There are different ways to view images in a stereopair and they all 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. A nice pictorial explanation for stereocards can be found on the Getty website.
Freeviewing is the ability to view stereoviews without a stereoscope and can take a bit of practise. 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, any more separation can make it impossible to fuse them without a special stereoscope, so please note that some screens and formats cannot be used for free-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 freeview these it is recommended that you look through the stereopair, 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 freeview smaller images and starting with simple diagrams on a smartphone is an ideal way to learn to freeview so I’ve added two parallel stereoviews made by Jules Duboscq and Frederick Hale Holmes in the early 1850s to help practise 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 to use without being too bulky are the London Stereoscopic Company’s Lite OWL viewer, which can also be used with books and stereocards. Some people find them tricky to focus by being unable to judge the correct distance, in which 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 and allow you to focus easily by moving the lenses back and forth. The LSC’s online shop with a description of their 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; a Holmes stereoscope or a Brewster stereoscope weren’t made to accommodate smartphones, etc.
Stereopairs can also be freeviewed 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 viewer goes crosseyed so each eye sees the corresponding image and the brain fuses them in to a single 3-D image. Some people recommend putting something like a finger or a pen in front of your nose and going cross-eyed to focus on that, then 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. It is unusual however to find stereoviews in this format and I’ve not seen modern stereoscopes made to help with this technique however you will find that most people can either parallel-view or cross-view more easily (some lucky people can do both) so some stereo photographers prefer displaying their stereoviews as cross-views. I’ve edited the Duboscq and Holmes stereoviews so they are in the cross-view format for you to practise with.
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 Stereo Photo Maker. Anaglyph glasses are cheap and easy to find online such as through 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 and rely on polarising lenses on two separate projectors. Active projectors with active glasses 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.
The London Stereoscopic Company project their presentations using two projectors with very specific filters and glasses to almost eliminate any ghosting and producing excellent quality 3-D, which I’ll discuss in a later blog.
I hope you have fun viewing stereoscopic 3-D images and using the instructions to try and free-view.
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