Making fine prints in your digital darkroom
Scanning film
by Norman Koren

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Table of contents

for the Making Fine
Prints series

Getting started | Light & color
Pixels, images, & files | Scanners
Digital cameras | Printers | Papers and inks
Monitor calibration and gamma
Printer calibration | Scanning
Scanners and bit depth | Scanner software
Scanning objectives | Scanning steps
Scanning example: HP Photosmart S20
Basic image editing
Black & White | Matting and framing
Tonal quality and dynamic range in digital cameras

Color Management: Introduction | Implementation
Profiles with MonacoEZcolor | Evaluating profiles




for Image editing with
Picture Window Pro
Introduction | Making masks
Contrast masking
Tinting and hand coloring B&W images
Example: Sunset, Providence, Rhode Island

In this page we discuss how to scan images for maximum quality, using the Hewlett-Packard Photosmart S20 2400 dpi 35mm film scanner as an example. Scanner hardware and specifications are discussed in a separate page on Scanners.
 

For more scanning tips and techniques, go to my reviews of the Canon CanoScan FS4000US 4000 dpi 35mm/APS film scanner (great hardware: very sharp with excellent color; a few quirky software deficiencies; supported by Hamrick VueScan) and the Epson 2450 2400 dpi flatbed scanner (not as sharp as dedicated 2400 dpi film scanners; excellent color, wonderful for medium format and 4x5, but limited for 35mm; also supported by Vuescan).

Scanners and bit depth

Scanning is a process that converts prints or images on film-- negatives or slides-- to digital format, i.e., pixels (picture elements). A scanner consists of a fixture for holding the film or print, a light source, a CCD sensor, and associated electronics and software. Either the film or the CCD moves. An analog-to-digital converter converts the analog electrical signal in the CCD to a digital signal with 10 to 14-bit precision (typical values). n bits represents 2n discrete levels, so 10 bits represents 1024 levels; 14 bits represents 16384 levels.

Most standard image file formats store pixels with 8-bit precision (256 levels): 24 bits per pixel for color files (8 bits each for R, G, and B), 8 for B&W. This is more than sufficient for full toned prints with gradations as fine as the eye can see, but it is well below the capability of most scanners. Information-- tonal detail-- is lost in converting a 10+ bit scan to an 8-bit file. If the pixels in the scan correspond closely to the desired tones in the print-- if little editing is required-- this loss is not significant. But if significant editing is required-- and editing is almost unavoidable in images made outside highly controlled studio environments-- there can be a loss of tonal gradation and subtlety. This degradation can be avoided by using file formats with 16-bit precision (48 bits per pixel for color). All n scanner bits are transferred from the scanner; the remaining 16-n bits are padded with zeros. No information is lost-- images can be edited extensively without loss of tonal quality. See Why you need a 48-bit color/16-bit B&W image editor for a more detailed explanation. File formats are summarized in the table below.
 

Image file bit depth
Bit depth
Bits (bytes) per pixel,
B&W
Bits (bytes) per pixel,
color
Discrete levels
8 8  (1) 24 (3) 256
16 16 (2) 48 (6) 65536

Most scanner software, but not all image editing software, supports 16/48-bits. Picture Window Pro (my favorite) offers full support, and Photoshop (the full version) offers limited support. Photoshop Elements and LE, the "lite" versions supplied with many scanners and printers, do not support 16/48 bits.

Scanner software

Scanners operate under the the control of software, which is supplied by the manufacturer and by two vendors. Here are some things to look for when comparing scanner software packages:

Scanning objectives

Scanner software gives you considerable control over the appearance of the scanned image-- its resolution, cropping, color balance, saturation, brightness, contrast, and sharpness. Adjustments are made prior to the final scan. Three of them, resolution, exposure, and cropping, are applied the scan itself. The remainder are applied after the scan but before saving or transferring the image.

Color and tone adjustments are done with a slightly different intent, depending on whether you are saving the image with 8 or 16-bit precision.

Resolution depends on the scanner's maximum optical resolution and the quality level you require at your maximum intended magnification.

Every scanner has a maximum optical resolution, specified in pixels per inch (ppi, often called dpi). (See the Scanner table.) Some scanners publish interpolated resolution numbers that are much higher. Don't bother scanning at these higher resolutions-- it wastes time and storage space and offers no advantage. You can always resize up later if needs be (for example, to send a print out for LaserJet printing).

