over-exposed.

    2. But this is not what we have commonly know about exposure in photography. Reflected light fools the camera. For that reason, we get underexposure in snow and overexposure in a black panther, for example. Use a light meter or grey card, as we are told! But then, that will not guarantee getting the right exposure.

    So how do you reconcile the 2 different observations in 1 and 2?

    Ok, its getting late now... Need to take off my philosophical cap now....

  2. #42

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

    Thanks everyone for more input... especially Jed for the neat techinical explanation.

    Please correct me here if I'm not quite getting it right... a handful of you mentioned about Dynamic Range. In my understanding, we talk about the limited dynamic range of the sensor (or film) when we fail to record all the different tones of a scene cos the shadow and highlight range is too wide for the camera to capture.

    But to me, that's because the light falling in the shadow region is obviously a lot less than the highlight region. For eg, if you try to capture a building's interior (lit mainly by artifical lighting much less powerful than the sun) with strong sunlight outside the window, you have to decide whether you want to expose for the interior (causing the outside to look washed out) or expose for the outside (causing the interior to look very dark).

    In my example though, even though there is dark (the black shirt) and light (the white shirt), the SAME amount of light is falling on them. Can we still use the Dynamic Range reason to explain why the white can possibly get blown out while the dark is recorded well by the camera?

    To explain further:

    If you take an incident light meter and measure the light falling within the different parts of the interior and the light outside the room, you're going to get a lot of different exposure readings. Our eyes can capture all the details well cos they have a much wider exposure latitude than the cam sensor (or film). But if I use an incident meter and I read the exposure from the friend wearing the white shirt, and next I do the same for the dark shirt one, I will get the SAME exposure reading cos the SAME amount of light is falling on them. How does the Dynamic Range reasoning apply here?

    Ok, so I set the cam to that exposure given by the light meter. What I'll see on the LCD screen is that the white looks overexposed while the black shirt is ok. I think it's more than just about dynamic range at play here... No? The white is able to reflect more light into the cam for a given exposure. So even thought the same amount of light is INCIDENT on the 2 different colors, one is actually brighter than the other.

    Another reason I brought this issue up is that, I remember many years ago, I did a simple test, as I was coming to grips with learning about exposure. I took a light incident meter, and metered a patch of green grass on a bright sunny day. When I set the same exposure readings, ie ISO, shutter speed and aperture on my cam and took the shot, I noticed the grass looked underexposed.

    Well actually, if I don't tell you when the shot was taken, you might think it was well exposed on say a cloudy day. (It appeared darker green.) But I know the grass should be brighter (lighter green) than that, so to me, the shot was rightly underexposed.

    In the above example, there isn't really any "dynamic range" issue to talk about. Cos in my viewfinder, I framed a complete patch of grass and nothing else. The incident meter it seemed, is not able to interpret the correct expsosure.

    So as some said, it's not just the amount of light falling on the subject that determines what is the correct exposure. Which means to say, the incident light meter may not give us the correct exposure once the subject deviates from middle grey.

    Put it further, the question now is, does the reflectance of the subject have a part to play in us deciding on the exposure? In the grass example, its surface is quite smooth, it reflects more light than on a less sunny day. The incident meter won't "know" that cos it only measures light coming from the sun, and not how much more light is radiating from the grass which does cause more light to enter the camera lens.

    Other than the dynamic range explanation, does the above argument also explain why the white shirt gets easily overexposed on a bright day compared to a darker colored one? There's something about lighter colored surfaces being overexposed first in bright lighting.... It's their reflectance.
    Last edited by Priscilia; 8th February 2010 at 09:57 AM.

  3. #43

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

    This is a problem on dynamic range..simply put, camera sensors ( not all, look at S5pro) does not have enough dynamic range unlike the human eyes--techinical limitations. Thus in a scene of , say a wedding, the groom wearing a dark suit and the bride wearing the usual white bridal dress, the dress will be blown if u meter the groom or vice visa.

  4. #44
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    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Priscilia View Post
    Thanks everyone for more input... especially Jed for the neat techinical explanation.

    Please correct me here if I'm not quite getting it right... a handful of you mentioned about Dynamic Range. In my understanding, we talk about the limited dynamic range of the sensor (or film) when we fail to record all the different tones of a scene cos the shadow and highlight range is too wide for the camera to capture.

    But to me, that's because the light falling in the shadow region is obviously a lot less than the highlight region....

    In my example though, even though there is dark (the black shirt) and light (the white shirt), the SAME amount of light is falling on them. Can we still use the Dynamic Range reason to explain why the white can possibly get blown out while the dark is recorded well by the camera?
    Yes you still can. Even if incident light is the same, what the camera (or sensor) captures in the end is actually reflected light. Incident light after hitting the white or black shirt, already becomes reflected light.

