Thursday, March 27, 2014

Using Photoshop to Improve PDF Image Files

Ever had a scan of a document that you can’t read because it’s either too light or too dark? I get that all the time in my research of military sources. It’s a real pain.

But wait! There may be a solution (or semi-solution, anyway). Photoshop to the rescue!

For a long time I’d wondered if a PDF can be opened in Photoshop with an eye on improving the image quality. Recently I had the opportunity to try it on some files of WWII submarine war patrol reports, and it worked pretty well.

Little background: When a U.S. submarine went out on a combat patrol in WWII the captain was responsible for turning in a detailed, day-by-day account of the voyage (which was often 6-8 weeks long). If he attacked an enemy ship he had to note all sorts of details, including the serial numbers of the torpedoes and how good the food was. A typical patrol report ran 40-60 pages. By the end of the war there were nearly 1600 of these on file. In the 1970’s the Navy began to microfilm this pile of paper, and they didn’t always do a very good job, resulting in page images that were dark or light and hence, sometimes nearly unreadable. Now all the microfilmed reports are available on line at:
The PDF file I was working with, the five patrols made by USS Harder (SS257), was 325 pages long. That’s a big file, in terms of page quantity. And I only needed to improve four or five of those pages.

Here’s the technique:

Open the PDF in Photoshop. Up comes a window showing a thumbnail image of each page, with page numbers below the image.

Choose the pages you want to fix by clicking on the thumbnail. You can open as many as you want; just hold down the shift key and click. Click OK and the pages will load. The file bar will string each of the image names in descending order.

Click on the page you want to fix and it will open in the Photoshop window.
The screen shot shows a page that is almost too light to read legibly

How to fix (or at least improve - the easiest way is to go to IMAGE>ADJUSTMENTS>LEVELS. 

A window with three sliders will open. Black level is on the left, White on the right, Gray in the center. Make sure the “Preview” box is checked. Now, slide the sliders around and watch the changes to the document. Fiddle until you can’t get a better result. Click OK.
Image too dark? Same technique.

Caveat: you can almost always improve a page image, but it’s rare that you’ll be able make it look like it just came off the typewriter.

You can now crop the image, if you need to.

Then save the file under a different name (you can’t save a manipulated page back into your original PDF).

Here is the improved version:

Typically, I print these guys out because I like having hardcopy to read from when I’m writing my article (in this case for World War II magazine). But you can just as well read them off the screen.

If this post was helpful or interesting to you, please let me know. I’m always looking for ways to improve the blog. 
Disclaimer: The descriptions of web pages are accurate as of the date of the post. Like everything else in this digital world of ours, they can change in the blink of an eye.