Greetings,
I recently scanned some photos trying to get an idea of how dense various parts of the negative really were in an attempt to evaluate my workflow so that I was making negatives that could be either scanned or optically printed one day. I came across a result that made me think about how much influence the software driving the scanner was making. To try to eliminate the scanner software I thought of making a scan with significantly large areas of clear glass and black matting. Those two areas (if significant in size) would be treated by the scanner software as the upper and lower bounds - the two ends of the histogram.
I assume the software will interpret any value in between in a linear fashion. I have turned off all controls on the scanner software and scan to a non-compressed tiff file. Scanning as a 16bit per channel -vs- 8bit per channel has no bearing on the density range. Exactly the same graph will result with a heck of a lot more shades of grey in between.
To relate the scanner results to real life I scanned a calibrated 4x5 Stouffer step wedge that has 31 steps.
Attached to the post are three pdf files. One of a step wedge with areas of clear glass and black matting, one of just the 4x5 step wedge itself and one graph showing the difference between the two.
Six observations / conclusions:
A. The scanner begins to be non-linear at about a density of 1.75 to be generous. This would mean (to me) that one should not develop a negative to a density of greater than 1.75 or so to linearly digitise it. In reality we might choose to go beyond this a little.
B. The scan that has the absolute limits set seems to have a more linear response. It may not matter in reality. Certainly, the histogram in Photoshop has room to move for the slider at the high and also at the low ends. To me a scan that establishes the 'absolutes' of 'white' and 'black' is one that does have much less capability of software interpretation.
C. The film enthusiast should not develop film to have a greater density than about 1.75 if they wish to have a linear scan made to digitise their image.
D. To check the neg density range make sure to scan with large enough clear glass and use a black matt. The digital values -vs- densities are clearly seen in the results I've posted (assuming softwares and scanner hardwares produce a similar result ...)
E. A scanner will scan a bit more density range (linearly) than one could print optically without dodging and burning. Straight print -vs- straight (linear) scan the scanner has more range but only just. A diffuser enlarger density range of 1.2 is a digital range from around 220 (density of ~1.47) to 40 (density of ~0.27) on the chart.
F. Given the fact of (E) one can expose generously and pull development (with a goodly strong developer) knowing that in the event of a slightly denser negative, it is able to be scanned very well. Low density negatives cannot be rescued by either digital scanning nor optical printing.
One question:
How, on the basis of these results, can a scanner manufacturer claim to be able to scan a density of 4.0 ??? The scanner is blocked up at 255 with a density of around 2.4 ! (and it is highly non-linear from 2.0 ...) Perhaps the only thing I can think of is the 'exposure' setting (if there is one). Perhaps letting the CCD collect more light over a longer period of time ? Given my question, it makes no sense whatsoever to buy a higher end scanner based on the 'fact' that it can scan to some fictitious density ...
If others could post their results from their step wedges it would be a great comparison. I'll go out on a limb and bet the results will be very similar across scanners and softwares which means if people include large areas of clear glass and also use a black matt they can interpret their digitised results pretty accurately (no need to buy a Stouffer step wedge after all ...).
And BTW I have contacted Epson about their Epson scan software that is limited in its file size. I have asked them to consider those of us who would scan the whole glass platen without limitations but I don't think they'll change. I think too the Digital ICE might only work on small film sizes in the Epson film holders.
All to the good !
Cheers,
Steve
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