    So in the end, just remember metering is inanimate. In the end the photographer have to use judgement on what the final exposure should be. And after that, what sort of PP to do to achieve the final image he/she has in mind.

    There is no right or wrong answer. Just what the photographer is trying to achieve, the photographer will make the judgement call on exposure and PP. Photographers have to make this call simply beacause our eyes have a much much bigger dynamic range than any sensor or film out there at this point in time.
    Last edited by daredevil123; 8th February 2010 at 10:04 AM.

  5. #45
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    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Priscilia View Post
    But to me, that's because the light falling in the shadow region is obviously a lot less than the highlight region. For eg, if you try to capture a building's interior (lit mainly by artifical lighting much less powerful than the sun) with strong sunlight outside the window, you have to decide whether you want to expose for the interior (causing the outside to look washed out) or expose for the outside (causing the interior to look very dark).

    In my example though, even though there is dark (the black shirt) and light (the white shirt), the SAME amount of light is falling on them. Can we still use the Dynamic Range reason to explain why the white can possibly get blown out while the dark is recorded well by the camera?
    Short answer, yes. For all intents and purposes essentially a brightly lit dark subject is the same as a poorly lit bright subject. So if you can understand the difference between dark indoors and bright outdoors, then it's a similar situation except in your example you are keeping the light level constant so the only thing that varies is the amount of light being reflected by the various objects/shirts. In the case of a light shirt, that reflects a lot of light, in the case of a dark shirt, that reflects very little light. But yes, a light shirt is "brighter" than a dark shirt, because it reflects more light.

    You have to have a look at the whole concept of colours and how they come about, and how something looks "red", and how something looks dark and something looks light.

    As daredevil123 puts it well, once the same amount of light falling on a scene hits different objects it becomes reflected light (ie reflected off the various objects) and this is affected by various things including the type of material, how much of each colour light it absorbs, etc.

    And when it comes to light shirt getting blown before dark shirt getting blocky, I think the answer to that lies in the average digital sensor's dynamic range layout (ie a lot more below mid grey than above mid grey).

  6. #46
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    Default Re: Exposure question - How do you explain this paradox?

    note to TS: do not delete your thread just like that. it's rude.
    sigh.

  7. #47

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

    i don't get the problem here, camera meter fooled easily? that happens all the time.

    exposure not correct even though it's correct for one and not for another? then nail the right exposure that retains the highlights, get the white shirt right, most cameras will allow you to get the shadow details there, albeit with noise coming into the picture. your camera just got fooled into giving you the blown highlights, which is not the best output to be honest.

    i don't find this a thought-provoking question actually. it's not always about getting the answers to all your questions, but asking the right ones as well. cheers.

  8. #48
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    Default Re: Exposure question - How do you explain this paradox?

    Short reply on "correct" exposure.

    The "Correct" exposure is when the camera records exactly "How" the eye sees it. However, under different lighting conditions, the eye responses differently, and it is not possible to always exactly recreate how the eyes sees it and assign the correct exposure based on current technology. Based on research by manufacturers, the best estimate is to meter for 18% grey. That is taking the average for many scenes, 18% grey is deemed to be the most common situation encountered.

    However, it is still not possible to exactly recreate the scene as seen by the eye.

    Because, the eye has the ability to do "Localised" exposure adjustment. that is, in the region that is dark, that part of the sensors/photosites will automatically increase the "gain", For bright regions, the sensors will reduce the gain.

    Model camera usually apply a flat gain (ISO) to ALL photosites, hence, the output is almost always different as seen by the eye.

    Still, some model camera have those "D-lighting" or whatever technology that tries to do localised exposure compensation. But maybe that technology is still not seasoned enough to adjust the output so as to match how mother nature does it.




    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.

    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?

  9. #49
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    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Priscilia View Post
    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....
    It's entirely a dynamics issue and weighting problem. You'll need to really understand the mechanics of camera metering and the types of metering to comprehend exactly what's going on. I for one am not going to spend the next 5 years explaining to you the methods of camera metering as you'll need a lot of understanding of physics (wave propogation, light, and reflected light transmission) as well as electronics and sensor technology, none of which come without a major learning curve.

    Jed's done a nice job of summising most of the problems in a nutshell so there's no need for me to elaborate further.
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  10. #50
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    Default Re: Exposure question - How do you explain this paradox?

    Most of the time, the camera does a good job in deciding the right amount of exposure.

    In case of extreme lights, we need to do some exp-com to get what we want.

    Well, some cameras does it better than others. Some cameras would try as far as possible not to clip to be on the safe side. Others would average it out, and pretty often, you get your subject having some nasty 100% white areas.

    Got to know your camera. Nikons, Canons etc have different tendencies. Even among Nikons, there is quite a bit of difference in their tendency to clip.
    I love big car, big house, big lenses, but small apertures.

