pass to the film plane. Therefore, the mirror does not need to move when a picture is taken. Having an uninterrupted view through the finder is especially helpful when trying to pan the camera and keep moving objects in the viewfinder, or during sports photography. Although the viewfinder does not blackout, it can however go very dim as the lens stops down to the taking aperture, so this advantage is greater at wider apertures. In the RT mode the camera will stop down prior to shutter release. This acts as a full-time depth-of-field preview, allowing the photographer to scan the scene to see what the apparent depth-of-field will be in the image at the aperture set. This enabled fine tuning of the settings prior to taking the picture. A disadvantage of a pellicle mirror is that the light diverted to the viewfinder means less light is available to expose the film — in the case of the EOS RT, the lens effectively loses 2/3 of a t-stop. A less reflective mirror might be used, but then the viewfinder would be darker and harder to use. The advantages of a stationary mirror are less important now that modern cameras can move the mirror back and forth very quickly, reducing the shutter release lag and increasing the amount of time the viewfinder is usable. Cameras such as the Canon EOS-1D and Canon EOS-1D Mark II are able to take 8+ frames per second while moving the mirror back and forth, which is very close to the 10 frame per second rate of the pellicle mirror-equipped 1N RS. The EOS-1v with PB-E2 power booster (2000 release) and the EOS-1D Mark III are capable of sustaining 10 frames per second burst rates without the use of a pellicle mirror. Finally, the pellicle mirror also degrades lens resolution. Early tests performed by Modern Photography magazine showed that Canon film cameras with conventional mirrors produce sharper images than Canon cameras with a pellicle mirror. Nevertheless, Sony has implemented a very similar technology on its current digital interchangeable-lens DSLR-like camera line, called SLT cameras. References Shell, Bob. "The Canon EOS System." Hove - FOuntain Books, 1990. ISBN 0-906447-47-X. External links CANON EOS FAQ Version 2.4 EOS RT - Canon Camera Museum Photography in Malaysia - Canon EOS RT R