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How to understand 18% gray?
why 18% gray is in the mid of the full scale of brightness from
darkest to brightest? what's the math behind it? |
#2
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How to understand 18% gray?
On Sun, 25 Nov 2007 21:37:51 -0800 (PST), Steven Woody
wrote: why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? It really means that 18% of incident light is reflected. Human perception of brightness is logarithmic, though, and not linear. Basically, logarithmic means we are multiplying instead of adding. Compound interest is logarithmic. So is the frequency of the notes on a piano. So is the decibel scale for measuring sound pressure. Likewise with the light plotted on a typical histogram. You can make (or imagine) a set of brightness patches by starting with 100% reflection and halving each time. The next is 50%, then 25%, 12.5%, 6%, 3%, and 1.5%. That's 7 patches. It is a logarithmic sequence; the factor is 1/2. The first patch is white, the last very dark gray, virtually black. In the middle it will be middle gray, 12.5% actually, a bit darker than 18% gray. You can fiddle with sequences like this to make as many patches as you like, with different gradations of brightness between them. Using a factor of 0.75, we get 13 steps, with the middle being 18%: 100 75 56 42 32 24 18 13 10 7.5 5.6 4.2 3.2. Ed |
#3
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How to understand 18% gray?
In article
, Steven Woody wrote: why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? It can be mid-way for exponential gamma. The exponential gamma has its origins in old tube TV circuits but it remains in use because it models visual perception reasonably well. It shifts more digital levels into areas where the eye is most sensitive. Few things in the natural world are linear. |
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How to understand 18% gray?
"Steven Woody" wrote in message ...
why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? See: doug.kerr.home.att.net/pumpkin/Scene_Reflectance.pdf and doug.kerr.home.att.net/pumpkin/Exposure_metering_18.pdf Cheers -- cmyk |
#5
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How to understand 18% gray?
"Steven Woody" wrote: why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? While the "logarithmic" answer people have given is correct, it's easier to see if you remember that a one stop difference means a twice the brightness. So if you have mid-gray at 18%, you get five (that's all, folks) useful tones on a print: 4.5%, 9%, 18%, 36%, and 72%. If you toss in 0% and 100%, that makes 7 (although you probably won't be able to see the difference between 4.5% and 0). David J. Littleboy Tokyo, Japan |
#6
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How to understand 18% gray?
"David J. Littleboy" wrote in message ... "Steven Woody" wrote: why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? While the "logarithmic" answer people have given is correct, it's easier to see if you remember that a one stop difference means a twice the brightness. So if you have mid-gray at 18%, you get five (that's all, folks) useful tones on a print: 4.5%, 9%, 18%, 36%, and 72%. If you toss in 0% and 100%, that makes 7 (although you probably won't be able to see the difference between 4.5% and 0). David J. Littleboy Tokyo, Japan Perhaps a usefulness not yet explained or stated is for matching colour. I use a white, black and grey card (home made) in the first shot of a scene. I use it with the "levels" function of Photoshop and so far (maybe 1000 frames) it works better than nearly any other post shoot white balance. I don't believe digital photography has the same use for a grey card as film shooters have. Douglas |
#7
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How to understand 18% gray?
Steven Woody writes:
why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? It isn't exactly in the middle, really. Any given scene has a darkest tone you want to record and a brightest tone you want to record, and the middle tone for that particular scene is halfway in between. Now, it's not halfway between in linear-light space, because the eye responds logarithmically. Suppose you have a scene whose brightness ranges from 1 to 100 on some arbitrary (but linear) light meter scale. To the eye, a brightness of 50 is "one step" darker than 100, and 25 is an equal-sized step darker than 50, 12 is another step darker again, and so on. So the midtone for this particular scene is actually a brightness of 10, in the sense that it is just as much brighter than the minimum (10X) as the maximum is brighter than the midtone (10X). One general way to calculate this is: midtone = sqrt(min_bright * max_bright) This value is called the "geometric mean" of min and max brightness. You can also average logarithms: midtone = exp( (log(min_bright) + log(max_bright)) / 2) The magic value of 18% comes from somewhere else. Someone did a study and found that the average reflectance of a large number of photographic scenes was 18%. So if they set their incident light meter to assume that light from the scene was 18% of the light falling on the meter, they would (on average) get a good exposure. Similarly, if they had an 18% reflectance grey card, and measured the light on it with a reflected light meter, that would also be a good exposure. It happens that, in many scenes, the average scene brightness is actually pretty close to the middle tone (halfway between darkest and lightest areas of interest), so it's also useful to expose as if it was the middle tone, giving equal range above and below it. But that's not necessarily true, and spotmeters and the zone system are tools for more accurately figuring out what the tonal range of your scene is, and how to place that relative to what your film (or sensor) can capture. Dave |
#8
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How to understand 18% gray?
Steven Woody wrote:
why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? Go back 100 years or so and ask. -- Neil reverse ra and delete l Linux user 335851 |
#9
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How to understand 18% gray?
Steven Woody wrote:
why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? The gray card is a development of the "artificial highlight" method of exposure metering. Before the gray card was invented, Kodak recommended using a 90% white card and setting your light meter at 1/5 of the normal film speed rating. The gray card simplifies this because you don't need to change your meter setting. 90/5 = 18 Peter. -- |
#10
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How to understand 18% gray?
"Douglas" wrote: "David J. Littleboy" wrote: "Steven Woody" wrote: why 18% gray is in the mid of the full scale of brightness from darkest to brightest? what's the math behind it? While the "logarithmic" answer people have given is correct, it's easier to see if you remember that a one stop difference means a twice the brightness. So if you have mid-gray at 18%, you get five (that's all, folks) useful tones on a print: 4.5%, 9%, 18%, 36%, and 72%. If you toss in 0% and 100%, that makes 7 (although you probably won't be able to see the difference between 4.5% and 0). Note that Dave M. has provided a much better discussion of this issue than the above. Perhaps a usefulness not yet explained or stated is for matching colour. I use a white, black and grey card (home made) in the first shot of a scene. I use it with the "levels" function of Photoshop and so far (maybe 1000 frames) it works better than nearly any other post shoot white balance. I don't recommend this. I have two Kodak gray cards here that have radically different color compositions: one is somewhat blue, the other quite warm. David J. Littleboy Tokyo, Japan |
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