  11. #51

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

    the magic is in the lightwave, its position, direction and angles. distance too ( for artificial ight )

    the variables are the subject/objects where the light fall on them, next will be the reflectance of these materials.

    without light, everthing will be black and there will be no white.

  12. #52

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

    I liked Jed explanation the best.

    And what I am going to write is a variation from what Jed had said. Sorry if I had not read all the mails in detail.

    Incident light measurement does determine the correct exposure, provided your dynamic range is not exceeded - then everything will be rendered correctly.

    If the subjects you're photographing, say the black and white case, are in contrasy lighting situation, then the incident light tells you how middle grey will turn out, and if the dark is beyond the lower limit and the white is above the higher limit, then you can get both blown highlight and lost shadow.

    You do have roughly 5 stops of dynamic range so it the light falling on the subject creates a larger difference between highlight and shadow then you will get something blown, highlight or shadow up to you to choose.

  13. #53

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

    Quote Originally Posted by Jed View Post
    Hang on.

    I've just come back from a job so I'm not totally clued in yet (black shirts, v white shirts, no less). But I'm a little confuzzled about what the exact nature of the problem, or the paradox is.

    Firstly, an incident light reading basically measures the amount of light falling on a given scene. This means that using that setting under that lighting, middle grey should reproduce as middle grey.

    Now with the dark shirt/light shirt example, is that dark shirt exactly the same number of stops darker than the light shirt is brighter? If the dark shirt is not as dark as the light shirt is light (eek for the expression) then it stands to figure that it's entirely probable that the light shirt will not reproduce as well. Have a further look at the zone system for a more thorough examination of this.

    Add to that the fact that dynamic range of the sensor is very pertinent. Firstly if your tone curve is linear (which it isn't) then if the dark shirt is exactly the same number of stops darker than the light shirt is brighter, and you have accurately metered for middle grey, then they should reproduce similarly. But, especially when it comes to digital sensors, the tone curve tends to be S shaped, headroom in the shoulder area is poor, and dynamic range tends to be limited, with particularly poor performances in the highlight areas especially when any single channel comes close to being clipped.

    Finally, you also have to bear in mind that most highlight clipping on the camera triggers if any of the RGB channels is clipped, rather than if all are clipped. It's entirely possible that a light shirt that might appear to be clipped is actually within tolerances.

    Now also consider, if you reversed the lighting situation. So instead of the two shirts in sunlight, you have them in a dimly lit room. The dark shirt is likely to block up, while the light shirt would likely be properly exposed.

    I'm still not entirely sure where the paradox comes in, but I suspect it's somewhere in the whole "correct exposure" concept. Incidence metering takes care of your mid point, after that a scene with lots of light tones will appear light, a scene with dark tones will appear dark, and if the dynamic range of the scene exceeds the sensor's capabilities then you will end up with clipped highlights, or clipped shadows. Because of sensor design at the moment, clipped highlights are more likely than clipped shadows generally speaking, with more latitude in the shadow areas than in the highlights.

    Have a quick look at any dynamic range graph on DPReview for example, or any other reliable test site. As an example I had a quick look at the D3000 review on that site, and at ISO 200 the useable shadow range is 5.1EV, and the useable highlight range is 3.6EV (below and above mid grey respectively). That's an extra 1.5 stops of latitude in the shadow areas, and why your dark shirt is AOK while the light shirt gets clipped.
    Superbly detailed explanation, and clears up a lot of confusipn.

    Jed, could I check something ... you mentioned that a sensor's response is curved. I always told it was linear, or at least much less curved as compared to film so much so that it appears almost stright?
    Last edited by Dream Merchant; 9th February 2010 at 09:38 PM.

  14. #54

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

    I think besides the DR issue, u shud also look at the incident metering mode. Is this the correct mode to use? Is it really fool proof?

    if i'm not mistaken, the incident mode takes its reading from all over the area. Whereas when u meter thru the lens, its metering is "funneled" in, narrowed down to your composition of the scene & not from all over the place. There lies some differences. Eg. when u shoot the 2 guys using wide angle (35mm) that encompasses more scenery & then shoot the same guys, same lighting, same distant using a telephoto lens (105mm). Do the exposures read the same? Try it & u'll know what i mean.

    Also dark objects need to appear dark & light objects light, as how ur eyes see them. U want to shoot a dark maroon shirt & expect it to turn out pinkish white?

    I dun see any paradox here, juz confusion. Shoot more & think less

  15. #55
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    Default Re: Exposure question - How do you explain this paradox?

    Quote Originally Posted by Dream Merchant View Post
    Jed, could I check something ... you mentioned that a sensor's response is curved. I always told it was linear, or at least much less curved as compared to film so much so that it appears almost stright?
    Most camera tone curves are S shaped as I explained somewhere in one of my super-ridiculously-long posts. So not straight certainly. Easiest reference is to have a look at one of the more recent DPR reviews. Also, some cameras with the right software will allow you to set your own tone curve.

