Are these related? In my search for a film scanner I see some manufacturers specify DMax (like Epson) and others (like HP) bit depth, but I haven't noticed both specified together.
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DMax and bit depth
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I see Dmax as the overall range and bit depth as the number of increments within this range. To me, a good scanner should offer the option of 8 and 16 bits per channel.
OP
OP
One of the scanners I'm looking at ( an HP G4050) specifies a bit depth of 96 bits. How many "channels" are usually present?
Three, one for R, G and B each. This is apparently an 32 bit per channel scanner.
No, I think 96 is probably 24x4: r+g+b+i, 24 bit each. i=intensity, which is kinda like a grey scale applied to the other colours, thereby expanding the total # of colours. Through the "i" channel you get brighter and darker variants of each of the other colours. At least that's my recollection, and offhand I am not finding any explanatory articles to link to...
my understanding is that Dmax is now determined by the bit depth. When I started photography Dmax was a measurement of maximum density and Dmin was a measure of minimum density. About 5 years ago I measured the density of a strip of exposed film and then scanned it on a a scanner that claimed a Dmax of 3.8 or 4.2 (or some astronomically high number), but according to the densitometer reading of the film the scanner had a Dmax of around 2.6 or 2.8. If I recall correctly Black and white and transparencies can achieve a dmax in the mid to high 3s and c41 tends to max out at around a Dmax of 3.0. If all that I am saying is correct then a properly exposed scene will exceed the capabilities of the scanner I tested. I may be wrong on the exact numbers, but my experience is that most scanners fall well shy of reading into to the denser areas of film. On the flip side they tend to do an excellent job in the thinner areas.
Dmax is number I tend to ignore when reading the specs of most digital devices.
Dmax is number I tend to ignore when reading the specs of most digital devices.
I don't think so JD, offhand. I think the bit depth and DMax (also DMin) are quite separate specs. That said, if you have really thick parts of a neg (or slide) then the gradients near those regions will typically be quite fast there, and high bit depth becomes very important to getting that last bit of tonal detail.
Anyway, DMax-DMin is important, people usually neglect DMin, assuming it to be zero. But because of noise and other factors, Dmin isn't zero. What alas is not spec'd for a typical scanner is the noise level per pass. You can have high DMax and high bit depth and still get a lot of noise in the thickest and thinnest parts of a slide or neg.
In my experience the biggest challenge for any scanner is velvia. It can present ridiculous density range to the scanner, something like 4.2 DMax and DMin near zero. Compared to that neg film is easy peasy as long as you have the bit depth to handle how many tones are represented in the much tighter density range.
Anyway, DMax-DMin is important, people usually neglect DMin, assuming it to be zero. But because of noise and other factors, Dmin isn't zero. What alas is not spec'd for a typical scanner is the noise level per pass. You can have high DMax and high bit depth and still get a lot of noise in the thickest and thinnest parts of a slide or neg.
In my experience the biggest challenge for any scanner is velvia. It can present ridiculous density range to the scanner, something like 4.2 DMax and DMin near zero. Compared to that neg film is easy peasy as long as you have the bit depth to handle how many tones are represented in the much tighter density range.
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I don't think Dmax has anything to do with bit depth. You can have a scanner maxing out at a density of 4.0, but that doesn't tell you if the equipment can deliver 8, 16 or 24 bits per channel. Dmax is a measure of the overall range of tones and bit depth is a measure of the number of different tones in between.
Okay here is a link that offhand looks fairly well written:
Tutorial: Dmax: Optical Density - Pixmonix
Tutorial: Dmax: Optical Density - Pixmonix
Okay, this is a new one to me, HP speaks of a '6 color' 96 bit palette, i.e. 16 bit per channel x6.
http://www.hp.com/hpinfo/newsroom/press_kits/2007/ces/bg_dp_6color.pdf
I wasn't aware of this. Interesting. So the 96 mentioned above is probably not rgbi at 24 bits/channel as I previously guessed.
http://www.hp.com/hpinfo/newsroom/press_kits/2007/ces/bg_dp_6color.pdf
I wasn't aware of this. Interesting. So the 96 mentioned above is probably not rgbi at 24 bits/channel as I previously guessed.
Reads OK. Dmax and bit depth are two different kettle of fish.
Just imagine the file size!
OP
OP
On reading the reviews, the file size seems to create problems for a lot of computers (or the software).
Yeah, filesize is a huge problem! This is why I was saying somewhere, the other day, that there really should be some intermediate image processing (mostly downsampling) done between the scanner and the computer. In other words the scanner would go full tilt, but that resulting file would be downsampled. Our scanned files probably contain a vast excess of completely useless information. We could program in thresholds for the number of detectable tones and such and simply throw away all extra bits and pixels that do not contain real information....
