Planar Lenses

The latest edition of Camera Lens News, from Carl Zeiss, has just been published and includes a feature on Planar lenses. Given that the Planar design was developed by Paul Rudolf for Carl Zeiss, CLN is the ideal source of information on this topic. Here is a summary of some of the points covered.

Sectional view of Planar lens.

This ‘blog is based on a Carl Zeiss article that is available online at the link given below. Supplementary information has also been provided where appropriate for readers who simply want to know a little more about the iconic Planar design. Readers who want the full Planar story are advised to consult the very detailed paper that is referenced in Issue 40 of Camera Lens News: the full article can be found online at http://blogs.zeiss.com/photo/en/wp-content/uploads/2011/07/en_CLB_40_Nasse_Lens_Names_Planar.pdf

The important feature about the Planar design, as its name suggests, is the flat (planar) image field that it projects. The significance of this is not obvious until it is realised that lenses tend to project curved image fields, which means that the sensor also has to be curved if image sharpness is to be maintained right across the picture. Curved sensors, whether those sensors are film or solid-state, are not very practical whereas a perfectly flat sensor is achievable. For this reason, lens designers try to create flat image fields - and that is where the Planar design is so strong.
Planar lenses are symmetrical, which means there is the same arrangement of elements to each side of the centre of the lens (where the aperture is located). As was the case for so much in early optics, the basic arrangement used was devised for use in telescopes (before photography was invented). Rudolf's contribution was to realise that the symmetrical arrangement could be modified to incorporate paired elements that had the same refractive index (light-bending power) but different dispersion characteristics (separating of different colours).
This meant that Rudolf's Planar offered superior colour correction, which in turn made it well suited for use in process lenses (used for copying applications). As is often pointed out in What Digital Camera's lens tests, high-contrast test targets tend to reveal chromatic aberration much more strongly than many real-world images - and this is no different to any other copying situation that involves stark black-and-white patterns or text.
Inevitably, there were problems with the Planar design: in particular, the multiple surfaces and strong curvature of the elements caused ghosting from internal reflections and also meant a significant level of light loss due to these same reflections. Ironically, later designs increased the number of elements but the development of newer, high refractive-index, glasses allowed the curvature to be decreased.
This adaptability, coupled with the option to provide a wide maximum aperture, explains the longevity of the Planar (and its various, sometimes non-symmetrical derivatives). Dr Hubert Nasse's historical and technical overview (linked above) highlights Stanley Kubrick's use of a 50mm f/0.7 Planar lens in the film Barry Lyndon in order to record scenes lit by candlelight!
Sadly, that record-breaking lens could only just cover an APS-C sensor (not full-frame) and weighed nearly 2kg. It was also very expensive: one went to auction last year and sold for 90,000 Euros! Furthermore, the distance between the back of the lens and the sensor (film) was just 5.3mm - meaning that there would be no room for any mirror box, so use on a reflex camera body would be totally out of the question.
There is much more information in Dr Nasse's paper so if this brief summary has whetted your appetite please read the full version - and register to receive notice of future editions of Camera Lens News at http://blogs.zeiss.com/photo/en/?page_id=2.

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