Scanning Tips


A few basic tips to improve the quality of your scans.

Scan photographs as grayscale
In most scanning software, the halftone scanning function is used for art that has already been scanned and has a dot pattern.

Scan at the right resolution
Most new users scan at too high a resolution. Too high a resolution will create huge files and increse processing time on your rip or your digital printer

Use the correct resolution for the application
The scanning resolution for photographs is different than the resolution for line art. The resolution for photographs is a function of the final line screen that the photo is to be printed at, and varies depending upon whether the photo is to be enlarged or reduced.

If you are enlarging a photo 120% and printing it using 133-line screen, there is a simple formula you can use:

(line screen) x 1.55 x (enlarge/reduce percentage) = Scan Resolution.
In this example, the scanning dpi would be 227 (or as close as it can be) (133 x 1.55 x 1.1 = 227 dpi).


Scanning line art
Line art should be scanned at the highest possible resolution. Even if your scanner is only a 300 dpi scanner, you can interpolate a grayscale scan and get 1,200 dpi results.

First, scan the art at the highest resolution your scanner allows. In Photoshop, choose Adjust>Levels, from the Image menu. While looking at the graph of the image, reset the top set of three sliders. Move the black slider so that it is just to the right of the peak of the lift-hand hill on the graph. Move the white slider so that it is just to the left of the peak of the right-hand hill. Click the OK button.

Choose the image Size from the Image menu. Make sure the File Size check box is not checked. Without changing the dimensions of the image, change the resolution to 1,200 dpi.

Choose Bit map from the Mode menu. Make sure the Output Resolution is set for 1,200 dpi.

Save the resulting file in TIFF format with LZW compression.

The scan will have been saved at the higher 1,200 resolution.

Plan before you scan
Know the size you want the halftone to be in the finished piece. If you plan to reduce the image 50 percent, it will double the resolution. Reducing it 33 percent triples it.

Use a retouching program
Clean up your scan in a retouching program such as Photoshop before using it in a layout program. The size and resolution should be set before placing them in a layout program.

Crop the image while you are in the retouching program. White space creates data in the file and will make the file larger and take longer to rip or print. You will want to avoid using Quark or PageMaker to crop or rotate a scanned image.

Scan the photo at the right Gamma
Gamma is a way of expressing changing a photo’s overall brightness and contrast. It’s a curve on a graph that changes a photo according to a number, such as 1.4, 2.2, or 0.8. The number is shorthand for the curve. It mostly changes the mid tones of a scan. In most photographs, the mid tones (the skin, for example) have most to the important detail. By either lightening or darkening these areas, you can bring out this detail.

If you make the change as the photo is scanned in, you will not degrade the quality of the image by adjusting it. If you scan a photo in and change the gamma afterwards, you will lose detail and tonality in one way or another.

The operator should increase brightness and contrast for most photos. The most common problem with desktop scanners is creating scans that are too dark. Most scanning software allows you to set gamma, and you can usually start with a 1.4 gamma and go higher if need be.

If your scanning software doesn’t have a gamma adjustment, you can use brightness and contrast controls; increase both for more gamma, decrease both for less. This technique will not work quite as well.

Adjust the photo to boost its tonal range
Make the darkest part of the photo as black as it can be and the lightest part as white as it can be, while making a smooth transition from black to white. Set the tonal range by using the Levels dialog. Set the black point slider just to the left of the end of the graph of the image, and the white point slider just to the right of the start of the graph.

Experiment with moving the mid tone point slider (gray) to improve the image further.

Be sharp on screen
Make the on-screen image just a little sharper than you think is needed. It will soften in production. Use the Sharpening routine in the retouching program.

Add Noise
Add Noise to a scan to minimize the transitional steps between your grayscales. This technique smoothers a low resolution scan.

Calibrate your monitor
It is hard to get your monitor to match the printed piece. One trick for black and white photographs is to make sure the screen is giving you a good idea of the overall contrast of an image.

One calibration suggestion found on the Internet was to tape a printed version of a photograph up next to the monitor and open the image in Photoshop. The printed piece will show some darkening and increased contrast.

To determine how much calibration is needed, use the Printing Inks Setup dialog (choose Preferences>Printing Inks Setup from the File menu). Make sure “Use Dot Gain for Grayscale Images” is checked, and start with 10% dot gain (you will have to exit Printing Inks Setup to see the previewed dot gain). Try different amounts of dot gain until the screen is close to the printed piece. Dot gain will vary from one press to another. You will have to redo the process depending on the press used to produce the work. The press operator can give the desktop publisher information on how much dot gain to expect.

Printing Inks Setup changes the way the image is displayed on the monitor, not the image itself. This will cause the photo to look different on another monitor. After you “calibrate” a monitor, don’t change the brightness and contrast knobs.

When you calibrate any monitor, allow at least 30 minutes for the monitor to warm up so you can get a more accurate reading.

 

Making Images Larger

Enlarging images can be more problematical. As explained above, when you enlarge a pixel-based image--in whatever application you use--you also enlarge the pixels until you reach a point when the pixels themselves become visible to the naked eye. Although making enlargements in your page layout or drawing application is less demanding on disk space and makes output faster, you may notice loss of output quality if you enlarge more than about 180%, although the degree of quality loss depends just as much on the halftone screen ruling you will use. To minimize loss of quality when enlarging an image, you must "resample up".

We recommend that you read our articles on increasing your picture size.


 

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