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Yes and no, depending what we are talking about
The scanner is analog and digital device at the same time. The Dmax you can get from the film depends on the Dmax you can get from analog parts of CCD array AND from analog/digital converter. There is no simple relation, however, IF the scanner's CCD works in linear fashion, then Dmax you can get from 16-bit A/D converter is not bigger than LOG(1/2^16) ~ 4.8 (for 12-bit A/D converter is not bigger than 3.6D). Manufacturers tend to advertise this number, not the real Dmax.
Do not confuse A/D converter resolution with the TIFF file bith depth. This is indeed different kettle of fish.
Thanks for the clarification.
I assumed, we're talking about the bit depth of the image file. What does a manufacturer refer to when advertising a scanner as '48-bit color scanner' for example?
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Well, I suspect the manufacturer means 16-bits per color channel.
So what I am reading here and from the article is that the manufactures are citing a Dmax that is a theoretical max based upon bit depth. Which is more or less what I thought. From a practical stand point, the scanner needs to read into the denser areas of your film. You can't know whether or not a given scanner can do that based upon their stated dmax.
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If either your equation or JaZ99's are used by the manufacturers to create their Dmax claims then the Dmax is based upon bit depth or am I still misunderstanding?
I'm not sure about that, because I've seen many scanner claiming 16-bit scanning without Dmax claims going as far as 4.8D. Anyway, I don't need 4.8 Dmax capability as much as I need 16-bit scanning. I need smoothness of tones for normally developed negatives, and they have a Dmax of less than 1.8D while still returning a 16-bit scan. But then again, maybe that's a hoax. How can I tell?
I'm afraid, we're still confusing A/D converter resolution with the bith depth of the image file.
I'm afraid, we're still confusing A/D converter resolution with the bith depth of the image file.
If you want higher DMax from a scanner, you just use a brighter bulb. Simple. Has nothing to do with bit depth.
If we start talking about how tones you can extract from the range between DMax and DMin, then bit depth comes into play. But DMax alone... there's no physical problem making that 4 or 5 or 6 or 8 or 20... and you can sample that at 2 bit or 8 or 16 or whatever you please.
Am I missing something??? Are we talking about different issues?
In any case: caveat emptor, DMax-DMin is the important number. Not DMax alone.
If we start talking about how tones you can extract from the range between DMax and DMin, then bit depth comes into play. But DMax alone... there's no physical problem making that 4 or 5 or 6 or 8 or 20... and you can sample that at 2 bit or 8 or 16 or whatever you please.
Am I missing something??? Are we talking about different issues?
In any case: caveat emptor, DMax-DMin is the important number. Not DMax alone.
Yeah...there appears to be a lot of confusion here...and the geek in me feels the need to try and clear some of it up. And manufacturers do a lot to obscure facts, so that doesn't help us, either.
Here goes:
1) There are 2 bit-depths at play here, the bit depth of the output file, typically 8-bit or 16-bit *per channel (RGB + optional Infra-Red), and the bit depth of the A/D converter in the scanner...anywhere from 8-16 bits (or 24, with the HP, apparently). We're interested in the bit depth of the A/D converter, a 16bpc output file can accommodate any A/D converter up to 16 bits. i.e. it's easy to put 3/4 a gallon of milk into a gallon container.
2) 8 bits = 256 distinct levels (2^8)
10 bits = 1024 distinct levels (2^1024)
...
16 bits = 65536 distinct levels (2^16)
3) What really matters is dynamic range (Dmax-Dmin). But for practical purposes, lets assume the Dmin of slide film=0, so that Drange = Dmax, since on many scanners, the scanning time or exposure can be increased so that Dmin of the film yields a full-scale reading from the A/D, effectively rendering Drange=Dmax. It seems when most manufacturers talk about Dmax, the either mean Drange, or they assume Dmin=0, which is effectively the same thing.
4) The A/D converter operates linearly. All gamma encoding, ICC profiles, etc. are all applied after the scan. So for an 8-bit scanner, the very brightest thing it can sense will yield an output value of 255, and the darkest, 0. This is how you get 256 steps for an 8-bit scanner. Therefore, the darkest thing it can scan is 256 times darker than the brightest thing it can scan. A brightness range of 256 = 8 stops = 2.4 logD. As you can see from (1) every additional bit of bit depth = a doubling of the number of levels, which results in a halving of the minimum brightness level, so it's easy to equate 1 bit = 0.3 logD of brightness range, and that's theoretically correct. Now, this theoretical 8-bit scanner can scan through something that's denser than 2.4 logD, say 3.2. But in order to do so, everything lighter than 0.8logD must all register as full-scale (completely white), because the dynamic range (Drange) can *never* exceed 2.4 for a theoretical 8-bit scanner, 3.0 for a theoretical 10-bit scanner, etc.
