I have a Toho FC-45X and four lenses. Three of the lenses work quite well, pretty much as I expect. That includes a f/6.8 90 mm Grandagon N. Because I was having some trouble using the 90 mm lens for photographing buildings where I couldn't get back far enough, I got a f/4.5 75 mm Grandagon N. I had checked and several people recommended it highly. But my lens seems to have serious loss of image quality as I depart from the center of the field, particularly if I use any movements. I expect illumination dropoff, but I'm seeing loss of fine detail and an out of focus look at all distances on the edge. I have to do some systematic tests to verify this since there are all sorts of ways to go wrong, which will probably be worse for a wide angle lens. Also, I suppose the illumination drop off could be a factor because of loss of contrast. I don't have these problems with my 90 mm lens.
The specifications say the image circle is 195 mm, which should allow me up to 20 mm shift either way in the long direction. In some cases, it is possible I shifted more than that, which is one reason why I have to do systematic tests. Just looking at the image on the gg under a loupe doesn't suffice because it is pretty hard to see just what is going on at the edges of the field with a wide angle lens, and in any event, after stopping down to taking apertures, it is hard to see much of anything.
The lens is a new lens, so mishandling should not be an issue.
Any input would be appreciated, either with respect to what I should expect from such a wide angle lens or how to go about doing the systematic tests. In particular, how much enlargement should be possible with such a lens before it is obvious that there is loss of image quality at the edges of the field?
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