"Exposing to the right" is over exposed, what now?
in [Photoshop]
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From: Doug McDonald on 8 Oct 2009 10:37 Floyd L. Davidson wrote: I have performed tests on my Canon 30D camera to see what happens if I expose less. I exposed 1/3 and 2/3 stops less than before. At 2/3 stop less than before yes, even at contrast setting zero, I do see a teensy bit of space between the top of the red histogram and the top of the display. This means that if I'm willing to lose 2/3 stop of unclipped data at the bright end, Floyd is "effectively" right. But if I want that extra 2/3 stop, I'm still correct. I should add that my method works --- I use -4 in the camera, use the histogram, and find my raw images are very nicely exposed, no clipping (except as desired for specular highlights) and the data right up to the clipping level. Doug McDonald
From: Doug McDonald on 8 Oct 2009 10:40 Floyd L. Davidson wrote: > Hence if the right edge is set correctly with high > contrast or low contrast, either way it is exactly the > same exposure and neither is more accurate than the > other. > > You continue to say that low contrast makes it easier to > see where the edge is, but that is only true for special > cases, and for an equal number of special cases high > contrast would make it easier! > In NO CASE would high contrast make it easier ... at least on a Canon 30D. Mr. Davidson: Let me ask again: What is the serial number of the Canon 30D you have checked this on. If you have not done so, SHUT UP. We admit that you are right if somebody using a Canon 30D is willing to allow 2/3 stop more "slop" than is really necessary. Doug McDonald
From: Paul Furman on 8 Oct 2009 12:31 Floyd L. Davidson wrote: > Paul Furman <paul-@-edgehill.net> wrote: >> Floyd L. Davidson wrote: >>> That is exactly the point I *am* making. WB changes >>> the histogram in a useful way. The Luminous-Landscape >>> article did not talk about WB at all. They discussed >>> changing the camera's setting for contrast, and that >>> simply does *nothing* useful for the histogram/exposure >>> issue. And if you doubt that the contrast setting will not >>> change the histogram as stated, *try* *it*. I gave an step by >>> step description of a very easy way to show exactly what >>> does and does not happen. Why argue from supposition when >>> you could actually learn something about photography... >> >> I am interested in learning. I tried fiddling with the camera contrast >> settings to compare histograms and the difference was very minor, at >> least for the conditions I tried just now. A second try showed a significant difference. >> I was thinking of >> post-processing where increasing contrast blows highlights & blocks >> shadows. Maybe not the very final edge of 100% saturation but contrast >> adjustments can make a big difference in what appears to be blown or >> blocked. What you are describing sounds more like a gamma adjustment >> than contrast where only the middle part shifts. Scratch that comment, they both have an effect. > "Gamma is equivalent to contrast. I dunno, maybe your linked definers are thinking of a different context. Common sense and experience tell me gamma is adjusting the middle-value, more like brightness, without changing the the ends. Contrast adjustments tend to hold the mid-point and use an s curve to raise highlights & lower shadows or the inverse. A gamma adjustment curve is a fairly simple bowed shape, not an s shape. Sure a gamma adjustment has an effect on contrast but that's a side effect, not the goal. Gamma adjustments boost contrast in either the highlights or shadows, not both. > This can be observed > in traditional film curves, which are displayed on > logarithmic scales (typically, density (log10(absorbed > light) vs. log10(exposure)). Gamma is the average > slope of this curve (excluding the "toe" and > "shoulder" regions near the ends of the curve), i.e., > the contrast." > http://www.imatest.com/docs/glossary.html > (The above is a web page owned by Norman Koren.) > > "Gamma -- A way of representing the contrast of an > image, shown as the slope of a curve showing tones > from white to black." > http://www.hp.com/united-states/consumer/digital_photography/articles/scanner_glossary.html > > That might help make things clearer as to why changing > contrast does not make the histogram any more or less > accurate for exposure. > > When the histogram indicates precisely the correct > exposure, changing the gamma (contrast) moves the curve > between black and maximum white values, but it doesn't > change the value of either. Note too that it might > spread the histogram out from the center (if the gamma > curve is moved by picking a point at its center), or it > might move towards ether the right or the left (if the > gamma curve is moved from point closer to the ends > instead of at the center). > > In