Ernst Dinkla
Well-known
The front cell focusing lenses got my interest again when I recently put a 101mm Kodak Anastigmat Special on a folder camera that normally had a 105mm lens there. It was possible to increase focal length of the 101 lens that it could focus on infinity. I expect it will be degraded in quality at that distance though.
In an earlier tinkering mood I asked questions at what object distance a front cell focusing Tessar type is optimal in quality. Or put in another phrasing; equal to a normal unit focusing Tessar of the same focal length. Interesting in case one should put a lens like that on a camera that can do unit focusing. The best answer I got is as follows:
>>Roland Haid , Apr 04, 2003; 02:34 a.m.
Dear Ernst,
according to J. Stüper, Photographische Kamera, 1962, a lens with front focusing is designed to have optimum performance at a distance of 40x focus_length, hence 2m for an 50mm Triplett or approx. 4m for a 6x9 camera. Thus, good performance for such a design is then provided between 20x focus_length and infinity. If a Tessar design is set to 4m and fixed then, the quality will be much the same for far distances, because Tessar's are robust in variation in object distance.
On the other hand, a lens with front focusing which is optimised for 40x focus_length the spherical aberration will be overcorrected for far distances, this is not a problem because for landscape photographie the lens is used with smaller stops most times and the increase is small. If you fix the front lens at infinity setting, the overcorrected spherical is (not exactly) fixed for all distances and subsequently reducing the quality. Best regards, <<
So for the Kodak lens above that distance set and fixed should be 4040 mm = 4 meter. When I measured the aperture to film distance in the original camera it was less than the 101mm. Measuring and some computing done I figured out that the focal length of that lens (101mm) is given for its optimal quality distance of 4 meter and the focal length varies roughly between 104mm for short object distances to 98mm for infinity. I always understood that a lens focal length is based on image distance when focused on infinity. I can understand that with front cell focusing the lens designers start from another idea, they have to find the optimum in image quality somewhere in the middle of the focal range and compromise for infinity and close-by quality. It is understandable that they describe the focal length of a lens like that at its best image quality and not when compromised for infinity. I could check whether that was common practice by testing some other front cell focusing lenses in a symmetrical image object distance measurement where the total distance between object and image is 4x the focal length
I wonder whether the observations I made and the conclusions drawn are correct and what other aspects are interesting in relation to front cell focusing lenses. There have been front cell focusing lenses used on unit focusing cameras to soften the image but not very succesful, with Tessar types the result is not better than slightly off-focus would give.
Ernst
In an earlier tinkering mood I asked questions at what object distance a front cell focusing Tessar type is optimal in quality. Or put in another phrasing; equal to a normal unit focusing Tessar of the same focal length. Interesting in case one should put a lens like that on a camera that can do unit focusing. The best answer I got is as follows:
>>Roland Haid , Apr 04, 2003; 02:34 a.m.
Dear Ernst,
according to J. Stüper, Photographische Kamera, 1962, a lens with front focusing is designed to have optimum performance at a distance of 40x focus_length, hence 2m for an 50mm Triplett or approx. 4m for a 6x9 camera. Thus, good performance for such a design is then provided between 20x focus_length and infinity. If a Tessar design is set to 4m and fixed then, the quality will be much the same for far distances, because Tessar's are robust in variation in object distance.
On the other hand, a lens with front focusing which is optimised for 40x focus_length the spherical aberration will be overcorrected for far distances, this is not a problem because for landscape photographie the lens is used with smaller stops most times and the increase is small. If you fix the front lens at infinity setting, the overcorrected spherical is (not exactly) fixed for all distances and subsequently reducing the quality. Best regards, <<
So for the Kodak lens above that distance set and fixed should be 4040 mm = 4 meter. When I measured the aperture to film distance in the original camera it was less than the 101mm. Measuring and some computing done I figured out that the focal length of that lens (101mm) is given for its optimal quality distance of 4 meter and the focal length varies roughly between 104mm for short object distances to 98mm for infinity. I always understood that a lens focal length is based on image distance when focused on infinity. I can understand that with front cell focusing the lens designers start from another idea, they have to find the optimum in image quality somewhere in the middle of the focal range and compromise for infinity and close-by quality. It is understandable that they describe the focal length of a lens like that at its best image quality and not when compromised for infinity. I could check whether that was common practice by testing some other front cell focusing lenses in a symmetrical image object distance measurement where the total distance between object and image is 4x the focal length
I wonder whether the observations I made and the conclusions drawn are correct and what other aspects are interesting in relation to front cell focusing lenses. There have been front cell focusing lenses used on unit focusing cameras to soften the image but not very succesful, with Tessar types the result is not better than slightly off-focus would give.
Ernst