Pentaprisms have five sides. Pentaprisms are used in the viewfinder of EOS SLR and DSLR cameras. So why does the prism shown above have eight sides?
When you look through the viewfinder of an EOS camera the impression is that you are looking directly at the subject. However, this isn’t the case. Rays of light from the lens enter the front of the camera and hit a mirror angled at 45°. Here, the light is reflected up to a focusing screen. An archaic term for a reflected source of light is ‘reflex’, which is where the reflex in single-lens reflex (SLR) camera comes from.
A lens projects an inverted (upside down) image. Reflecting the light from a mirror flips it back again. So if you looked directly down in the focusing screen you would see an image the right way up – but reversed left to right. Anyone who has used a waist-level finder ona twin-lens reflex camera will be familiar with the effect.
The purpose of the prism is to flip the image back again so that the left side is back where it should be on the left, and the right on the right. But it does not need the full pentaprism to do this. To save space and reduce the bulge in the viewfinder area, three of the pentaprism points are cut off, creating three extra sides. Although the prism is now an octahedron, optically it acts as a pentaprism and it retains this name.
The distance from the eyepiece to the focusing screen is only a couple of inches, even allowing for the convoluted light path. Your eye can’t focus on anything at this distance, so eyepiece lenses are added to make the screen appear to be one metre from your eye.
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