SLR viewfinder and interchangable lenses are two separate and independent technologies ususlly employed in high quality cameras, both film and digital. The Oly E-20 uses one, the SLR viewfinder, but not the other, interchangeable lenses. It has a very good, big 4x (35-140mm equivalent on a 35mm camera) fixed lens instead. Another design feature perhaps unique to the E-20 among DSLRs is it's beam-splitter which affords LCD preview. Here's the description from the review at imaging-resource.com
"the E-20 features an SLR (Single Lens Reflex) optical viewfinder and an LCD monitor. The optical viewfinder's SLR design utilizes a "beam splitter" method rather than the traditional mirror design. Oddly, although the image is carried to the CCD and viewfinder simultaneously, there's still a brief "blackout" each time the shutter trips. I'm not sure why this is, since the optical path looks as though it should support light going to the viewfinder and CCD at the same time. One benefit though, is that the beam splitter design should reduce vibration significantly for shooting under dim lighting conditions with long telephoto lenses, since there's no mirror slamming up as the exposure begins. Another benefit is that, with no mirror to flip up, the maximum cycle time can be very fast."
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