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Everything you always wanted to know about the semi-transparent mirror technology (but were afraid to ask)

19 January 2015
Szymon Starczewski

2. Semi-transparent (pellicle) mirror

The semi-transparent mirror – its name is a bit misleading. In fact such a mirror used in photography reflects usually about 1/3 of light so it would be better to call it a beam splitter; still I agree that it is not exactly media-friendly. The Canon Pellix, presented in 1965, was the first reflex camera where such a mirror type was used. The Canon company featured also other similar models in its line-up: the F-1 HS, the EOS RT and the 1N, launched in 1994. Nikon also used to produce versions of the F2 HS and F3 HS cameras, equipped with such mirrors. The F3 was launched in 1998 so not so long ago (or so it seems to me). Sony employed the idea of semi-transparent mirror in many models of their cameras, from the A33 to the A99.

The term for the mirror (pellicle or pellical mirror) is not accidental. It is derived from Latin where pellis means skin, reflecting well the thinness of the device. Starting from the 1965 Canon Pellix the producers used thin plastic film with vapour deposited dielectric layers constituting the reflecting coating. There are two good reasons why not to use a conventional glass mirror.

The first: reflected rays of light double and those which pass through the system are shifted. The problem is illustrated below.

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Everything you always wanted to know about the semi-transparent mirror technology (but were afraid to ask) - Semi-transparent (pellicle) mirror

In a case of a thick mirror the rays of light are shifted by a quantity depending on the incident angle, index of refraction of the plate and its thickness. What’s more, the dispersion of the refraction index of the plate causes blue and purple rays to be shifted more than red ones. The reflected rays for a change are used for light metering, focusing and preview. Even if you employ really good anti-reflective coatings on the other side of the plate you won’t be able to eliminate the residue reflection and, consequently, a double image. If the phase AF sensor detects light on an object with a weak contrast and there is a sharp, well-contrasted edge in the residue image hypothetically you can observe a situation where the camera misfocuses completely.

The second serious flaw of thick semi-translucent mirrors is the fact that they introduce optical aberrations into the system. A numerical analysis of optical aberrations can be found further on.