In choosing scan resolution, first determine the largest expected print magnification,

Magnification = M = print dimensions/film dimensions.
For example, if you plan to print an uncropped 35mm image (24x36 mm = 0.945x1.42 inches) on paper no larger than 8½x 11 inches (10.4 inch image length), M = 10.5/1.42 = 7.3x. If you crop the image so the narrow side is printed 8.2 inches wide, M = 8.2/0.945 = 8.7x. 8x is a typical magnification for 35mm prints on 8½x11 inch paper. For an uncropped print on 13x19 inch paper (18.4 inch image length), M = 13x.

The number of image pixels per inch required for sharp prints has been discussed in Selecting and using printers and in Understanding image sharpness Part 3: Printers and Prints. Here is the summary.
 

Image resolution (pixels per inch) sent to printer
Print Pixels per inch (PPI)
Subjective description
100 Adequate for very large images viewed at a distance.
150 Good for big enlargements-- 13x19 inches or larger.
200 Excellent sharpness for all sizes; nearly as good as 300 PPI.
300 As sharp as the eye can see; no advantage to increasing PPI.

The scanner ppi required to obtain a given print ppi is,

Scanner ppi = M * Print PPI
This is the minimum value for setting your scanner. For example, if you want to print a 35mm image onx11 inch paper at 200 ppi quality, use a scanner ppi of at least 200*8 = 1600 ppi. I set the CanoScan FS4000US scanner, which has resolution settings of 4000, 2000, or 1000 ppi, to 2000 ppi. I set it to 4000 ppi only if I expect to print larger than 8½x11 inches. I set the Hewlett-Packard S20, which allows resolutions of 2400, 2100, 1800, ... ppi, to at least 1800 ppi. Since I like to have extra margin in case I decide to print larger, I often select 2400 dpi.

Remember that images scanned at high resolution for making large, high quality prints can be quite large. 48-bit images require twice the storage of 24-bit images. A full frame 35mm image (0.945x1.42 inches) scanned at 4000 dpi in 48-bit color (6 bytes per pixel) requires 128.8 megabytes. Huge. To edit files this large, your computer should have plenty of RAM. 1 GB is not unreasonble (and not expensive at today's prices).
 

Image size and resolution confusion
The true size of a digital image is its pixel size, for example 5,749 x 3,740 pixels. Digital images are also specified by their size in inches and Resolution in dpi (dots per inch) or ppi (pixels per inch, which is technically correct, though dpi is widely used in its place). For example, 1.44 x 0.93 inches at 4000.00 dpi resolution. Don't be overly concerned by these numbers. In this case they refer to film in the scanner; they have no effect on the monitor display. Different image editors treat them differently. Picture Window Pro ignores them and lets you select print size manually. These properties can be changed without changing the image pixels. Pixel size is what really counts.

Resolution has many definitions, depending on context. Sometimes it refers to digital sampling rate; sometimes it refers to the highest spatial frequency where detail is visible. And there are instances where it's downright misleading.

In scanners Resolution refers to the sampling rate, usually specified in pixels or dots per inch (ppi or dpi). But it can also refer to the ability of a scanner to resolve detail. For example, 2400 dpi dedicated film scanners tend to resolve detail better than inexpensive 2400 dpi flatbed scanners. I try to avoid the word "resolution" in this context and use "sharpness" or "MTF," but it can be confusing.

When we discuss printer ppi above, we refer to the the number of image pixels per inch on the print. This is not the same as specified printer resolution, for example, 1440 or 2880 dpi for Epson printers. These numbers are somewhat misleading; they are the spacing between ink dots, which overlap. It takes several ink dots to reproduce one pixel. Image editor and printer driver software convert image pixels into printer dots.

Sharpening should be applied sparingly at scan time. Oversharpening should be rigorously avoided.

Sharpening is discussed below and in Understanding image sharpness, Part 2. An oversharpened image has exaggerated edges, which distort histograms, making tonal adjustments more difficult. It can result in an artificial appearance and in exaggerated grain. It is difficult to evaluate the precise degree of sharpening at scan time. For this reason it's best to apply it late in the image editing process. Modest amounts may, however, be applied at scan time. For the Epson 2450, which has an extremely soft image without sharpening, I usueally leave the Unsharp Mask box checked. I've had no problems with oversharpening.

