I have a 7" Ross Homocentric barrel lens f 1:5.6.Can you tell me anything about it?

-- James Tremills (tremills@chebucto.ns.ca), June 11, 2001

Answers

The Ross homocentric is a derivative of the symmetrical type lens consisting of two air spaced elements (+ - in front and - + in the back)in each half. Part of the symmetrical deigns that swept lens making a hundred years or so ago (inlcuding designs like the Dagors, Protars etc). The Ross Homocentric can be regarded as a logical development from the Rapid Rectilinear - the lenses which were previously cemented in the RR are now separated and have their order interchanged. The curves that had to be the same in the RR so they could be cemented may now be different, allowing the designer greater degrees of freedom. Advantage was taken of this to produce an anastigmat with a vastly better field curvature and astigmatism correction than that of any RR lens (the zonal spherical aberration is also comparitively small). The construction approximates a wide angle but the Homocentric is not a true wide angle lens - some covering power is sacrificed to obtain a quality of performance over a more restricted field.

The advantage of symmetrical designs is the automatic correction of coma, distortion and lateral chromatic aberration (when used at 1:1). However, the corrections are not terribly upset at infinity distances. The Homocentric was supposedly capable of being used as a convertible i.e., either half can be used as a longer focal length lens. Unlike the Protar (which was corrected for coma in each individual cell), the Homocentric probably will need to be stopped down quite a bit when using individual elements to reduce some of the aberrations.

Hope this helps. Cheers, DJ.

-- N Dhananjay (ndhanu@umich.edu), June 11, 2001.