Challanging assumptions about scanning B&W

[Ed Richards]

Back to my point on workflow. If you do adjustments in the scanner, you have to rescan to adjust the values. If you do it in Photoshop/PWP, you can save a master file that can be readjusted in the future without rescanning. Whether rounding errors matter or not, it is very useful to be able to readjust the B/W points and gamma as you work with the curves and local contrast, which you cannot do if it done in the scanner.

Kirk - I would not use a curve at all but use levels and gamma to get into the ball park. When I tried converting raw scans with curves, I got exactly your problem. As for noise, that drum scanner is going to beat flatbeds no matter what we do to optimize them.

Paul - I would be really surprised to find that the scanners have a wider internal data path. If so, why would they internally limit it to 12-14 bits as almost all do, and not at least broaden it to 16? I doubt that the sensors have that much dynamic range. Some scanners do change exposures when reading denser negatives. The two you have might be doing that. They might also just be scanning slower because the software is doing more calculations and they are bogging down on data transfer - I do not think they have much in the way of a buffer, and I know they dynamically adjust scanning speed to match data output rates. As for the insignificance of the rounding - in the worst case, which is pretty common, the original data in areas such as the sky is spread over a very narrow range and is being scaled out over a much wider range. If the entire range of original data is spread over just a few bytes worth of range, then rounding errors become much more significant as they compound.

[tim atherton]

" I would not use a curve at all but use levels and gamma to get into the ball park. When I tried converting raw scans with curves, I got exactly your problem. As for noise, that drum scanner is going to beat flatbeds no matter what we do to optimize them. "

I'd really urge anyone who is making raw scans and then converting them in PS to try out the NegPos plugin I detailed above (+ read the various documents he has up - especially if you are into the math of the stuff... he has a LOT of detail on that).

I have found it generally does by far the best job of either doing by "hand" (levels, gamma etc) or Silverfast Ai, or any of the other scanner software I have tried such as Vuescan

[Bruce Watson]

Ed,

It occurs to me that this discussion isn't going to help you much. What you need to do to resolve the issue is to run some tests on your own equipment. As the wise man said, "Why guess when you can know?"

This won't be hard or take long. Toss a favorite negative in the scanner. Scan it twice. Once in your "raw" mode. Once pulling in your white and black points and setting the constast curve. Process both in your favorite image editor making the best image you can each time. Make a print from each (why? because this is about prints, not what you see on your monitor) at your maximum size. Compare the prints side-by-side under the same lights.

If you like one better than the other, that's the method to use. If it's a tie, do it the way that easiest for you. Let us know what you decide.

[Ed Richards]

> NegPos plugin

I have read their documents and they seem consistent with what I am doing. Their plug in is designed for color - are you using it for black an white?

> It occurs to me that this discussion isn't going to help you much. What you need to do to resolve the issue is to run some tests on your own equipment. As the wise man said, "Why guess when you can know?"

I have been doing that for the past month:-) What I found is that you get a much better adjustment using levels and then curves, rather than curves alone, even in PWP which has a much better implementation of curves than does Photoshop. I think this is what people are doing when use the scanner software to preprocess their data by setting black and white points and gamma. I am betting if they try the same thing in Photoshop with levels and gamma before curves on a real raw file, they will get the same results they are getting with scanner processing. The fact that it reduces rounding errors is interesting, but probably only philosophical.

The real advantage I am finding is the iterative use of levels and curves together, using prints for fine tuning. You cannot do this iterative tuning if you preprocess in the scanner. I also change my mind about how an image should look, so I like to be able to readjust it without rescanning. Now this may just be me - I am sure the experienced folks know exactly where they want to be in the final print and what B/W points and gamma they need to get there before they scan the negative.:-)

[Kirk Gittings]

Ed your final point may be the real key. I am an old Zone System guy who "previsualizes" my images. I pretty much know right where I want to take the print before I ever expose the film. An old analoge way of workig that I have brought with me into scanning and priting.

[Chuck_1686]

Trying to understand this I'm confused about some scanner basics. I'm assuming the scanner sensor feeds some type of A/D converter. This converter might be adjustable by software to use the maximum range based on the negative density. If this is true it would seem like you would want to do scanner adjustments for each negative. I suppose that if this stage of the scanner was not adjustable and scanner software worked on the digital data then doing the adjustment later would be better.

[steve simmons]

There is a long and extensive article on scanning black and white in the Sept/Oct 05 issue of View Camera if anyone wants some additional info

steve simmons

[Paul Butzi]

Ed your final point may be the real key. I am an old Zone System guy who "previsualizes" my images. I pretty much know right where I want to take the print before I ever expose the film. An old analoge way of workig that I have brought with me into scanning and priting.

Kirk, are you saying that you don't do any tonal adjustment post-scan at all?

Wow. If that's a workflow that works for you, I'm impressed.

I'm like Ed - the very first thing I do to an image is add a 'levels' layer, adjust the overall levels (usually to be just a teensy bit 'soft' relative to where I want) and then add a 'curves' layer, which I use to adjust to overall tonality.

[Kirk Gittings]

Paul,

"Kirk, are you saying that you don't do any tonal adjustment post-scan at all?" I wish that were true as things would go allot quicker.

