Robert D Feinman

Sharpening for Print Quality

When optimizing a scanned image for printing, it is important to sharpen  to restore details lost in the scanning process.
This is not "cheating" since proper sharpening can actually restore detail  lost by the mismatch between the scanner's pixel size and the grain of the slide or negative.
Sharpening performs an operation that cannot be easily duplicated photographically. It subtracts the bluriness around the edges.
Here is a raw scan at my scanner's max resolution of 1600dpi, reproduced as actual pixels.
In the final step we will view this at print size, but this enlargement makes the changes from sharpening easier to see.

Color Original
Color Original

We used the sharpen filter on the entire image with settings of:
Amount 165%
Radius 2.7 pixels
Threshold 7 levels
Under most circumstances the radius should be set to about 1/100 inch.
So since we are planning to print this as a 300dpi image a radius of 3 pixels  would be 1/100 inch. We used slightly less to minimize undesirable side effects.
By setting the threshold to 7 we avoid  sharpening the film grain, especially in the sky.
The image is much "sharper" than the original, but note the fringe at the top of the hill where it meets the sky.
If this was a scan destined for web display the image would be at 72dpi and a radius of about 0.7 pixels would probably be best.

Overall sharpening
Overall Sharpening

To explain what has happened we have created a difference image of the previous two and greatly increased the contrast. This shows only the altered pixels in colors. Unchanged pixels are black.

Notice that there is a fringe on both sides of the ridge of the hill and the fence bars, etc. Also small highlights in the foliage have become donuts.

Sharpened Difference
Difference from Sharpening

Here is the image with sharpening applied only to the luminosity (lightness channel). This is achieved by selecting the image in the channels  pallet and control-click on the thumbnail. This creates a selection where the brighter parts are more selected than the dark.
We then apply the sharpen filter to the selection. This tends to sharpen the parts of the image that the eye sees more clearly without overdoing the rest of the image. Notice the effect is less that the overall sharpening even though the same values were used.

Luminosity Sharpen
Luminosity Sharpen

Once again we show the difference between the original and the previous  sharpened image.
Notice the same general features have been sharpened, but the halos are not as big.

Luminosity Sharpen Difference
Luminosity Sharpen Difference

One of the worst problems with sharpening occurs at sharp color boudaries such as the horizon.
In order to minimize this we use the color range tool to select the sky and invert the selection. The sky now will not be sharpened and the ridge of the  hill will not bleed into the sky.
We sharpen with the same values as before. Notice the fence looks sharper  but there is not a halo near the sky.

Color Range Sharpen
Color Range Sharpen

Here is the difference image. Notice that the ridge is sharpened only on the lower side. This avoids the ugly fringes that are seen more and more these days and that shout "manipulated image!".

Color Range Difference
Color Range Sharpen
To emphasize this even more, here is the same image sharpened with a pixel radius of 4. The fringes, especially near the sky are much too  obvious. Appropriate sharpening does make the image "sharper", but it is still impossible to put detail where it doesn't exits. Better a slightly soft image rather than one full of fringes. Stick to the limits of your original image size, scanner limits, and print size.
I find that the rule of thumb is to divide the true scanner resolution by 300 to get the maximum degree of magnification for an image to appear "sharp" in the print.
So for my scanner at 1600dpi I can enlarge about 6 times. This gives a 6x9 inch print from a 35mm slide and a 13x16 inch print from my medium format 6x7 camera.
Some people are satisfied with lower sharpness and use 260, 220 or 200 dpi in the print.
They can then get prints up to 50% larger.
To each his own taste...


Finally to keep things in perspective, here are the images reproduced as they would appear if printed out at 300dpi. Since they are being displayed online the resolution has been reduced to only 72dpi. They now size properly in the browser, but some of the sharpening has been lost by the image reduction.
Remember the values and examples given here are aimed at producing high quality paper prints, the values are quite different for screen only display.
See Selective Sharpening for another example.

Appropriate sharpening does work!

Raw Scan
Overall Sharpened
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© 2002 Robert D Feinman