There are different ways to view images in a stereo pair and they usually rely on each image being viewed separately by the corresponding eye, so your brain can fuse the two 2-D images into one 3-D image.
FREE-VIEWING STEREOVIEWS
Free-viewing is the ability to view stereoviews without a stereoscope and can take a bit of practice. It is important to note that the images shouldn’t be more than 7cm apart for parallel-views because this is just over the average separation between the human eyes. A larger separation can make it impossible to fuse two images without a special stereoscope, so please note that some screens and formats cannot be used for free-viewing.
PARALLEL-VIEWING
Most Victorian and modern stereoviews are in the parallel-view format, which has the left eye image on the left and the right eye image on the right. To free-view these it is recommended that you look through the stereo pair, as if staring to infinity, and relax your eyes until you can see three images. The image in the middle should be in 3-D and you need to gently bring it into focus, without changing the convergence of your eyes, to see the depth.
If you’re using a phone or a stereocard, you can initially have it close to your eyes to get the third image, then slowly move it further away to focus. Some people put the side of their hand, or a piece of paper, vertically on their nose and between their eyes to create a separation to help with the convergence. Others find it easier to free-view smaller images and starting with simple diagrams on a smartphone is an ideal way to learn to free-view. If you’re viewing parallel-views by crossing your eyes, the image will appear inverted, with the background coming towards you and the foreground receding (please see the examples below and in the cross-view section to try and spot the difference).
I’ve added two Victorian parallel stereoviews to help practice parallel-viewing with:
Parallel-view images can also be viewed with stereoscopes which use lenses or mirrors to focus each eye on each separate image.
For the smartphone, I’ve found the best lenticular stereoscopes (viewers with lenses) to use without being too bulky are the London Stereoscopic Company’s Lite OWL viewers, or the studier London Stereoscopic Company’s Steampunk OWL viewers, which can also be used with books and stereocards. Some people find these tricky to focus as they are unable to judge the correct distance to hold the card away from the viewer (which should rest against your nose and forehead, like wearing glasses). In this case, I’d recommend the London Stereoscopic Company’s OWL viewer and OWL VR kit, which hold against the book or hold the smartphone in place. They allow you to focus easily by moving the lenses back and forth. The LSC’s online shop with a description of its viewers can be found here.
All kinds of antique and retro lenticular stereoscopes can be found, for example, on eBay, in antique or second-hand shops, at auctions, etc., and they often work very well. Just keep in mind which format you’d like to use the viewer for and check it’s suitable. For example, Stereo Realist slides can’t be viewed with a View-Master viewer; an original Holmes stereoscope, or a Brewster stereoscope, wasn’t made to accommodate smartphones, etc.
For viewing parallel-view images on a larger screen or monitor, I use a mirror stereoscope made by nvp3d. I find they are of excellent quality and their mirrors don’t distort anything. Loreo makes a less expensive ‘Pixie 3D Viewer‘ (which are unfortunately being discontinued, but you may also find them on eBay). These are both made specifically for computer monitors and TV screens.
CROSS-VIEWING
Stereo pairs can also be free-viewed by cross-viewing; this is where the left and right images are swapped so the left eye image is on the right and the right eye image is on the left. The person viewing them goes crosseyed, so each eye sees the corresponding image, and the brain fuses them into a single 3-D image. Some people recommend putting a finger or a pen in front of your nose and going cross-eyed to focus on it, quickly replacing it with a stereoview, and then, staying cross-eyed, focusing your eyes to bring the two images into the third, 3-D image, in the middle. This technique allows you to free-view much larger images than by parallel-viewing.
It is unusual to find stereoviews in this cross-view format, and it’s not common to see modern stereoscopes made to help with this technique. You will find that most people can either parallel-view or cross-view more easily (some lucky people can do both), so some modern stereo photographers prefer displaying their stereoviews as cross-views.
I’ve edited the diagram stereoviews so they are in the cross-view format for you to practice with:
When free-viewing, always try and check if the stereoview is parallel-, or cross-view, to make sure you’re seeing the amazing depth as it should be. If you’re unsure, try and find the closest part of the image to you, like the spire in the above image, and make sure it is coming towards you, not receding. You can flick between the two different versions of the above card to spot this.
ANAGLYPHS
Another way of viewing images is by using filters, so again, the left image is filtered to the left eye and the right image filtered to the right eye. The most common way of using this technique is with anaglyph glasses using red and cyan filters.
Stereo photos can be converted into anaglyphs on smartphone apps such as 3DSteroid/ i3DSteroid and computer programs such as StereoPhoto Maker. Anaglyph glasses are cheap and easy to find online, for example from eBay and Amazon. I sometimes find there can be ghosting with anaglyph images, depending on the subject, which can be quite distracting, but it is a cheap and easy way to display larger 3-D images.
Anaglyph comics and magazines were quite popular previously and can be readily found on eBay, but some of the early French ones had the red and cyan images swapped. I’ve found wearing the anaglyph glasses upside-down and looking rather curious works with these. It’s important to note that the usual way to wear anaglyph glasses is with the red lens on the left eye and cyan/blue lens on the right eye.
OTHER VIEWING METHODS
Polarising glasses are another way of filtering the corresponding image to the correct eye. This can be done by using polarising lenses on two separate projectors, with each projector projecting a separate perspective. 3D TVs also use polarising glasses.
Active projectors are used with active glasses and use a single special projector. Battery-powered active glasses have LCD shutters over each lens which darken to block the lens of each eye sequentially, in sync with the 3-D itself, so only certain frames are seen by one eye and others by the other eye, creating the 3-D effect.
WIGGLEGRAMS
I’m going to shatter some dreams and surprise others when I point out that roughly 6+% of people have stereoblindness, which is the inability to see in 3D using stereopsis. There are several physical reasons for this, but the most common is strabismus. I have previously shown some stereoblind people lenticular prints and Wigglegrams to give them the impression of what stereoscopic 3D is, and in most cases, it works.
For a tutorial on how to make Wigglegrams with Photoshop, please see this post. You can also make them automatically in 3DSteroid Pro and i3DSteroid (Stereo>Wiggle), however, I cannot get these phone apps to keep my manual alignment, so the Wigglegrams are VERY clunky. The free StereoPhoto Maker software for Windows and Macs also lets you export files as Wigglegrams (File>Make Animation Gif).
I hope you have fun viewing stereoscopic 3-D images and using the instructions and examples to try and free-view.
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