I made a closeup diopter viewing lens for using a larger viewscreen like a small viewfinder. The commercially-made viewing loupes typically put the lens 2 inches from the viewscreen. This is too close for me, as the resolution of the viewscreen isn't enough at 230,000 pixels. With newer models that have 921,000 pixels, this wouldn't be such a problem.

My viewer is positioned 3.2 inches from the viewscreen and works very well, even though I can see each pixel. It consists of three diopter lenses from reading glasses stacked together. One is 3.25X and the others are 3.5X. The out-of-pocket expense for this was $0.00. To read the details about how I made this, go to this link to a photo on my Flickr album:
http://www.flickr.com/photos/22121562@N00/5509000218/

Be very careful not to direct your viewfinder lenses towards the sun. If you have them set up correctly the sun's rays will be brought to a sharp focus on your finder screen. Your screen can be damaged in a matter of seconds under these conditions. Don't ask how I know. You might find your out of pocket expenses to be more than $0:00.

Some of the better movie cameras had eyepieces that opened when you pressed against them.

I've decided that stacking three diopter lenses to get a position 3.2 inches (80.6 mm) from the viewscreen, was just too close. So, I've made another one using just two 3.5X diopter lenses stacked together. The distance from the screen for this one is 4.6 inches (116 mm). The image looks much sharper and the individual pixels aren't nearly so visible. I can also see the whole screen more easily.

I can't account for many commercially-sold viewing loupes being positioned just 2 inches (50.4 mm ) from the viewscreen and yet produce sharp images. My camera has a 230K pixel viewscreen. A new model with 921K pixels might allow it to be viewed more closely, but it would still be more difficult to take in the entire screen.