But, I do allot of post-scan "fine" adjustment. The original vision of that image never changes for me, but that original vision may "previsualize" allot of computer work to realize that original vision. As for my workflow issue it is just that if you scan the tones in the ballpark of where you want them in the final output then you create allot less artifacts in the overall process as you manipulate tones and contrast.

[Struan Gray]

I'm no expert, but I love playing one on the internet.

Scanners work by taking a voltage from a PMT preamplifier or a CCD cell readout amplifier, scaling it with an analogue gain circuit and then digitising it with an analogue-to-digital converter chip (ADC). There may be some oddball ADCs that adapt their scaling on the fly, but all the standard ones in the catalogues have a fixed input gain and a fixed output bit depth.

The voltage is a linear measure of the amount of light that passes through the film. You can't have negative light, so that voltage will vary between zero and some maximum value. If you are scanning a transparency the maximum value will be set by the clear film of a blown-out highlight and there will be a miminum that is close to zero - often within the noise level - from the densest shadows. If you are scanning a negative the maximum will be from unexposed portions of the film (i.e. whatever base+fog lets through) and the minimum from the densest highlights.

The linearity of the light measurement means that there is no benefit in terms of image data quality from applying a gamma curve or a custom tonal curve at scan time. There may well be workflow benefits, but the data doesn't care if it gets scaled in the digital domain inside the scanner or inside the computer. One caveat: often cheap scanner firmware is optimised to be small and fast, so tonal manipulations in the computer can in principle offer you more control, less noise and fewer artifacts.

If you are scanning transparencies things are simple. Almost all films have similarly clear bases, so there is no need to fine tune the analogue gain before digitisation. Just choose a value that puts zero absorbtion by the film at the maximum of the digital dynamic range and go. Your bit depth and noise performance will then determine the darkest shadows in which you can reliably detect detail.

Of course, if you have an underexposed tranny it would be nice to be able to turn up the analogue gain. This way you match the dingey highlights to the maximum of the digital dynamic range, and all the lower zones get expanded up in proportion. There is a noise penalty associated with this, but that can be somewhat counteracted with multisampling, and it's usually more acceptable than the results of a purely digital scaling of the values. This is why setting the white point can be a good idea from the viewpoint of the quality of the scanned image data. Some scanners change the analogue gain when you set a white point, others (like the Nikon 35 mm Coolscans) make it a seperate control. Some simple experiments with a dense slide will tell you if your scanner is one of those that makes this worthwhile.

Note that setting a tranny scan black point is almost certainly of no use at all. In theory you could subtract the black point equivalent voltage in the analogue domain and then scale only the black-to-white range of light/voltage values onto the dynamic range of the ADC, and thus gain some more image quality. Laboratory instruments do this routinely, but I don't know of any photographic scanners that do, and certainly none of the consumer flatbeds. If your scanner does turn out to have an analogue offset combined with its gain circuit setting a black point will enhance scans by supressing noise in the shadows. The benefit will be greatest with scenes that have plenty of light but little contrast such as high key fashion and foggy seascapes.

With negatives the issues are inverted: setting a black point will help in a scanner that adjusts its analogue gain to match; setting a white point probably does little good; setting gamma or curves at scan time is of no benefit. Note that traditional darkroom concentration on base+fog is less of an issue here since digitisation is intrinsically linear - there is no paper toe to avoid. However, base+fog will exacerbate the problem of there being no analogue offset, so expect problems with low contrast originals.

Also, negatives are inherently less dense than transparencies. This makes the issue of analogue gain more acute because if you simply rely on scaling in the digital domain, you've thrown away most of the dynamic range of your ADC and you become heavily reliant on good noise performance. The very cheapest scanners keep a fixed gain for both negs and transparencies, which is usually a disaster for the former, but doesn't bother reviewers who are fixated on Dmax and scanning slides. Most consumer and professional scanners will increase the analogue gain (or equivalently, the exposure time on the CCD) to compensate, which is why scanning negs as transparencies and inverting in photoshop is usually a bad idea: the switch to negative mode in the scanner software/firmware is what ups the gain.

Finally, colour negatives are orange. Good scanners - even in the consumer realm - will use different exposure times or gains for the different channels, and this can affect their tonal quality, especially with respect to noise.

So, all that said, what are the consequences for scanning B+W negative? The most important thing is to find a way to force your scanner to adjust its analogue gain to match the negative stock and your particular image. That almost always means scanning in a negative mode, not positive, and it sometimes helps to set a black point. Some scanners don't adjust gain for B+W negs, but do for colour ones, which is why scanning B+W as colour negative can be a good thing. In colour neg mode many scanners will have much better noise performance in the red channel. Naturally, they are usually the ones that are sharpest in the blue channel: your choice :-)

Finally, low base+fog is a good thing. N+ processing for low-contrast scenes is also a good thing. N- processing is redundant.

I use TMAX100, expose for the shadows, and let the highlights end up where they will. I will push a neg of a low contrast scene if tonal seperation is important to me - i.e. if I don't want a low-contrast final print. On my Epson 3200 I get best results scanning in colour negative mode and taking the red channel, but that's because I'm more interested in avoiding noise than maximising sharpness. 16 bit is always worth it, even though it's really only 12 bit on the Epson and I often reduce the bit depth very soon after scanning. YMMV.