What is Planar Lens Design?

Possibly you have seen Lens manufacturer Zeiss to write the phrases like Planar and Tessar. What these terminologies mean and how they work? Planar Lens Design is registered trademark taken by Paul Rudolph for Carl Zeiss. In the same way, Tessar is also a design by the same physicist Paul Rudolph in 1902. We will not discuss about Tessar in this article rather limit our discussion within the topic – What is Planar Lens Design. The Planar has in its basic form six lenses. Usually Planar Lens Design is used for Normal Lens. For Medium Format DSLR, Planar Lens Design is used. Tessar can be used for all type of lens including the compact lens used for advanced Mobile Phone Cameras – you can check, Nokia N73 ME has Carl Zeiss Tessar lens.

What is Planar Lens Design?

Planar is an important variant of the double Gaussian lens. Double Gaussian lens is the basic design which is still used in many cameras. Carl Friedrich developed Gauss to improve the lenses for refracting telescopes. He calculated a lens combination that was free from central and marginal rays of spherical and chromatic aberration. Gauss combined a positive front with a rear negative lens. These lenses were rarely used in astronomy and repeatedly produced commercially.

The Planar Lens Design has the basic shape of six lenses in four groups. Compared to the original four-element of the Double Gaussian lens, additional degree of freedom for the optimization of the imaging performance is gained. A Planar usually has two converging lenses at front, then two diverging lenses behind within them the aperture, and then another two converging lenses. Often the inner converging lenses are cemented together with the diffusing lenses.  The two diverging lenses are relatively strongly curved. The lens system as a whole is largely symmetrical.

Planar was introduced in 1896 by Paul Rudolph and patented. In particular it was possible with the planar, not only to avoid the spherical aberration and the chromatic aberration, but also the astigmatism or other aberrations. The Planar continues to have a slight curvature of field, from which the name derives – Planar.

Before the introduction of tempered lens, the surfaces of lenses with multiple lens groups had a partially strong tendency for internal reflections. Between each pair of lens groups two glass-air transitions can occur. If these reflections happens in the photographic plate, the unwanted light causes flat contrast. Therefore, at least four of the six lenses of the Planar are cemented together. Despite the higher number of lenses, the reflection slope of the Planar is therefore only slightly larger than the reflection slope of the original double Gaussian double with only four solitary lenses. Meyer developed heavily curved menisci lens later, which have a very low reflection angle.

What is Planar Lens Design?

These developments deviated more and more from the “ideal” symmetrical structure. Planar Æ’/0.7 / 50 mm is possibly the brightest lens on this earth. Overall there 35 copies. Examples of developments under other brand names are Pancolar, Xenon, Biotar.