I did a quick comparison of a Fujinar 250mm f4.5 lens (a 4/3 tessar design) and a Fujinon SF 250mm f5.6 lens (a 3/3 triplet designed for soft focus) to see how they compare when well stopped down. The film used is Shanghai GP3 100 in 5×7 format with a Rittreck View 5×7 camera. Note that the soft focus “strainers” were not used for any images with the soft focus lens.
The first pair of images were made at f22. Note that the second image has slightly less angle of view indicating that the SF lens has longer focal length. (That’s not significant, just an observation.)
The image quality from both lenses is very good, with similar high level of sharpness (note the writing on the building just to the right of centre, and the detail on the building on the centre left edge) remembering that that this is 5×7. They are difficult to tell apart.
A minor operational disadvantage of the SF lens is that focus will shift when the aperture is closed, and Fuji recommends that the lens aperture should be set for exposure then closed to one or two stops more for focussing, then reset for making the exposure. I followed this procedure with the SF lens but noted that after about f32 focussing becomes difficult, and did not close down further than f22 while focussing.
I took a second set of images at a different location. This scene was strongly backlit which caused some flaring in the soft focus lens at different times, most notably for smaller apertures the flare shows as a circle in the centre of the image. (It is possible that at wider apertures the flare covered the whole image, and was corrected somewhat in scanning and editing.) Shading the lens with a hood, a hand, hat or dark slide would probably solve the problem, as would avoiding strong backlight scenes (but where’s the fun in that).
For comparison, an image was made with the lens as wide-open as I could manage which was f8-11 (the fastest shutter speed on the Copal 3 shutter is just 1/125). Even at this aperture the image sharpness is very good but there is a hint of softness when enlarged, which is interesting.
Lastly an image with the lens stopped down about as far as it goes, half-way between f64 and f90. The flare is obvious in this image.
I’m happy to conclude that the Fujinon SF 250mm lens produces good image sharpness when stoped down to f22 and smaller, at least as good as a tessar design lens, with the proviso that strong backlight can cause flare, and that the lens must be stopped down to the taking aperture when focussing. The SF lenses are abundant on the used market, many missing one or both of the soft focus disks, and are half the cost or less of a similar plasmat lens. When well stopped down that Fujinon SF 250mm lens could make a cheap long lens for pictorial work on 4×5 and 5×7.
Fuji specifies the SF 250mm lens image circle as 300mm which is just a few millimetres from covering 8×10: a test on that format is pending.