4A) Now, if you're scanning, say B&W neg film, which may only have a density of 2.0, any decent 10-bit scanner and up should be able to handle it without much problem. In this case, the scanner is capable of a greater brightness range than the subject presents. So film base will be white (or nearly so), but the maximum density on your film will render as some dark gray. No problem, simply set the black point (a la PS "Levels" tool) and you're good to go. The scanner has basically "exposed-to-the-right" (for all you digi-cam users out there), using the least-noisy bits of the A/D, and setting the black point then (mathematically) "stretches" the data to cover the full 16-bit range.
5) So theoretically, a 16-bit A/D in a scanner has a Dmax (Drange) of 4.8 logD. But from a practical standpoint, that number is lower, due to any number of factors, which together we refer to as "noise". An excellent sampling system may only have 1 bit of equivalent noise, meaning that the average noise level in a 16-bit scanner is 65536 times smaller than the full-scale signal. An ordinary system may have 2-3 bits of noise, and a poorly-designed system may have as many as 6 bits of noise (meaning that the average noise level is now only 1024 times less than the full-scale signal, effectively creating a 10-bit scanner). Now, some noise is random, so if we acquire each sample multiple times (or do multiple scan passes in perfect registration), we can reduce the random noise in a system. But there is also non-random noise in the system. Hi-ISO fans of DSLR's may know this as "banding" or "fixed-pattern" noise. Nothing short of re-designing the scanning system will eliminate this noise.
So a disreputable scanner manufacturer might say 16-bit output file = 4.8 Dmax.
A lazy manufacturer (or a good one driven by it's marketing department) may say 16-bit A/D = 4.8Dmax (theoretically correct, but less, in practice, due to noise).
A good manufacturer not driven by it's marketing department will actually test it's design and report a number that comes out of that test...but I don't know if there are any standards that govern that, kinda like "Watts" for a speaker or amp.
Here are some examples, all with 16-bit A/D's, Dmax quoted by the manufacturer:
Nikon LS-5000 : "Density Range: 4.8D"
Epson V750 : "Optical Density: 4 Dmax "
PrimeFilm 7250Pro3 : "3.6 Dynamic Range"
Plustek 7600i : "Dynamic Range: 3.5"
Nikon appears to be the marketing scofflaw, here. Though, in practice, it probably does out-perform the other 3 in this regard (Dmax/Drange).
Anyway...that's a pretty dense post...I hope somebody benefits from it...
--Greg
Here goes:
1) There are 2 bit-depths at play here, the bit depth of the output file, typically 8-bit or 16-bit *per channel (RGB + optional Infra-Red), and the bit depth of the A/D converter in the scanner...anywhere from 8-16 bits (or 24, with the HP, apparently). We're interested in the bit depth of the A/D converter, a 16bpc output file can accommodate any A/D converter up to 16 bits. i.e. it's easy to put 3/4 a gallon of milk into a gallon container.
2) 8 bits = 256 distinct levels (2^8)
10 bits = 1024 distinct levels (2^1024)
...
16 bits = 65536 distinct levels (2^16)
3) What really matters is dynamic range (Dmax-Dmin). But for practical purposes, lets assume the Dmin of slide film=0, so that Drange = Dmax, since on many scanners, the scanning time or exposure can be increased so that Dmin of the film yields a full-scale reading from the A/D, effectively rendering Drange=Dmax. It seems when most manufacturers talk about Dmax, the either mean Drange, or they assume Dmin=0, which is effectively the same thing.
4) The A/D converter operates linearly. All gamma encoding, ICC profiles, etc. are all applied after the scan. So for an 8-bit scanner, the very brightest thing it can sense will yield an output value of 255, and the darkest, 0. This is how you get 256 steps for an 8-bit scanner. Therefore, the darkest thing it can scan is 256 times darker than the brightest thing it can scan. A brightness range of 256 = 8 stops = 2.4 logD. As you can see from (1) every additional bit of bit depth = a doubling of the number of levels, which results in a halving of the minimum brightness level, so it's easy to equate 1 bit = 0.3 logD of brightness range, and that's theoretically correct. Now, this theoretical 8-bit scanner can scan through something that's denser than 2.4 logD, say 3.2. But in order to do so, everything lighter than 0.8logD must all register as full-scale (completely white), because the dynamic range (Drange) can *never* exceed 2.4 for a theoretical 8-bit scanner, 3.0 for a theoretical 10-bit scanner, etc.