Summary A good scan from a slide or negative has the following properties.

I strongly recommend scanning with 16-bit precision for B&W and 48-bit precision for color; there is no excuse not to. Virtually all scanner software supports 16/48-bits, and Picture Window Pro, a superb 48-bit image editor, costs under $100 US-- a fraction the price of a scanner. 48 bits allows multiple editing operations without tonal loss, something I frequently do to get the print just right. Once editing is complete, you can safely store files in 8/24-bit formats. 
 

Scanning workflow

Scanners and software packages differ in details, but the overall workflow is similar.
  1. Clean the film. This is a critical step-- it's where you get to apply the proverbial ounce of prevention. Clean the film carefully, examining it closely under a bright lamp. I usually use quick bursts of canned air (available at most office supply stores) on each side. If dust remains, I go over the film with a clean 1 cm wide bristle brush, then give it another burst or two of canned air. Here is Greg Brakefield's technique: "I gently (and I mean VERY GENTLY) brush the media with a Kodak Camel Hair brush and then follow-up with a Gentle wiping with an Ilford anti-static/dust cloth. So far the results have been superb with NO Scratches."

  2. .
    Dust appears as white specks on negative scans and black specks on slide scans. It can be removed in the image editor by cloning, which isn't difficult if there are only a few specks, but can become tedious for extremely dusty images-- some of my old negatives have stubborn embedded dust I can't seem to remove. Polaroid's excellent Dust & Scratch Removal Utility, which operates at a standalone program or as a Photoshop plug-in, is a big time saver for images with lots of scratches and dust specks. I don't bother with it when I have just a few dust specks. You can download it for free. A somewhat intimidating questionnaire asks for your scanner model and serial number. You don't need to enter them.
    .
    In scanners with infrared dust (IR) removal (ICE, FARE), the presence of dust is sensed by an IR light beam, and the offending areas are filled by interpolation. This saves a lot of trouble with little degradation of image quality. IR dust removal doesn't work with Kodachrome or B&W film.
     
  3. Open the scanner software.Some scanner software operates standalone; some is opened from within the image editing program via the TWAIN interface. From Picture Window Pro, select the scanner (if you have more than one) by clicking on File, Select Source..., then open it by clicking File, Acquire... or by clicking on the TWAIN Acquire icon.

  4.  
  5. Insert the film into the scanner. Most scanners have film holders, but film goes directly into the HP Photosmart S20.  If you have a film holder, be sure the film is clean after you insert it in the holder.

  6.  
  7. Scan the thumbnails. This step scans the film at low resolution and displays either small, low resolution thumbnail images for for each frame or the entire film area. Epson's TWAIN 5 software for the 2450 gives you either display, depending on the Automatic Thumbnail Preview setting in Configuration: Preview.

  8.  
  9. Select the image(s) to be scanned. Depending on the software and preferences, either select specific thumbnail image(s), or select an area of the film by dragging the mouse.

  10.  
  11. Scan or open the preview image image. The preview image has higher resolution than the thumbnail but less than the final scan. It should fill most of the screen. Depending on the software, the preview image is either derived from the initial scan or from a second scan.

  12.  
  13. Adjust the image. This is where you control image appearance: resolution, exposure, cropping, tone (brightness and contrast) and color (balance and saturation). Settings for resolution, exposure, and cropping are applied during the scan, but most tonal adjustments and all color adjustments are applied afterwards-- they are actually part of editing or post-processing. Remember, the adjustment intent is slightly different for 8/24 and 16/48-bit formats. Depending on the software, the "Brightness" setting can control exposure or alter the image after the scan.

  14. .
    Be sure to select the correct film type (color positive or negative; B&W negative). I've seen occasional recommendations for scanning film with software set to other than the correct film type: color negatives as positives, B&W negatives as color, etc. I'm unconvinced. There might be some advantage in isolated cases for cheap scanners with defective software, but all you're likely to do is complicate your workflow.
     
  15. Perform the final (full resolution) scan. This can be slow for high resolution scans. If IR dust removal is employed, there may be two scanning passes. It is usually combined with color and tone adjustments and saving or transferring the image.

