Piezography BW ­ Scanning DPI and ideal workflow
Please report errors or omissions to Bill Bergh - bill@cone-editions.com

An oft asked question "What is the ideal scanning DPI"?  In the early months of printing a year ago, we were following the old axioms for the Epson driver, and always printed 240, 300, or 360 DPI images.  Since then we have experimented a great deal and learned a lot from our developers.

A number of recommendations have resulted from our experience.

1 - The Piezography driver loves resolution.
2 - The Piezography sizing routine is far superior to Photoshop's best "Bicubic" interpolation.
3 - Because Piezography renders 1% increments well - an spiky image histogram with the "fingers of death" will print poorly.
4 - Poor scans print poorly, bad unsharp masking skills are now revealed perfectly well.
5 - Using the Levels or Curve adjustment tools that artificially set the white and black points often stretch out the midtones and cause a poor print.

So what does that mean in practical application?

Ideal workflow -

1 - Scan at the highest OPTICAL DPI and bit depth your scanner will allow.  Hopefully your scanner will scan at a higher bit level than 8-bit.

2 - Do levels and curves for tonal changes at the higher bit depth.  Photoshop has limited tools available in the higher bit depths, but fortunately levels and curves are covered.  When converting to 8-bit greyscale, you will have a smooth histogram.  Aggressive levels and curves adjustments in 8-bit can cause the "fingers of death" spiked histogram as a result of math round off errors.

3 - If you need to resize the image, do to cropping or to make an image smaller or larger, make sure to "Uncheck" the "Resample" check box in the Photoshop Image Size dialog box.  Photoshop Bicubic interpolation softens the image and you lose edge detail.  When you "Uncheck" the "Resample" check box, you will notice that height and width and DPI are now linked.  As you change the height or width to the desired size, the DPI will increase or decrease depending on whether you are making the print size smaller or larger respectively.

4 - Let the DPI fall where it may.  Weird DPI numbers - even fractional DPI numbers are OK.  An image DPI of 573.4629 is no problem for the Piezography driver.

5 - So what is the optimal resolution?  I can say that 72 DPI is way too few.  We sort of drew a line in the sand and said that minimum is 240 DPI.  I have pushed down a bit below this to get a larger print and the result was good.  Could it have been better - yes probably.  Jon maintains that 720 is best.  Of course he has Larry and a Hell Drum scanner and can say to Larry - "Scan this image at the output size at 720 DPI", and the resultant file may be 200 megabytes.  The Hell can go to 8,000 DPI optical and 12,000 DPI interpolated.  Scanning interpolation can be as bad as Photoshop interpolation.  It is a software guy's theory on how best to add data that is not really there.  It is best to stay at the max optical DPI your scanner can scan.   You should do your own experiments to see if your particular scanner / software adds objectionable noise patterns if you are pushing to the max interpolated scan DPI.  Watch the shadow areas.

6 - Please note that there is a difference between scanned 720 DPI at the output size, and an DPI added artificially with a program like Genuine Fractals.  If you have an 8x10 image at 300 DPI, there is no benefit from using  Genuine Fractals to create an 8x10 at 720 DPI.  This program works well, but the intended use is to try to boost the image to a good 16x20 image from the original 8x10.

7 - The maximum amount of pixels you can scan will depend on the max resolution of the scanner and the size of the film.  A 4x5 negative yields a much larger file than 35 mm. when both are scanned at the maximum.  Once in Photoshop all you have are pixels (PPI).  Pixels have no size until the moment of printing when you have defined how large the image will be at a specific DPI.  That is why it is meaningless to talk about size with pixels, and you will often hear people refer to the actual file size.    It is more meaningful to say you have a 100 Megabyte file versus a 30 Mb file than to say that you have an image that is 8x10.  If they tell you they have a 8x10 - then you have to ask - "At what DPI?"

8 - Artificially setting the black point and white point in Photoshop will often stretch out and slightly posterize the midtones, and usually is reflected in a bad looking histogram.  This is a leading trick in all the Photoshop books and is bad advise.  It is better to properly scan the image and let the data fall naturally where it should.  At the scanner you have better control - but it is still not a great idea to force the endpoints to something that is really not on the film.  At http://www.piezography.com - Jon has a technique for selecting specific tonal ranges of an image - that can be used to good effect to darken the shadows, or conversely to select the blown out highlights so that you can add a very small amount of gaussian blur, and then add 2 or 3 percent grey.  Excellent method to fix blown out highlights (ex. an area of clouds that are on the edge of the image that have blown out to paper white so that the edge of the image is lost.)