How To Use a Stereo Viewer
In this guide, we will explore how to view the stereograms shown on my Into The Third Dimension page as well as other stereograms you may find across other forms of media. Before we begin, you should pick up a stereo viewer. I recommend the Lite-Owl by the London Stereoscopic Company. It is the one I use on a regular basis to view stereograms on my phone and in books. Although, I should say that while it is possible to view stereograms without the aid of special lenses, I myself had little success with the technique. But, if you are curious to try out this technique yourself, check out this guide.
Before we get into how to use a stereo viewer, it's worth explain how it actually works. I won't get into the optical physics of how the human eyes interpret 3D images in this guide but you can read about that more [here]. Now, as you are aware, we live in a 3D world and so our eyes are capable of seeing 3D objects but why is that exactly? Well, it's due to our depth perception which comes from the fact that humans have two eyes spaced some distance apart (~2.5in). That is very important because it means whenever we look at anything, we are really looking at that thing from two points of view - namely, you're left eye sees the left view while your right eye sees the right view. Your brain is then able to interpret both views into a single image with depth.
Now, the stereo viewer itself is just a plastic device with two acrylic lenses. The lenses are similar to what you may find in cameras and telescopes. They have a specified focal length that helps magnify the image contents and make viewing stereograms a bit easier. Because not everyone's eye vision is the same, you may have to fiddle with different methods. If you wear glasses, I've found not wearing them when using the stereo viewer, to be easier than it was when I kept them on. That's just my personal take. Finally, To use the stereo viewer, relax your eyes and look straight into the lenses while also looking at the stereo image. What starts with two side-by-side images, should then become a single centralized 3D image. Be aware that this will take some practice and you may get a headache at first. You may need to adjust the distance between you the image page or slightly tilt the lenses to adjust the focus or a combination of both. A technique that I found helpful is to first be in a well-lit room and away from many light reflections. Next, hold the lenses directly on the bridge of your nose and while being as far as you can (either sitting or standing) from the image page. Slowly make your way towards the page over to the stereo images. As you look at stereograms across the web, books, or in cards, you may need to readjust your viewing form as not all stereograms are presented in the same size/format. But, you will eventually get the hang of it, I promise. When viewing a centralized 3D image, you may still see snippets of the left and right images. This is normal and should be fairly easy to ignore. Remember to rest your eyes every now and then.