  16.  
  17. Save the image to a file or transfer it to the image editor via the TWAIN interface. This is usually done in the same step same step as the final scan, but I'm listing it separately because IR dust removal, color, and tonal adjustments are done between scanning and data transfer. Save high quality images in the TIFF (.tif) format. The JPEG and GIF formats used for Internet display involve a loss of quality. You can always save an image in another format after you've edited it.
Is your computer having problems recognizing your scanner? I've run into troubles with both USB and SCSI devices. Deon van der Westhuysen of Cape Town, South Africa has written a cute little standalone program called RescanHardware that requests Windows to check for hardware that has been added (or turned on) since the computer was last restarted-- like SCSI scanners. It's equivalent to opening Device Manager and selecting "Scan for hardware changes." I haven't tested it yet. Click here to learn more and download it.
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Scanning example

The example uses Hewlett-Packard's PhotoSmart S20 scanner software opened from the Picture Window Pro image editor.

Open Picture Window Pro, then open the HP scanner software (via the TWAIN interface) by clicking File, Acquire... or clicking on the scanner icon. This brings up an empty Preview Scan screen. Opening the scanner software in this way allows 48-bit color files to be acquired; only 24-bit files can be acquired when the HP software is operated in standalone mode. Be sure Photo Type is correct, then clean the film and insert insert it into the scanner. You can set Scan Resolution now, but you can also set it later, before the final scan. The thumbnail images (in this case, for a strip of four negatives of mountains near Silverton, Colorado) appear in the Preview Scan screen, shown below.

Thumbnail images for the HP S20
Select the image to edit. The Image Adjustments screen appears, a portion of which is shown below. This screen includes a brightness Histogram, shown separately.
Preview image for the HP S20
The PhotoSmart software makes its best guess for color balance and saturation, which is set in the Color box, and for brightness and contrast, set in the Exposure box. These guesses are not great. Color balance is pretty good-- just a tiny bit magenta. Color saturation is poor. Contrast is low, indicated in the first brightness Histogram: only about 70% of the potential tonal range is used; there are no deep black tones. The changes I made were, More about sharpening: Sharpening increases contrast at boundaries. In a boundary that goes from dark to light, sharpening darkens the dark portion and lightens the light portion of the image immediately adjacent to the boundary. This is illustrated in the figure on the left. Sharpening improves perceived sharpness and detail in prints, although it doesn't increase the actual information content. Too much sharpening makes grain and film defects more visible, and can lead to edge artifacts. But the right amount definitely enhances the image. Sometimes the amount of sharpening needed for edges results in a rough, grainy appearance in smooth areas, which can be especially troublesome in skies. To combat this, image editing programs have a function called "unsharp mask" (from the old days when it was done with unsharp low contrast film images). "Unsharp mask" sharpens only if the difference between adjacent pixels exceeds a manually set threshold. Only edges get sharpened; smooth areas are passed over; grain is not exaggerated.

I try to avoid oversharpening-- exaggeration of edges-- during scanning. The illustration, above left, shows slight oversharpening. Oversharpening can distort histograms, which can be extremely valuable for setting highlight and shadow levels. In extremes (sometimes seen in digital cameras), it can result in a disagreeable artificial appearance.

The results of adjusting the image are shown in the screen below and in the second histogram. The effects of sharpening are exaggerated in this screen-- they're more gentle in the full sized image.

When you are satisfied with your settings, click Accept. This brings back the Preview Scan screen. Make sure all the images you want to scan are selected and you have the correct scan resolution, then click on Final Scan. A 2400 dpi scan is rather slow. Quality, not speed, is the objective. The 48-bit scanned image will appear in Picture Window. You should save it as a 48-bit TIFF (uncompressed) file, which can be as large as 46 MB (larger if you're scanning a panoramic image)-- that's why you need a big hard disk drive. If you want to save it as a JPEG file (the compressed format used for Web images), you'll have to convert it to 24 bits.

In Part 3 we describe image editing-- the heart of the creative printmaking process.

Links

A few scanning tips by Wayne Fulton. A detailed, verbose approach covering all types of scanners. Much more detail on scanning reflected media and scanning for purposes other than making high quality prints.

Scanners



Images and text copyright © 2000-2014 by Norman Koren.
Norman Koren lives in Boulder, Colorado, founded Imatest LLC in 2004, previously worked on magnetic recording technology. He has been involved with photography since 1964.