I.R.I.S. Readiris 14 for Windows User Guide - Manual (Page 152)

Readiris TM 14 – User Guide
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OW TO PROCESS LOW
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RESOLUTION IMAGES
A resolution of 300 to 400 dpi is required to obtain optimal results
in Readiris. When you are faced with low-resolution images, the
best thing you can do is re-can the images in an appropriate
resolution. If that's not possible, there are few tips and tricks that
help you obtain better results:
Tip 1
When scanning images of an unknown resolution (which is the
case with digital cameras) or images of which the file header
displays an incorrect resolution, make sure to select the option
Process as 300 dpi:

Click the down arrow beneath the Scan button.

Then click Settings.

Select the option Process as 300 dpi.
When this option is selected, Readiris treats the images as if they
had a 300 dpi resolution.
Tip 2
When scanning images with a digital camera, make sure to select
the Digital camera option:

Click the down arrow beneath the Scan button.

Then click Settings.

Select the option Digital Camera.