any case, it changes the contrast but if there are > image values at the maximum white value they will not be > moved. The maximum white value may be already off the edge so you are talking about part of the middle of the curve, which is effected, showing more detail with low contrast. >> An interesting related issue I don't understand is how the exposure >> slider works in Lightroom or ACR. I don't know how to duplicate that >> effect in photoshop with curves, levels, etc. Those all do like you >> describe, moving the middle parts of the histogram but there isn't an >> easy way I can see to shift the whole exposure. Hmm, the middle slider >> on levels comes close but still doesn't match the effect. > > I don't use those programs so I can only provide general > instructions about how that is typically done. I'll > leave it to someone who can be more specific. Understanding this would help in simulating exposure changes in an editor - to better understand when shooting & interpreting the histogram. -- Paul Furman www.edgehill.net www.baynatives.com all google groups messages filtered due to spam
From: Floyd L. Davidson on 8 Oct 2009 14:18 "DRS" <drs(a)removethis.ihug.com.au> wrote: >"Floyd L. Davidson" <floyd(a)apaflo.com> wrote in message >news:87ab02ql1h.fld(a)apaflo.com >> "DRS" <drs(a)removethis.ihug.com.au> wrote: > >[...] > >>> I have always said it changes the indication of exposure. It is you >>> who made the false claim that I and others were saying it had >>> anything to do with changing the actual exposure, which we have >>> repeatedly denied. >> >> *All* *five* *show* *the* *exact* *same* *exposure*. > >Yes, how many times do I have to tell you that before you get it? > >> Not one of them shows any 1 f/stop difference as you >> claim. > >Yes, they do, in terms of *accuracy*. You simply do not know how to read a histogram, and until you learn there is little point in talking to you on any topic that requires it. >> The only thing that moves 1 f/stop are the tall peaks in >> the graph that have *nothing* to do with exposure. It >> is well to the left of the right edge, and it is that >> right edge that indicates exposure. > >The right edge moves. It doesn't. You are *not* looking at the right edge, you are looking at a peak that is left of the edge. The peak moves, the edge does not. >>> And because of the different contrast settings the 5 histograms show >>> the exposure differently. The exposure hasn't changed but the >>> accuracy of its representation has. Which is all that has been >>> claimed. >> >> They *don't* show the *exposure* differently. > >Yes, they do. Everybody can see it except you. Everyone who knows how to read a histogram can see the same thing that I do. >>> None of the 5 histograms indicate overexposure. What they do >>> indicate, to different degrees of accuracy, is the room for exposure >>> compensation. Only 1 can be the most accurate and it is the -4 >>> histogram. >> >> All of them indicate there is no room for any >> "compensation". Increasing the camera exposure will >> result in clipping, and each of those histograms shows >> that. > >No, they don't. Try looking at what is there. > >[...] > >>> In this instance that indicates specular highlights, which as has >>> been noted by several people, may be blown without spoiling the >>> image. That is a choice by the photographer. >> >> The tall peaks are *not* a "specular highlights", and > >I didn't say they were. Do you ever pay attention? You can't even follow >when I'm addressing your argument! There are *no* specular hightlights in the images shown. >> You know, logically if what you said made sense the >> article would have discussed it in some way, but it does >> not even hint at it. If changing the contrast displayed >> a more accurate *exposure* (right edge location), why >> didn't they point to it? > >"What does all this mean? In order to display the full dynamic range of >your camera in the histogram displayed on the back, you must set the >contrast to minimum. You now can see the same dynamic range in the >histogram on the camera as you will see it in your RAW processor on your >computer." Nice switcheroo, but the article that shows the histograms says nothing like that. Here is what it says, in it's entirety: "Contrast adjustment Adjusting the tone alters the shape of the 'S curve' used to map the linear image data captured by the sensor into the correct gamma. A lower contrast setting maintains more of the original data's dynamic range but leads to a flatter looking image. A higher contrast setting stretches the grayscale (dark to light) of the image and could lead to clipping of both shadow detail and highlights. The Neutral Picture Style was used for the samples below." -- Floyd L. Davidson <http://www.apaflo.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd(a)apaflo.com
From: Floyd L. Davidson on 8 Oct 2009 14:29
