This is one of those questions I feel like I should be able to answer, but can't quite figure it out.
Originally, this fixture used a lamp that was in effect a point source. Later fixtures used a spherical
reflector to gather the lost light and redirect it through the source. With all the light striking the
lens either from the source or re-directed through it, all the rays of light were coming at the lens from the same focal point.
However, an MR 16 uses a parabolic reflector. In this case, the light comes not only from the point source of the filament itself, but from the light redirected by the parabolic reflector. Now, I would assume the filament of the MR 16 you selected is not at the focal point of the reflector, as this would be a mini beam project and I'm not even sure if those exist. What I don't know, and what would help, is the angle of the MR-16's you chose for this project. Either way, with the MR 16, the rays of light are focused at two different points, being the point source that is the filament, and wherever the focal point of the reflector is.
In the mean time, I do offer this hypothesis: the beam of these box spots will still be focusable, however, I would suspect the final beam will be wider than the original equipment.
A thought on that
Fresnel: That pattern reminds me of the Fresnel lenses designed to spread the light into an oval. However, that does appear to be a rather early
lense, particularly in the case of the very large central "bulls eye" and the uneven rings, neither which have I seen in other Fresnel fixtures that I can recall.
Another thing I have been wondering, is a box spot really called a box spot because it is, infact, shaped like a box? If the box was changed to a cylinder, wouldn't it no longer be a box spot? Seems to simple for
theatre.
In addition, in all the Kliegl,
Altman and
Strand literature on line, why is it that I cannot recall seeing a Fresnel box spot, but rather only PC versions. Did the other major manufacturers of the day offer Fresnel box spots, and if so why have I yet to see reference to this. Clearly, Fresnel box spots do exist, as I have pictures I have saved off ebay showing box spots with both Fresnel and Step Lenses, in addition to PC lenses, but where they sold that way, or was the Fresnel a later addition to update older equipment?