Originally Posted by
Bob Salomon
1 the faster wide angle lens, from any given manufacturer, covers a larger circle then the slower one.
2 because of the larger circle the fall off starts further out from the center so it is less then the slower lens over the same film area.
3 Schneider, Rodenstock and Heliopan made center filters for large format wide angle lenses. Nikon and Fuji didn't.
4 from any given manufacturer the MTF curves, distortion curves, fall off curves are better with the faster lens.
5 none of the Rodenstock, Fuji, Nikon or Schneider wide angle lenses perform optimally wide open or near wide open. And all center filters require that the lens be stopped down at least two stops for the center filter to have any effect.
6 all of them are optimized for f22 and are diffraction limited at f22. For star tracks that is probably a moot point. But are you only going to shoot star tracks?
7 if a lens is a stop faster at a given focal length then it's front element is significantly larger in diameter. This adds more weight, just for the lens element. Since the lens element is so much larger the diameter of the lens barrel is also substantially larger and, since it is metal, it also adds weight. Then, if you check the number of elements you will find that the faster lens has more elements, that adds weight. For each element, or groups of elements there is mounting and centering parts within the lens, this adds weight. In the case of the Rodenstock 90mm 4.5 Grandagon - N, it is mounted in a 1 size shutter while the 6.8 Gradagon-N is in a 0 shutter. This adds weight. The formulation and specifications as well as the performance of the faster lens is always different then it's slower cousin.
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