That's a very useful technique known as Local Contrast Enhancement.  I've been using it for a few years, albeit in Photoshop.  Here are some descriptions:

http://luminous-landscape.com/tutorials/contrast-enhancement.shtml
http://www.lonestardigital.com/photoshop_quicktips.htm
http://www.cambridgeincolour.com/techniques/local-contrast-enhancement.htm

I've used the Comparator tool in Qimage many times, but usually to compare images taken at slightly-different exposures, or to confirm point of focus.

But I just used the Comparator to determine that my Nikon 18-55 kit lens exhibits severe diffraction at smaller apertures.  The texture of a leaf and the wood trellis behind it are clearly rendered at 5.6.  At f22 or 32, the fine detail suffers badly.

First time I've used the Comparator in this context.  The EXIF info displayed at the top of each image is a great convenience.

The only improvement I can think of would be the ability to load more than two images into the Comparator.

Thanks for the tip on the Swap function.  I'll give it a try.

Nikon used to publish (and perhaps still does, though I can't find any) detailed lens specs which identified the sweet spot, often f8, if I recall.  Beyond that, you got greater DOF, of course, but more diffraction.  I remember seeing lens reviews which indicated that even Micro-Nikkors exhibited diffraction when stopped down beyond f8 or f11, even though they could be stopped down to f32.

But you're certainly correct that many lenses show improved performance when stopped down a few notches from their wide-open aperture.

Oh, well, I've been meaning to try a focus-stacking program, anyway.

I have another Undocumented feature in Qimage that comes in very handy every week.

Let me set the scene.
You just printed a wonderful set of three 11 x 14s all neatly cropped and one with a nice matching colored border, with your "Photo by Terry M" in the lower left corner.
Two days later, you found you needed to do the same job or part of that job again due to damage to a print by forgetting that you left it on your chair and you sat on it, or someone liked the prints and wanted a set also.
Oh Gee Whiz! I must set the size, what was the border I used? What were the names of the image files? What folder were they in, I can't remember.

Watch this!
Click File. Open Automated Job Log.
There are all the print jobs in a file, from latest on....
Look for the date of two days ago, click, and read the filenames, and size etc.
If you found the correct one, highlight it, click OPEN at the lower right, and you are ready to print.
There's your PHOTO BY, you border, your size, your setup, everything.
Put the same paper into the printer as you used, even the printer paper selection, the profile, the driver settings will be all ready to go.

There is one Caveat.
You will have to have turned this feature on earlier for it to be logging the prints.
Make sure it is turned on by going to EDIT Preferences, Job Logging, Status, Keep all Jobs. (or other preference)

Hope this one helps a little.

Fred

Owen,
It looks like you and I are the only ones passionate about our beloved Qimage. 

Here's another somewhat obvious, yet obscure feature of Qimage.

Print Cropping!
As we all know, print cropping is decided after you decide what size print you are making.
If we intend to make an 8 x 10, and we go into the Full Page Editor screen, clicking on the Cropping tab, we see our image in an 8 x 10 frame.
Upper right corner gives us a movable image and a zoom bar to adjust the crop to exactly the extract we want, keeping the 8 x 10 ratio.

Let's suppose that besides printing an 8 x 10, aunt Gladys wants a 5 x 7 for the coffee table.
Simple!  Just tell Qimage that the next print will be a 5 x 7.
The print crop box now changes to a 5 x 7 shape. This is obviously not nearly the same ratio as the 8 x 10, and will call for a new look at the print, it's zoom, and cropping attributes.
You make that print.!

Next day.
Oh Boy, you forgot. Aunt Gladys called and wants two of each instead of one of each.
How will you ever get these to be exactly the same as yesterday's prints?
Easy! Qimage remembers the crop for each print and all you do is set the print size for 8 x 10, and add the same image.
Qimage says, I remember printing that. Here's the way it looked last time in 8 x 10, and I even have the 5 x 7 size ready to repeat too.

Fred

I believe you get an 8X zoom, and then click on HQ. You will be able to see the sharpness of any section of that image by sliding it around using the hand.
You will have to click the HQ button every time you move the image, or change the setting from Thumbs/small to All or Selected images, and you will get your HQ High Rez image all the time.
If you click and hold the mouse button either on the zoom button slider, or the blue word "zoom" it will read out the ppi for the selected print size as well as the zoom factor.

Have fun.
Fred

Thanks, guys.  Fred, I didn't know that you could see the zoom factor and the ppi with just a click...although I wonder what "8X" means.  Eight times what?

Terry, I'll experiment a bit with the slider.  8X appeared to be about the same as 100% on my monitor, and in any case will be close enough for what I'm trying to do, which is:

1.  Earlier in this thread, I mentioned how easy it is to use the Comparator to check for lens diffraction at small apertures.  It is of course very easy to set up on a tripod and shoot a subject at several (or all) different apertures, and it's child's play to use the Comparator to compare the photos side-by-side.  My Nikon kit lenses show noticeable diffraction at small apertures.

2.  I mentioned this finding on a Nikon forum and was promptly asked to post photos, so I started looking for a way to:
   a.  apply the exact same 100% crop (preferably) to about four of my test photos (no problem in QI)
   b.  create new email/web size copies (ditto)
   c.  arrange the new images side-by-side for easy comparison (Freehand placement in QI, right?
   d.  create a single new image from the four test images (Print to File, right?)
   e.  post the image to the forum (this I know how to do!)

An EXACT 100% crop isn't absolutely required, of course.  And I believe that I can use the Floating Text tool to identify the aperture used for each photo.

Now...what have I overlooked?  Your comments, please.