Are there lens with 1mm
focal length?
What is the shortest focal length possible for full frame sensor? And can it be rectilinear lens?
There is Sigma EX 4,5mm
, but it's for APS-C and still far from 1mm and it's fisheye lens, not rectilinear.
Largely it depends on sensor size. 4.5mm is for APS-C but there are shorter ones, even less than 1mm, for smaller sensors.
The issue for a rectilinear lens is that as the angle-fo-view gets wider, the construction becomes more and more difficult. Remember that a rectilinear lens must preserve straight lines and so the optics will get more complex and certainly larger and heavier to correct for aberrations.
At this time, the largest field-of-view from a rectilinear lens on an APS-C camera is 104°, obtainable by at the widest of a Sigma 8-16mm lens. This was the widest of any rectilinear lens until last year when Irix introduced their 11mm and Canon followed with the EF 11-24mm F/4L that can achieve 126° field of view.
With a fisheye, the mapping is completely different and you can have some than even project more than 180°, up to 220° as far as I know, although I forget who was the manufacturer. It is a third-party one.
I you're talking about "full frame" there is the Nikon 6 mm fisheye (https://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/fisheyes/6mmf28.htm) and the rectilinear Voigtländer 10 mm for Leica M mount: https://voigtlaender.com/10-mm-f-5%2c6-hyper-wide-heliar-vm.html