What is a Pellicle Camera Mirror?

Pellicle Mirror

The pellicle mirror is an ultra-lightweight ultra-thin, semi-transparent mirror used within a path of light of an optical implement, which splits a single beam of light into two distinct beams, both with diminished light intensity. Splitting the beam in two allows it to be use for more than one purpose at the same time, while the thinner film greatly decreases reflections most often connected with using glass beam splitters.

The pellicle thin mirror has often been used with single-lens reflex cameras (SLRs), in the beginning to provide through-the-camera-lens exposure measuring and maybe to diminish camera shake, although later (and most successfully) to provide fast series shooting, which would otherwise have been slowed by the reflex mirror movement, while sustaining constant finder visualization.

The traditional SLR camera contains a reflex mirror guiding the beam of light from the lens into the focus screen within the viewfinder, that is subsequently moved out of the path of light as the exposure is shot and resulting in the viewfinder going dark. This action creates a delay between the pressing of the shutter button and the specific exposure of the sensor or film.

The very first camera to use a pellicle mirror to split a beam was Canon's Pellix camera, announced in 1965 by Canon. The objective was to bring about exposure measuring through the lens (TTL), a feat pioneered in 1963 by the Tokyo Kogaku KK, on the Topcon RE Super. It features a CDS metering cell installed directly behind a reflex mirror with narrow slots made in the surface allowing light to get through to the cell.

The pellicle mirror of the Canon EOS RT
The pellicle mirror of the Canon EOS RT

Canon made improvements upon the concept by building the mirror fixed and semi translucent. The metering cell was then moved into the pass of light in back of the mirror by moving a lever located on the right front of the camera for stop down exposure readings, dimming the viewfinder for just a moment. Two thirds of this illumination coming from the lens was allowed to pass through this mirror, while the balance was reflected onto the viewfinder screen. This Pellix pellicle mirror consisted of a layer of 0.02mm ultra-thin Mylar with a vapor deposited semi reflecting film. Because the mirror did not blackout, the photographer was able to see the picture at the exact moment of exposure.

The subsequent 35mm SLR camera that used a pellicle mirror became the High Speed Canon F-1, presented during the Olympic games of 1972, with the objective being rapid series shooting, problematical at that time to accomplish with a mirror that moved. The mirror layout was identical to the Pellix. In 1984, Canon announced another adaptation of their new at the time" F-1", which accomplished a blazing 14 fps, a record speed for an analog SLR of the time.

In 1978, Nikon announced the high-speed F2H Nikon. The mirror was a pellicle instead of a traditional front surface mirror that swung out of the path of light as the exposure was acquired. As a way of identifying the F2H, it's shutter speed dial did not contain a T, B or even 1/2000; it did not have self-timer and it had an non-removable Type B focus screen.

Two additional Canon adaptations were manufactured using pellicle mirrors, They were the Canon EOS RT plus the Canon EOS-1N RS, while the RT was based upon the EOS 600 and EOS 630 while the 1N RS was based upon the EOS-1N.

As progression of SLR cameras have advanced since those early versions, fast sequence photography has obviously become achievable using normal moving mirrors in newer high-speed cameras, eliminating the susceptible pellicle mirror which was susceptible to dirt and dust. The mirror apparatus of traditional SLR cameras has been enhanced since the days the Pellix mirror made it's debut; the viewfinder turns dark for just split seconds, shutter lag is shortl, while the return mirror is plenty quick for fast shooting. DSLR cameras have the capacity to shoot ten or more frames per second by using an instant-mirror-return.

Other potential uses exist for a pellicle mirror in today's cameras. One is by using a pellicle to send a part of the inbound illumination into some sort of fast phase difference auto focus detection unit. Patents have been filed in this area. In August, 2010, Sony announced a pair of DSLRs with this type of design, although they weren't based upon a pellicle mirror, while in it's place a plastic half mirror although not as thin nor as lightweight as original one used by Canon.

"Translucent" mirrors

Sony has recently announced cameras featuring pellicle mirrors, which they have been erroneously describing as "translucent" mirrors. A translucent material will pass light, but it becomes so scattered that very little or no discrete image remains behind. A camera containing a true translucent mirror would create an indistinct light blob at its image plane.

What's the Benefits and Drawbacks?

The Benefits from a pellicle mirror:
The viewfinder does never goes dark, What happening can constantly be observed. There is no shaking mirror, which is really is great for macro shooting, where even a trivial amount of camera shake can create a blurred image. Photos can be captured a faster rate, as the mirror has no need to move up and down to capture each picture. For example, The Canon EOS 1N RS, can shoot 10 images every second and the system is quieter without having "mirror slap".

The Drawbacks from a pellicle mirror:
The pellicle mirror loses 1/3 of stop of light as some light must to be moved to the viewfinder. The mirror must be kept spotless, or the image light sensor along with other electronics, plus the image quality, most obviously will all decline, while keeping a pellicle mirror clean is not an easy task.

Although these may be matters of preference that could be important to one photographer, although meaning nothing to others. While the viewfinder doesn't darken, then there's no visual signal that the shutter even fired. This could turn into a big issue shooting in a noisy situation (war, rock concert, etc) where one cannot hear the shutter trip. Zooming from beginning to end of a 36-exposure film roll in a little over three seconds could become a practical or financial issue.

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