Updated 2017-07-08: add link to youtube video on Magic Eye free-viewing technique and book on learning to see in 3D (Susan Barry).
When you view 3D stereo photos (parallel, side-by-side, MPO formats) on your phone with my Google Play Store Android 3D/VR stereo photo viewer app, you would normally use a stereoscope or Cardboard viewer.
You don't need a stereoscope or Cardboard viewer after all, instead you can use the "free viewing" technique.
My Samsung Galaxy S6 phone has a horizontal display width that approximates the width of my glasses. Because my phone display width is about 4.5 inches, and the center to center distance between the left and right eye images on the phone nearly matches the distance between my eyes, I can free view 3D parallel side-by-side photos and actually see them in stereo.
Maybe this works for me because I'm near sighted. With my glasses off I place the phone very close to my eyes, relax my eye focus beyond the phone, and then very slowly back away the phone, until the photo comes in focus and I see a 3D image. This is the same technique used to view stereograms, as found in the "Magic Eye" book series, which are multiple volumes of 3D illusions. Stereo photos are easier to free view than stereograms in my opinion.
When I use the 3D/VR stereo photo app, in VR mode, while free viewing, I can zoom into the photo and pan left and right to see other areas of the image close up. In the Settings menu I turn off Lens Distortion Correction because I'm not using Cardboard VR lenses. This option makes the photo distortion free in VR mode.
I sometimes also use a Bluetooth connected mouse to move the photo around and zoom into the photo. The TeckNet BM307 Wireless Mouse works great. When I use the mouse I don't have to tilt the phone to zoom in. Tilting the phone makes it harder for me to maintain my 3D vision when free viewing. Also the app Settings allow you to turn off the head movement option, but still use a mouse or other Bluetooth controller to move the photo and zoom in.
Another improvement on free viewing I found is to wear a pair of reading glasses bought from a pharmacy/drug store. With the reading glasses I can get closer to the phone and see a bigger image. I use +3.75 magnification to see a photo in focus less than 5 inches from the phone screen. If you are far-sighted you will probably need reading glasses to view the phone at a close distance.
And iPhone owners, you can use the free view technique with side-by-side photos too. You don't need a viewer app, although an app helps if you want to zoom into a photo. Expanding a photo with your finger to magnify a photo will not show the photo correctly in 3D stereo.
If you want to learn more about how to free view check out this link
http://www.angelfire.com/ca/erker/freeview.html
Or do a Google search for "how to free view (3D) images".
An excellent video explaining Stereograms, Magic Eye, and free viewing is on Youtube:
Magic Eye: The optical illusion, explained
Of course DO NOT free view or use a viewing app and stereoscope combination, if this gives you eye strain or eye discomfort.
As people get older some may loose their ability to see in 3D and have poor depth perception, making driving difficult. This is because the brain controls stereoscopic vision not the eyes. I wonder if stereoscopic vision is a learned experience from birth and whether it can be relearned when it diminishes or the ability was lost or never learned.
Since posting this blog I read a book by Susan Barry, "Fixing My Gaze" in which she explains how she learned to see in 3D stereo after middle age.