    I was thinking about the film comparison but not sure I can give you one off the top of my head really; there's probably an article somewhere. From my own experiences the issues are usually very muddied by different DR between film and digital which makes it difficult to compare precisely how the two stack up. Sorry if that's a bit of a copout answer!

  16. #56

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

    No worries Jed.

    You've already provided far more information than most have.

    The only reason I ask linear response was because of my involvement with large format film and the zone system. I once used a DSLR to meter and preview, got into a long 'discussion' on location with another photographer who trusted the digital review on my DSLR whereas my handheld light meter was telling me something else entirely.

    In the end, I went with what the other photographer insisted was correct and the results were horrendously under, which appeared perfectly balanced on the DSLR.

    Which led me to read look into long discussions about LFers using digital P&S cams as their light meters and so on.

    Since this whole topic is academic, it's interesting that no one pointed out that there are actually two sets of applied mid-points: 18% grey and 11% or 12% grey, and that different manufacturers calibrate to these two sets, so depending on what equipment one is using, that's another spanner in the works to come to grips with. And also that many seem to assume an incident reading was an available in-camera metering mode.

    The other points about an accepted midpoint only being a starting point, reflectivity (and I suppose even refractivity of certain materials) have been pointed out so it's redundant to bring those up again.

    The other thing I'm curious about is how to find out the linearity or curve of a particular light meter. In this case, handhelds, specifically, since I've read that some handhelds have linearity problems and give vastly different readings of different colors under the same exact lighting conditions.

    I'm also wondering if metering linearity, besides (possibly adjustable) tone curves and dynamic response might be factors that affect DSLRs. Purely on an academic level of course.

  17. #57

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

    Quote Originally Posted by Jed View Post
    I was thinking about the film comparison but not sure I can give you one off the top of my head really; there's probably an article somewhere. From my own experiences the issues are usually very muddied by different DR between film and digital which makes it difficult to compare precisely how the two stack up.
    I was wondering more about the characteristics of the differences, rather than the fact that differences exist. If I had a set of absolutes to work with, as in the case of film specs, I could try (while probably turning my brains to jelly) to reconcile the difference and hopefully use my DSLR to both meter AND preview for times when I will end up shooting with specific films.

    Or just stick to two different metering and lighting techniques entirely and be forced to live with polaroids.

  18. #58
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    Default Re: Exposure question - How do you explain this paradox?

    polaroids... Ur rich man
    Still got my last 3 shots left in for 600 for 2 mths. Wonder if the battery is already flat

  19. #59
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    Default Re: Exposure question - How do you explain this paradox?

    Hmm, I haven't touched my LF gear in literally years and it wouldn't surprise me if my lenses are now fungus ridden... I would go look but it's buried in the back of my store room!

    I miss those days, I miss 5x4 trannies, I don't miss lugging the thing around!

    I did in the early days briefly use a DSLR to preview the scene exposure wise, but I never found it particularly good because I always used the screen on the back of the camera and that isn't calibrated even to the camera's own images. Between competent use of an accurate light meter and a DSLR I would use the light meter anytime, if for nothing else because the screen on a DSLR is horrible :P I usually can't recognise the images on my screen later either!

    The other problem of course is different cameras are calibrated differently in terms of exposure and exposure response, and then have the aforementioned different tone curves. So for example my D300 has a tendency to overexpose that my D3 doesn't have, while overall the D300 has brighter shadows - and tbh I've been meaning to tweak the curve for ages now.

    So it's going to be a little bit difficult to compare a random DSLR with film (and what film? :P) And also, digital has more DR particularly in the shadow areas, so I always find film blocks up in the shadows far more quickly, but I'm not sure if that's down to differences in tone curves or just poorer DR (my inclination being the latter).

    Remember also if you've used any lens movements on the LF, you'll be needing to compensate for that.

    I wasn't aware that some manufacturers calibrate to 12% grey... do you know which ones?

    Can't answer your light meter question I'm afraid, the incident ones I've used are all pretty accurate and if they're accurate to even 1/3 of a stop then their performance is linear enough for me

  20. #60

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

    Hi, There are incident, reflected and ambient lights in outdoor shootings and artificial lights indoor. It depends on what camera type, high-end with high ISO, e.g. 1DMk4, D3S which are able to shoot harsh or low-light situations outdoor compared with low-end or lower priced cameras where they are not able to shoot bright and shiny objects. Shooting ambient light is easy for exposure. Cameras are equipped to shoot as many lighting conditions as possible. Some times, flare occurs etc. Exposures have latitudes and exposure compensations do the necessary corrections. Of course, there are difficult lighting conditions where the best camera is not able to expose correctly, that accounts for post processing.

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