There only seem to be some circa 1890 lenses of this kind.. any ideas why?
If I really really wanted one, what would be the difficulties in making my own or adapting a video lens to this purpose?
Also, what would an astigmatic lens do to the image -- make it fuzzy (again, there are some old varifocals I'd consider)? I imagine any crisp images would need strong filters on the front.
Define Varifocal: zoom, depth of field manipulation, or other?
The old variable focal length lenses werre comparatively simple. They actually consisted of a normal lens followed by a negative attachment in most cases, though some were sold with the two portions assembled into one barrel. In either case, there would be a method of varying the spacing thus changed the focal length of the combination.
Results varied. Most could be used for very distant pictures, but with the disadvantages of extremely small effective apertures and rigidity problems during the resulting long exposures. Additionallly, though some of these combinations were better than others, image quality was not up to that of the normal lenses then available. When Busch marketed their fixed focal length "Bis-Telar", it was recognized as providing superior photographs, and the variable focus market faded away although some makers kept them available for years.
It wouldn't be too difficult to make one. You could put a negative lens, preferably an achromat, from Surplus Shed or some such source, into an old focusing barrel. (Please don't destroy a good Petzval for the barrel.) Then the hard part would be marrying your attachment to a normal lens. I guess an even easier approach would be buying a variable magnification spotting scope. I think they are basically the same thing.
Besides astigmatism. the negative lens would introduce other nasties into the mix, but the examples of photographs made with the old variable focal length lenses I have seen (not all that many), haven't been all that noticeably fuzzy. This is probably because of restricted angle of view, and black and white film probably less than panchromatic. Filtering might or might not help depending on the mix of aberrations produced by the particular combination. One more important factor, those examples still existing were probably made by highly competent photographers.
You didn't exactly ask, but modern zoom lenses are infeasible for LF use because of weight, bulk, and expense.
The Plaubel Tele-Peconar series started before WWI, but the bulk of them were made between the wars. I guess the poor image quality possible back then killed them.
By the time computers, a wider range of glasses and multicoating made good zooms feasible, large format already had become a niche product for ultra high image quality applications, and a loss in IQ would not have been considered a acceptable tradeoff.
ah, these all make sense
thank you
given the reasons, I don't think i would consider making one at this point
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