| I can't figure that one out Sean...I think is has to have something to do with the angle of incidence of the light entering the lens. I still think that a gradiant spot filter at the front of the lens would simply produce a vignette, possible in addition to smoother bokeh. It would be an easy test to try, fortunately. Other than my guess about the angle of incidence though, I don't know enough about optics to say why putting the filter closer to the focal plane is better.
Obviously the minolta STF has it internally...the second aperture would probably block the gradiant edges of the glass, thus controlling the smooth transition effect.
I have revived the thread at dyxum and asked the author...
**Wouldn't it be cool to try this with a catadioptric lens? Or I wonder if it would just end up working like a regular ND filter given the path of light... |