Marco Antonetto, in his book Topcon Story, subdivides lenses into versions of each focal length/maximum aperture. His classifications are apparently based on the manufacturer's parts lists and on a large data base relating serial numbers to versions. This is the superior method, but suffers from the fact that not all changes are apparent without disassembly of the lens.

I decided to use a different method, based on external characteristics. My method has problems with some lenses, especially the 58mm f/1.8, but works with most of them. I also decided to change the terminology from 'versions' to 'generation' to avoid confusion between the two techniques. I suggest referencing Topcon Story as well as the following paragraphs when attempting to classify a lens.

The attributes I use for classifying lenses into four generations are illustrated in the figures below.

First Generation Attributes

• Red aperture index and Infrared 'R' in the depth-of-filed scale
  
• Milled out recess on rear plate for compatibility with the original R Topcon cameras.
  
• Slot-head screws for attaching the rear plate
  
• Two infinity marks on the distance scale
  
• Beads on the focusing bands
  
• Focal length specified in cm
  
• Lettering on decorative ring in lower case (Accurately, the first letter of each word is capitalized)
  

• Green aperture index
  
• Focal length specified in mm
  
• Lettering on decorative ring in lower case
  

• Most, but not all, lenses have a matte black finish
  
• Green aperture index
  
• Flat rear plate
  
• Phillips-head screws
  
• Single infinity mark on distance scale
  
• On most lenses the 'feet' distance scale is colored orange
  
• No beads on focusing bands
  
• Focal length expressed in mm
  
• Lettering on decorative ring is upper case (all capitals)
  

Even though the 500mm f/5.6 lens has a red aperture index, I classify it as a third generation lens because the lettering of the decorative is lower case and the focal length is expressed in mm.

It would be possible to subdivide the fourth generation based on the movement of the aperture release tab on the rear mounting plate. Some follow the previous generations with the tab moving in an arc, centered on the optical axis. Some of the very last lenses have a redesigned aperture tab mechanism and the tab moves in straight line (or at least in an arc with a much greater radius). I decided not to incorporate this feature in my classification because there is no change between lenses of the same focal length, the variation exists only between lenses with different focal lengths.

In any collecting endeavor there needs to be some metric for deciding what to collect. Numismatists and philatelists have an easy time (at least with regards to classification) since coins and stamps are well documented with year, issue, mint, etc. being unique and unambiguous. But when the collectable item is the result of a manufacturing process--such as cameras and lenses-- change will probably be incremental, with gradual modifications made through the life of the production run. Of course the basic models, such as focal length and maximum aperture, are clear and easy to identify. But what of the accumulation of minor changes that add up to clearly identifiable differences between the beginning and end?  It would be near impossible to accumulate every version, especially when most of the changes are minor--a gear is made from steel in place of brass, or an adjustable stop is replaced with a fixed, precision milled, one--especially when identification would require disassembly. What is needed is the selection of easy to identify attributes that allow sub-divisions over the production life of the item.

A further complication for the Topcon collector is the fact that Tokyo Optical stopped supporting it's camera products about 1977 and repairmen were forced to cannibalize parts from one lens to repair another, resulting in many lenses with mixed heritage. The rubber focusing bands may degrade with time and are replaced with those from a parts lens; the rear lens plate is often 'dinged' when a lens is dropped and needs replacement; slot-head and Phillip's-head screws are totally interchangeable. Usually the replacement is made with parts on hand and without regard to 'purity' of the original version.

Because Topcon is only now becoming collectable I doubt that there have been many deliberate forgeries--intermixing of parts to produce an unusual, or rare, lens. But such practice is possible and the collector should be wary of any lens that seems 'odd'.

Of course if a lens is obtained for the sole purpose of photography, then exact classification is of no interest--if the lens produces superior images then use it. But for those of us who enjoy both using and collecting, a rational method of classification is essential.