Often you will run into pages that have well defined areas of half tone and non half tone areas. Something like an image surrounded by text on a solid color background. White backgrounds will have the highest chance of this happening, because white doesn't require any color added to the paper. Black can sometimes be solid, but a lot of times other solid color backgrounds will still have some level of half tone used to get the desired color.
In this example there is text on the left that uses normal printing. And then an image on the right that uses half tones. If we descreen the entire image as a whole then the picture will look good, but the text will get some blurriness added as inherent by-product of the descreen process.
Close up of the different printing technique used. The text uses solid printing. But the picture uses small dots to make representations of colors.
The idea is to make 2 layers. The top layer will not be descreened and will have the text as we want it to look in the final product. We descreen the layer underneath. Then erase just the non-descreened part of the image on the top layer. This will allow just the decreened picture to be revealed in the final image. The blury text will be covered by the unprocessed layer on top.
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This shows the difference in the text with and without descreen.
Descreen the bottom layer. If needed, use the selection/marque tool to make sure you only erase the desired area.
If the particular software you are using does not allow you to quickly erase the selected area (like hitting delete key) then you can toggle the transparency of the bottom layer so you get the alpha checkerboard. Then you can see what areas you may have missed.
And then finally you have the final image that has sharp text and a nice looking picture
