The master key: a fisheye A special photographic lens Caracteristics: Very short resultant focal A standard back focal length (42-43mm) A special distorsion of image a circular picture, typically 24mm of diameter
Why can't we adapt directly a fisheye to a beamer ? Because with the original lens, the picture isn't correctly focalized, and the size of beamer's picture isn't compatible
Removing and replacing the original lens of beamer: don't work either Because the back focal length of the fisheye is too short. A standard beamer's lens has 80mm-100mm back focal length. Compare with the back focal length of the fisheye.
Still not working... Extend the back focal length of the fisheye with a divergent lens, like a barlow lens
Then... What works ? Beamer + Condensor + Diagonal flat mirror + Fisheye
« LSS Principae » to project an image with a fisheye...
« LSS Principae » We must build an intermediate image, with Correct image size: typically 24 mm in diameter Correct incident ray light, like to take a frame, but inverted...
« LSS Principae » To build an intermediate image from the beamer, we use the same process of microphotograpy, but inverted. We don't take a frame, we project a frame, by putting additionnal lenses, (photographic lenses), in front of the beamer
« LSS Principae » This intermediate image can be deflected by a flat mirror. Advantage: the beamer will work in a « made for » position with no dust and heat problems.
Technics: Fisheye specs Fullframe format (24x36mm) Diaphragm used to limit some optical abberations in some cases (typically used at F/D8 with a standard 50mm photographic lens as condensor) Focus unnecessary (doesn't change anything)
Technics: Condensor specs Focal length and F/D is of no importance BECAUSE: Specifications change with: The back focus length => change the incidence of the rays of light The size of the rear lens of the condensor (to prevent cutting image) Diaphragm on condensor cuts the image: don't use it !
Technics: Condensor specs: ideally Back focus length as short as possible for max field of view Diameter of rear lens minimum approximatively 24mm or larger
Technics: Condensor in practice If you're lucky... find A 75 mm F/D 1.1 Heligon, which is: 10-15mm back focus lens (vary upon models) A big rear lens Exist in some variants: TV, XR, etc...
Technics: Condensor in practice With actual commercial lenses A photographic objective, as Canon 50 mm F/D 1.4 USM To reduce back focal length, add a proxy lens, +4 D to the system
Distance between optical parts A constant: back focal length of fisheye: 42 mm Correct distance: add back focal length of fisheye and back focal length of condensor Typically between 84 mm (42+42mm) and 52 mm (42+10mm)
Adjustments with Beamer Use DLP technology to increase contrast Zoom parameter can change the field of view and focus Focusing the image can be made on the beamer. If you can't, change the distance between fisheye and condensor (probably too long)
Field of view depend of: Focal back lenght of Condensor Beamer objective (zoom and focal lenght) Beamer size of DLP (typical 0,55 or 0,65'')
TIPS Don't place the diagonal miror at intermediate focus, if you don't want to see dust…accurately Back focal length of the condensor can be measured, using a beamer and a projected picture on a piece of cardboard (as test pattern) You can reduce the focal back length of the photographic lens with a convergent lens in front of condensor (typicaly F=250mm = +4D) but it adds some distortions on the edge
TIPS Focusing the projected picture need a VERY precise alignement of condensor and Fisheye otherwise there will be always a part of the horizon blurred... (like telescope collimation) You can limit horizon blur and distortion with fisheye diaphragm, but with loss of brigthness Note: unecessery with Heligon lens