Originally Posted by Rick Olson
Actually, the G Claron IS a plasmat (6/4 construction). I suspect the greater coverage of the G Claron is due to the absence of field stops. A field stop mechanically limits a lens' field of view. Most older lenses (Dagors, Angulons, etc.) did not incorporate field stops. Without a field stop to mechanically limit coverage, many older lenses have a much larger circle of illumination than circle of acceptable definition. The problem with this situation is that different people will have different opinions about what exactly constitutes "acceptable definition".
A real world example - The Schneider Angulon is a derivative of the famous Goerz Dagor (6/2 construction). The powers of the elements were switched (it's often referred to s a reverse Dagor design) and the outer elements are oversized (compared to a standard Dagor of same focal length) in an attempt to increase coverage by elminating mechanical vignetting. Since the goal was as much coverage as possible, the Angulon design doesn't incorporate any field stops. Early Angulon ads and brochures touted coverage as high as 107 degrees for the Angulon (here's a Schneider catalog from 1939 that lists the coverage of the Angulon series as 105 degrees).
Yes, an Angulon will throw a huge circle of illumination, but just how much of that huge circle is actually usable. Well, again it depends a bit on personal opinion, but in my experience with several Angulons of varoius focal lengths and vintages, usable coverage is around 80 degrees at f22 and increases to about 90 degree in the f32 to f45 range. I've been deliberately vague here as this is based on my own personal criteria for "acceptable definition".
So, Schneider publishes these outlandish 105 - 107 degree coverage claims and photographers start saying the Angulon has very dismal corner performance - it's soft at the edges, etc. Because, frankly, it does go soft in the corners, very soft, if you try to use the full 107 degree coverage (or even much more than 90 degrees). This isn't good for Schneider. It makes them look bad and hurts the reputation of their products. So, after WWII they start to gradually reduce the published specs on their Angulon series. By the 1960s, the published specs for the Angulon are a much more conservative 80 - 85 degrees at f22. The lenses still throw this huge circle of illumination, but the expecatations are now a lot more reasonable regarding the usable coverage of these lenses.
In order to avoid this problem from recurring, sometime in the 1970s, Schneider started incorporating field stops into the deign of their general purpose taking lenses. They use the field stops to limit the circle of illumination to something at, or slightly larger, than the published coverage specs. So, with these field stops, they have set a hard limit on maximum possible coverage and have removed any ambiguity concerning what constituites "acceptable definition".
So, back to the G Claron. It doesn't appear to have any field stops incorporated to limit maximum coverage. In this way, it's still a bit of an "old school" lens. Beyond the published coverage spec, the G Claron's performnce tapers off very gradually. And the performance at the extremes can indeed be improved by stopping down, thus effectively enlarging the circle of acceptable definition. Most photographers who use G Clarons consider the usable coverage to be about 80 degrees at f32, and maybe a couple degrees more if stopping down to f45 or f64.
Another lens that apparently didn't incorporate field stops is the old 355/360mm Symmar (often called the Convertible Symmar). Older samples came in Compound or Ilex shutters, had an engraved focal length of 360mm and a maximum aperture of f5.6. I have a later sample (ca. 1971) that is engraved 355mm and has a maximum aperture of f6.8 (due to the physical limitations of the Copal No. 3 shutter). Schneider rated this lens to cover 70 degrees at f22. Like the G Claron, coverage continues to increase as you stop down. Like the 355mm G Claron, users have reported that these older Convertible Symmars cover 12x20 with a bit left over at small stops.
When the original single coated Fujinon-W line debuted in the early 1970s, Fuji advertised 80 degrees of coverage. I suspect these lenses either had no field stops, or less restrictive field stops than their Schneider and Rodenstock contemporaries (Symmar-S and Sironar-N). When the newer multicoated Fujinon-W series (called NWS in the literature, but still labeled as Fujinon-W on the lenses) debuted around 1980, I have no idea if they incorporated field stops to limit maximum coverage. They did reduce the coverage claims in their published specs to a more conservative 72 degrees (more, or less, depending on focal length).
I recently purchased one of these 1980s vintage 360mm multicoated f6.3 Fujinon-W lenses (like the one Emile has - I paid considerably more for mine, though). I haven't yet had a chance to test the coverage. Based on Emile's, and other, user reports, I'm sure it will cover 7x17 just dandy. How much more, I can't say. I'll find out eventually when I have chance to test it along with some of my other ULF lenses. I'm planning to add 14x17 as my next format. So, I want to see just how much my various lenses cover before deciding which to keep and which to sell.