Ross Homocentric 15" f8.0 brass lens

Background:

The famed and legendary optical company of Andrew Ross produced a series of Homocentric lenses dating back to 1902. The 

Aplanatic f5.6 4 element Gauss design and subsequently corrected, revised and renamed as the Homocentric lens.  This is best considered as a sub-family of lenses.  The British Almanac lists 3 series at f8.0, f6.3 and f5.6 with stopping down to f16.0.  

Distortion and lens curvature was noted at 0.5% of the focal length for the f8.0 variant; 2% for the f6.3 variant and 4% for the faster f5.6 group.   The f6.3 variant seems to have had the longest lifespan, possibly due to its middling compromise in optical design for its relative speed and availability in a number of shutters, including Compound shutters and Rimset Compur and Dialset Compur shutters.

A compound Homocentric variant was offered for sale.  Its blueprint was based on the Dagor construction at f6.8 often offered in 

shutters and bears more in common with being a renamed Dagor.Fast Homocentric lenses at f4.5 were manufactured up to 18" 

stopping down to f22 and will cover whole plate format in their barrel incarnation.f5.6 Homocentrics were manufactured from 3" to

24" with a 9.5" focal length being recommended for whole plate format.  All uncoated Homocentrics are prone to flare; thus the manufacturer's recessing of the lens elements within the metal barrel hood as shown above.

From the British Almanac:

Summary of Characteristics:

The Lens Vade Mecum comments that the slower f8.0 examples were preferred due to the thicker inner glass elements.  These are well corrected lenses, capable of good image coverage the 4 element Gauss design is prone to flare particularly with uncoated examples.  

The advantages of the Homocentric lenses relied on good image coverage and freedom from focus shift on stopping down.  For the tinkerer, all Gaussian elements are easily separated for cleaning. 

Ross Homocentric mounted on a Charten Whole Plate camera

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