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I Don't Understand the Luminous Landscape "Expose Right" Article
I Don't Understand Luminous Landscape "Expose Right" Article
Anyone wishing to understand the article could run this simple test ... pick a fairly monotone subject (sky, gray card, etc) and shoot it in RAW mode at ISO 400 metered properly, then set your exposure compensation +2 and shoot it again, then -2 and shoot it again. When you convert the three RAW files don't make any exposure adjustments to the first image, adjust the +2 image by -2 stops, adjust the -2 image by +2 stops. You should have roughly equivalent exposures by now if you convert these ... check the noise levels of the three "equivalent" exposures" and you'll see what he means. You could have gotten the same improvement that the +2 image gives by shooting as metered at ISO 100. Try it and see. |
#2
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The article is discussing the behavior of digital cameras not film. How
many levels you have in film is not so clear cut, you can deal with the SN of film and get some sense of it, Roger Clark has done a lot of work on this, see this link http://clarkvision.com/imagedetail/d...ignal.to.noise Scott |
#3
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In message .com,
"Bill Hilton" wrote: I Don't Understand Luminous Landscape "Expose Right" Article Anyone wishing to understand the article could run this simple test ... pick a fairly monotone subject (sky, gray card, etc) and shoot it in RAW mode at ISO 400 metered properly, then set your exposure compensation +2 and shoot it again, then -2 and shoot it again. When you convert the three RAW files don't make any exposure adjustments to the first image, adjust the +2 image by -2 stops, adjust the -2 image by +2 stops. You should have roughly equivalent exposures by now if you convert these ... check the noise levels of the three "equivalent" exposures" and you'll see what he means. This should be true, but some RAW converters treat RAW data based upon their absolute values, and not the values after a purely linear "exposure" adjustment. ACR, for example, nails the highest RAW values to 255 in the output, no matter if you use -4 "exposure". You could have gotten the same improvement that the +2 image gives by shooting as metered at ISO 100. Not exactly; ISO 400 at +2 EC is usually better quality than ISO 100 at 0 EC, if the RAW data doesn't clip. Try it and see. -- John P Sheehy |
#4
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Scott W wrote:
The article is discussing the behavior of digital cameras not film. How many levels you have in film is not so clear cut, you can deal with the SN of film and get some sense of it, Roger Clark has done a lot of work on this, see this link http://clarkvision.com/imagedetail/d...ignal.to.noise And figure 5 on this page: http://clarkvision.com/imagedetail/dynamicrange2 which does discuss film/digital I still find that one shocking, particularly given the implication that digital has less info in the shadows than digital highlights. But still digital has more information in the shadows than film. But that is not really what the luminous landscape article is discussing I think. They are talking about how RAW digital files carry more info in the highlights due to their being linear data of simple intensity. That's the best I can understand anyways. Brighter means bigger number in that linear scheme so there are more significant digits to assign finer gradations of tonal range. Once it's converted from RAW, that extra info is dropped. During the RAW conversion process, there is an opportunity to sneak up & expand that highlight information before dropping it. One result of this effect (if I understand correctly -and I'm unsure) is that an unmanipulated RAW image looks too dark & muddy. Most of the information is crammed in the highlights. A curve needs to be applied to the image to increase contrast back to what our non-linear eyes expect to see. Maybe someone can comment on whether the diagrams below correctly explain the situation. Normal scenes appear to our eyes with most of the complexity in the middle ranges and not a lot going on in the highlights or shadows. Digital RAW format contains extra highlight data which is generally useless unless you stretch it to the left before clipping to a normally curved format. So yes RAW has less info in the shadows but it has all it needs as long as you don't need to adjust the contrast later. I think film is in the same boat on the shadow end of the chart, in fact according to the figure 5 mentioned above, film has less shadow detail and less room for adjusting highlight detail. ______________________________ | | | | | | | l | | | | | e | | | | | | v | | | | | | | e | | | | | | | | l | | | | | | | | | s | | | | | | | | | | |_|__|__|__|__|__|__|__|__|__| shadow highlight 1. LINEAR RAW DATA ______________________________ | | l | | e | | v | | e | | l | | | | | s | | | | | | | | | | | | | | | | |_|__|__|__|__|__|__|__|__|__| shadow highlight 2. NORMAL (FILM, EYE, TIF, JPEG) -Bell Curve Or I could be misunderstanding. I'm just trying to learn as I go. |
#5
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paul wrote: One result of this effect (if I understand correctly -and I'm unsure) is that an unmanipulated RAW image looks too dark & muddy. Most of the information is crammed in the highlights. A curve needs to be applied to the image to increase contrast back to what our non-linear eyes expect to see. The raw file holds the light values for the pixel in a linear format, the normal jpg format is going to hold the data in a non-linear format to fit more dynamic range in the 8 bits. This is done with the gamma of the image, the light level is proportional to the pixel level raised to the gamma power. A common gamma that is used is 2.2, this is what photos that are in the sRGB color space use. This also turns out to be the gamma of most PC monitors, MACs use a lower number of 1.8. So it has nothing to do with our non-linear eye and everything to do with our non-linear monitors. Scott |
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John Sheehy writes ...
