I just had one of my all time lucky days and tripped over a beautiful Cooke XVa triple (maybe more, I'm hoping) convertible lens. Now, I understand that the proper way to use the elements is either together (12 1/4"), or singly behind the shutter (19" or 25 1/2") to obtain the three conventional focal lengths of the lens when used as it ought to be. However, as I have started playing around (I can't dignify what I am doing with the more exacting term of experimenting) with the various elements I find that a single element in front of the shutter still casts a crystal clear image on the ground glass that appears to be only a little less in focal length than that achieved with the element behind the shutter (I'd guess about 21-22" for the front element in front vs the stated 25 1/2" when mounted behind the shutter). I have no idea what this does to the aperture of the element compared to using it behind the shutter. It does seem to reduce the flange-focal plane distance more than the simple drop in focal length would predict. I have not yet tried simply reversing the elements, that comes later today. Nor have I developed any film as yet, so I have no idea what sort of horrendous aberrations I may have introduced that were not apparent to me on the ground glass.
So, my questions to the optical gurus that abound on the forum are what is supposed to happen when one misuses a lens in this fashion (aside from that I should probably be struck down by lightning for desecrating this treasure in such a fashion)? Is their a way of knowing/predicting what happens to the focal length, aperture, and focal/flange distance of the individual elements when they are mounted in front of the shutter rather than behind (and of course therefore reversing the configuration of the lens elements themselves compared to that intended by their design)? What might this do to the optical corrections of the elements, or in other words, what sort of unfortunate aberrations am I likely to introduce? Similarly, what ought to happen when the front and rear elements are reversed and used as a combination? The focal/flange distance of the single elements used appropriately behind the shutter is somewhat greater (~10-15%) than their focal length, presumably because the nodal point is now behind the lens plane with the single element. Might this relationship be reversed, with the nodal point in front of the lens plane, permitting a shorter focal/flange distance, with the element mounted in front (and therefore reversed) of the shutter (and thus achieving my hope of being able to use the front element of the lens on a camera with a lesser bellows draw)?
Any and all insight will be most appreciated. My attempts to search the forums have not brought me answers as yet.
Thanks to all,