O.K. So this is two posts in a row. I went and bought a macro lens (see FS and macro post) and then a 55mm Grandagon at the same time. I know, I know, buy one lens at a time, learn it inside and out, and then move onto the next FL. Sounds good in theory, but when some of these optics come up used for a great price it is hard to resist

So here is the Q. Without burning through a pack of film, is it possible to calculate the amount of fall required when using a rear base tilt (knowing the angle of tilt) in order to maximize the already limited image circle? The amount of shift available without tilts has already been tabulated elsewhere, so no worries there, but this all changes when tilt is applied. I find it impossible to tell from the GG when vignetting has occured, so unless there is a handy calculation available, it seems to me either trial and error, or taking a more rigorous approach and for each degree of tilt, increment fall and record an image. Is the relationship linear beween tilt angle and maxium fall, such that only two tilt angles need be evaluated?