ACR, for example, nails the highest RAW values to 255 in the output, no matter if you use -4 "exposure" We must be talking about something totally different, because this is total nonsense ... in the test shots I mentioned (monotone subject -- in my case a heavily overcast sky -- at 0, +2 and -2) a straight conversion using ACR of each image leaves me with these RGB values for the brightest part of the sky ... 60/67/79 for the -2 shot, 129/140/165 for 0, 223/229/246 for the +2. Think about it ... if what you say is correct (it isn't) then every monotone image would get mapped to white, which obviously doesn't happen. You could have gotten the same improvement that the +2 image gives by shooting as metered at ISO 100. Not exactly; ISO 400 at +2 EC is usually better quality than ISO 100 at 0 EC Run the test and compare the noise levels. That's what I did. ISO 100 looks the same as ISO 400 at +2 corrected for exposure with Capture One .... Bill |
#7
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jim evans writes:
Last fall I emailed Thomas Knoll this question. After he did not reply, early this year I emailed the question to Michael Reichmann. He did not reply either. I have posted a copy of the question I sent Reichmann at http://factsfacts.com/LuminousLandscape.htm. The table in the article shows the tonal values of an image shot with a digital camera as a geometric distribution, and implies that the tonal values of film are distributed linearly. Are the tonal values of film really distributed linearly in this context? First, the digital camera response isn't a "geometric distribution". The response is linear. But the terms in the left-hand column of your chart are logarithmic. The top row is labelled "within the first f/stop", which by definition contains all tones between 1/2 of maximum and maximum. Naturally, because the sensor and A/D converter are linear, half of the available codes are assigned to this half of the brightness range. The problem with this approach is that the *eye* isn't linear. It sees the step from full brightness to 1/2 brightness as being about the same size as the step from 1/2 to 1/4, which is the same as from 1/4 to 1/8. So if you have a limited number of bits available (e.g. 24 bit colour, with only 256 codes per colour), you're better off to use a nonlinear transformation to spread the available codes so that each stop of range gets something like the same number of codes. This is in fact done. Second, what do you mean by "the tonal values of film are distributed linearly"? Film itself is analog; it has no levels or steps. Digitized film *does* have levels, but the distribution of them depends on the software running the film scanner. Some film scanners output data that is proportional to film density, which is the logarithm of transmittance. In this case, there will be about the same number of code values per each one-stop range of input brightness. Some scanners output linear data - but it's linear in film transmittance, not original scene brightness. And there are other possibilities. Basically, your question makes no sense unless the right column is about *some specific film, scanner, and software* combination. Film itself has no levels. If you reply please do so as requested in my question to Reichmann. That is, please either agree that my "Film" column in the table is correct, or give the 5 values that should be in the Film column. Your question is unanswerable as posed. There is no answer that would apply to all film scanners. Dave |
#8
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On Fri, 25 Feb 2005 10:16:37 -0600, jim evans
wrote: I Don't Understand Luminous Landscape "Expose Right" Article ----snip---- If you reply please do so as requested in my question to Reichmann. That is, please either agree that my "Film" column in the table is correct, or give the 5 values that should be in the Film column. Jim: I don't understand why you are trying to make a comparison to film. That article is strictly about digital photography, one of the quirks of using an electronic sensor, and how to compensate for it. It has nothing to do with using film, it implies nothing about the behavior of film. There is no connection to using film. Drifter "I've been here, I've been there..." |
#9
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Scott W wrote:
paul wrote: One result of this effect (if I understand correctly -and I'm unsure) is that an unmanipulated RAW image looks too dark & muddy. Most of the information is crammed in the highlights. A curve needs to be applied to the image to increase contrast back to what our non-linear eyes expect to see. The raw file holds the light values for the pixel in a linear format, the normal jpg format is going to hold the data in a non-linear format to fit more dynamic range in the 8 bits. This is done with the gamma of the image, the light level is proportional to the pixel level raised to the gamma power. A common gamma that is used is 2.2, this is what photos that are in the sRGB color space use. This also turns out to be the gamma of most PC monitors, MACs use a lower number of 1.8. So it has nothing to do with our non-linear eye and everything to do with our non-linear monitors. I didn't get all of what you are saying but I was reading some other articles about a way of converting RAW in a linear format to 16 bit TIF. Done that way, the image looks almost pure black but when curves are applied, it can be adjusted to get more info out of high contrast images, less blown highlights. The thing is it usually messes up the colors so the technique is to make a B&W image with the linear conversion then do another normal one for color & merge the two. I think Dave is on to something about the logarithmic scale of stops versus linear digital capture though that seems such an extreme difference. Probably it has more to do with that than my bell curve histogram idea. |
#10
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jim evans wrote:
On Sat, 26 Feb 2005 10:41:49 -0500, Drifter wrote: On Fri, 25 Feb 2005 10:16:37 -0600, jim evans wrote: I Don't Understand Luminous Landscape "Expose Right" Article ----snip---- If you reply please do so as requested in my question to Reichmann. That is, please either agree that my "Film" column in the table is correct, or give the 5 values that should be in the Film column. Jim: I don't understand why you are trying to make a comparison to film. It has nothing to do with using film, it implies nothing about the behavior of film. There is no connection to using film. That article is strictly about . . . one of the quirks of using an electronic sensor . . . Versus what? If it's a quirk, what doesn't have this "quirk?" Film maybe? It's a tip on how to take advantage of 'bonus' data hidden in RAW digital files. |
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