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Scanning B&W negative (why scan in RGB ?)

Sassan Hazeghi , Oct 02, 2005; 05:43 a.m.

For scanning B&W (traditional silver) negatives, some people seem to prefer/recommend using RGB, rather than gray scale mode. Assuming a well calibrated scanner and a truly neutral (gray) negative (no purple tint and correct profile for the film base+fog) shouldn't all the color channels record identical reading ? If the dynamic range of a given channel is above that of the other two, I can understand the possible advantage of scanning in RGB mode and using that channel as the gray value (in which case, the gray scale scanning mode should be able to do the same on its own (use the most sensitive channel, rather than averaging the 3 color channels ?)

The file size of an RGB scan is so much larger than gray scale that I would like to scan my negatives in gray scale if there is no clear advantage to an RGB scan.

Thanks for any light that others can shed on this topic,



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Lex Jenkins , Oct 02, 2005; 06:01 a.m.

If you scan a b&w negative in RGB and split the channels into separate red, green and blue, you'll immediately see the difference. Not only is overall tonality dramatically different, so are noise and grain.

Scanning in RGB gives you more options to finesse the final output.

For example, you can selectively use noise reduction or grain reduction techniques, as well as sharpening and other adjustments, on just one channel rather than all three.

In other cases you may find that printing from just one channel produces the results you want.

Even if you decide to reduce the final image to grayscale, you will still get some benefits from making adjustments on the RGB scan.

John Kelly , Oct 02, 2005; 09:41 a.m.

I'm not convinced that RGB offers any advantage when you use Vuescan combined with printing "black only" with an Epson or using a proper B&W printer driver (as opposed to posting online or printing via minilab or printing directly from Photoshop). There seem to be only two proper printer drivers for B&W, and the only one that seems to make sense (according to people who report comparisons online) is QTR...quadtone.

One has tremendous controls in Photoshop with greyscale scans, more control than one has in a wet darkroom. However if a person loves digital tweaking, scanning RGB offers even more control...the value of the additional control is subjective..."more" isn't necessarily "better."

John Finch , Oct 02, 2005; 10:12 a.m.

Greyscale offers up to 256 shades of grey per scanned pixel. 256 levels between pure black and pure white. RGB offers potentially 16.7 million variations of colour per pixel. Therefore, potentially, you can store more information of your scanned image using RGB scanning. By the way, I scan the finished photograph, not the negative.

I think of my scanned photographs as digital negatives - for archiving my work to DVD. I want to store as much information about the scanned image as possible in my 'negative' and so always scan RGB. This captures every nuance of the picture inc. paper base colour... as well as the digital information of the greyscale of the image.

As software improves so will the ability of the program to use this digital information. I recommend saving as much info about the picture as possible and therefore using RGB.



Russ Rosener , Oct 02, 2005; 10:37 a.m.

This is a very good question. I use vuescan and Minolta film scanners. Vuescan seems very capable of producing a low noise, full scale grayscale scan. In fact, I have noticed more noise and grain in my RGB scans of B&W negs.

This is not to say that every negative will be better in grayscale. It very much depends on the film type, and output resolution whether grayscale or RGB is better. However lately I find myself scanning grayscale and adding color via Photoshop's Duotone function.

Efke 50. Rodinal 1:50, and Vuescan grayscale scan.

Steve Unsworth , Oct 02, 2005; 11:51 a.m.

I scan b&w as 16 bit grey scale. This gives many more than 256 levels, in fact it gives the same number as desaturated 48 bit RGB.


Eric Friedemann , Oct 02, 2005; 12:38 p.m.

This is a good question, as we're ultimately talking about shades of gray. However many distinct shades of gray you theoretically create in scanning, a only small fraction will be distinguishable in a finished print.

I've tried scanning the same images in grayscale and in RGB and have not seen a discernable difference in final output. I would note that I cannot apply PhotoKit Sharpener to a grayscale image. Then again, I cannot print using Quad Tone RIP unless an image is in grayscale.

So, I've been scanning and editing B&W images in Nikon's "Adobe" RGB. Then, I've been converting images to grayscale for printing and web use. This seems to be working pretty good.

Scanned in RGB, converted to grayscale

Steve Unsworth , Oct 02, 2005; 02:23 p.m.

Eric, I use the Photokit sharpener too. I scan in greyscale to save space and then convert to RGB before sharpening.

pericles lavat , Oct 02, 2005; 02:50 p.m.

You have to remember that when you scan in color, the image will be build in pixels at the rate of one third each chanel (RGB) So, if the final image is 9 MB then each chanel will take 3 MB. When you convert from RGB to gray scale or b&w, you eliminate 2 of the 3 chanels (usually leaves the red one) and you finish with an image with 3MB. The best solution is to only desaturate the image, the result will be a file that has the 9 MB and still will be an RGB file, with lots of info on it. In silver-based negatives, the original image is also a color one.

Eric Friedemann , Oct 02, 2005; 03:41 p.m.

Steve, you're right. I've converted some images to grayscale before final PKS output sharpening, then switched back into RGB for output sharpening without incident.

pericles, the question is whether all that excess data will ultimately be represented on a computer screen or in a print. Will your 9 desaturated MB of information produce an image that's sharper or richer in tones than a 3 MB grayscale image of the same scene? So far, I haven't seen any difference.

And again, the endgame for me is prints. If I started with an RGB scan and finish with a print from a desaturated RGB print, I'm nowhere, as my Epson 2200 produces suck-ass B&W prints without a RIP. With Quad Tone RIP, I'm getting some swell prints, but I have to print in grayscale.

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