Paul Furman <paul-@-edgehill.net> wrote: >Floyd L. Davidson wrote: >> Paul Furman <paul-@-edgehill.net> wrote: >>> Floyd L. Davidson wrote: >>>> That is exactly the point I *am* making. WB changes >>>> the histogram in a useful way. The Luminous-Landscape >>>> article did not talk about WB at all. They discussed >>>> changing the camera's setting for contrast, and that >>>> simply does *nothing* useful for the histogram/exposure >>>> issue. And if you doubt that the contrast setting will not >>>> change the histogram as stated, *try* *it*. I gave an step by >>>> step description of a very easy way to show exactly what >>>> does and does not happen. Why argue from supposition when >>>> you could actually learn something about photography... >>> I am interested in learning. I tried fiddling with >>> the camera contrast >>> settings to compare histograms and the difference was very minor, at >>> least for the conditions I tried just now. > >A second try showed a significant difference. > >>> I was thinking of >>> post-processing where increasing contrast blows highlights & blocks >>> shadows. Maybe not the very final edge of 100% saturation but contrast >>> adjustments can make a big difference in what appears to be blown or >>> blocked. What you are describing sounds more like a gamma adjustment >>> than contrast where only the middle part shifts. > >Scratch that comment, they both have an effect. > >> "Gamma is equivalent to contrast. > >I dunno, maybe your linked definers are thinking of a different context. I would expect that both Norman Koren and HP do know exactly what gamma and contrast are. The context is *exactly* the one we are discussing. >Common sense and experience tell me gamma is adjusting the middle-value, >more like brightness, without changing the the ends. A generalization, but not wrong. >Contrast >adjustments tend to hold the mid-point and use an s curve to raise >highlights & lower shadows or the inverse. Not true. Contrast is the slope of the curve. >A gamma adjustment curve is a >fairly simple bowed shape, not an s shape. Not necessarily. >Sure a gamma adjustment has >an effect on contrast but that's a side effect, not the goal. Gamma >adjustments boost contrast in either the highlights or shadows, not both. Not necessarily. >> This can be observed >> in traditional film curves, which are displayed on >> logarithmic scales (typically, density (log10(absorbed >> light) vs. log10(exposure)). Gamma is the average >> slope of this curve (excluding the "toe" and >> "shoulder" regions near the ends of the curve), i.e., >> the contrast." >> http://www.imatest.com/docs/glossary.html >> (The above is a web page owned by Norman Koren.) >> "Gamma -- A way of representing the contrast of an >> image, shown as the slope of a curve showing tones >> from white to black." >> http://www.hp.com/united-states/consumer/digital_photography/articles/scanner_glossary.html >> That might help make things clearer as to why changing >> contrast does not make the histogram any more or less >> accurate for exposure. >> When the histogram indicates precisely the correct >> exposure, changing the gamma (contrast) moves the curve >> between black and maximum white values, but it doesn't >> change the value of either. Note too that it might >> spread the histogram out from the center (if the gamma >> curve is moved by picking a point at its center), or it >> might move towards ether the right or the left (if the >> gamma curve is moved from point closer to the ends >> instead of at the center). >> In any case, it changes the contrast but if there are >> image values at the maximum white value they will not be >> moved. > >The maximum white value may be already off the edge so you are talking >about part of the middle of the curve, which is effected, showing more >detail with low contrast. Maximum white values cannot go "off the edge". They go *to the edge*, and no higher. The maximum value does not change, though the number of pixels that have that value might change. >>> An interesting related issue I don't understand is how the exposure >>> slider works in Lightroom or ACR. I don't know how to duplicate that >>> effect in photoshop with curves, levels, etc. Those all do like you >>> describe, moving the middle parts of the histogram but there isn't an >>> easy way I can see to shift the whole exposure. Hmm, the middle slider >>> on levels comes close but still doesn't match the effect. >> I don't use those programs so I can only provide >> general >> instructions about how that is typically done. I'll >> leave it to someone who can be more specific. > >Understanding this would help in simulating exposure changes in an >editor - to better understand when shooting & interpreting the histogram. Yes, you will certainly find it useful. -- Floyd L. Davidson <http://www.apaflo.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd(a)apaflo.com |