4A) Now, if you're scanning, say B&W neg film, which may only have a density of 2.0, any decent 10-bit scanner and up should be able to handle it without much problem. In this case, the scanner is capable of a greater brightness range than the subject presents. So film base will be white (or nearly so), but the maximum density on your film will render as some dark gray. No problem, simply set the black point (a la PS "Levels" tool) and you're good to go. The scanner has basically "exposed-to-the-right" (for all you digi-cam users out there), using the least-noisy bits of the A/D, and setting the black point then (mathematically) "stretches" the data to cover the full 16-bit range.
5) So theoretically, a 16-bit A/D in a scanner has a Dmax (Drange) of 4.8 logD. But from a practical standpoint, that number is lower, due to any number of factors, which together we refer to as "noise". An excellent sampling system may only have 1 bit of equivalent noise, meaning that the average noise level in a 16-bit scanner is 65536 times smaller than the full-scale signal. An ordinary system may have 2-3 bits of noise, and a poorly-designed system may have as many as 6 bits of noise (meaning that the average noise level is now only 1024 times less than the full-scale signal, effectively creating a 10-bit scanner). Now, some noise is random, so if we acquire each sample multiple times (or do multiple scan passes in perfect registration), we can reduce the random noise in a system. But there is also non-random noise in the system. Hi-ISO fans of DSLR's may know this as "banding" or "fixed-pattern" noise. Nothing short of re-designing the scanning system will eliminate this noise.
So a disreputable scanner manufacturer might say 16-bit output file = 4.8 Dmax.
A lazy manufacturer (or a good one driven by it's marketing department) may say 16-bit A/D = 4.8Dmax (theoretically correct, but less, in practice, due to noise).
A good manufacturer not driven by it's marketing department will actually test it's design and report a number that comes out of that test...but I don't know if there are any standards that govern that, kinda like "Watts" for a speaker or amp.
Here are some examples, all with 16-bit A/D's, Dmax quoted by the manufacturer:
Nikon LS-5000 : "Density Range: 4.8D"
Epson V750 : "Optical Density: 4 Dmax "
PrimeFilm 7250Pro3 : "3.6 Dynamic Range"
Plustek 7600i : "Dynamic Range: 3.5"
Nikon appears to be the marketing scofflaw, here. Though, in practice, it probably does out-perform the other 3 in this regard (Dmax/Drange).
Anyway...that's a pretty dense post...I hope somebody benefits from it...
--Greg
Sorry I guess I am dense
Dmax in film is a measurement of maximum density and in scanning manufacturers use Dmax interchangeable with Drange. Film with a dmax of 3.5 seems to defeat most any non-drum scanner (even though it states a dmax of 4.8) and the tonal range of film tends to be greater than the scaner. So when a manufacturer claims a Dmax s/he uses the bit depth of the A/D converter (but its not about bit depth?!) to come up with a number that is not a match in density or range of the film it is built to scan?
I'll just observe from here.
Dmax in film is a measurement of maximum density and in scanning manufacturers use Dmax interchangeable with Drange. Film with a dmax of 3.5 seems to defeat most any non-drum scanner (even though it states a dmax of 4.8) and the tonal range of film tends to be greater than the scaner. So when a manufacturer claims a Dmax s/he uses the bit depth of the A/D converter (but its not about bit depth?!) to come up with a number that is not a match in density or range of the film it is built to scan?
I'll just observe from here.
No, JD, you're not dense, you're actually thinking...the marketing departments don't like it when you do that.
The very short of it: Some scanner manufacturers equate 0.3 logD * [A/D bit depth] = D-Max spec. This is true, but only in the theoretical, perfectly noise-free ideal sense. Other manufacturers may publish some other number that claims to represent the Dynamic Range (or DMax), but without telling us the conditions under which the test was conducted...supercooled sensor in a Faraday cage? 128x oversampling? Who knows? I'm pretty sure you can't get 4.0logD out of a V750. Some manufacturers may publish realistic numbers, but again, without a described test method, it's difficult to tell the second case from the 3rd case, without testing each and every scanner.
It's all about spec-manship. How many scanners can really scan at 9600DPI? How many Shop-Vacs out there have 5HP motors that run on 120V/15A? Do the math, and you'll see that's not possible either.
--Greg
The very short of it: Some scanner manufacturers equate 0.3 logD * [A/D bit depth] = D-Max spec. This is true, but only in the theoretical, perfectly noise-free ideal sense. Other manufacturers may publish some other number that claims to represent the Dynamic Range (or DMax), but without telling us the conditions under which the test was conducted...supercooled sensor in a Faraday cage? 128x oversampling? Who knows? I'm pretty sure you can't get 4.0logD out of a V750. Some manufacturers may publish realistic numbers, but again, without a described test method, it's difficult to tell the second case from the 3rd case, without testing each and every scanner.
It's all about spec-manship. How many scanners can really scan at 9600DPI? How many Shop-Vacs out there have 5HP motors that run on 120V/15A? Do the math, and you'll see that's not possible either.
--Greg
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