Hi eveready, following a tread on dome optic/clown fish in the gallery section, Jean Bruneau and CeeDave aka Chris came up with this diagram. His textual approach to this complex phenomenom is one of the best, so I have contributed my illustration talent to his talentuous text. hope you enjoy,
Capital letters = camera/lens
Small letters = housing/port
( L ) : film plane
( F ) : lens mount flange
( P ) : entrance pupil
These should be made available by camera manufacturer.
( f ) : port flange
( e ) : extension flange
( c ) : center of curvature of the dome
( d ) : location of dome surface
( r ) : radius of sphere that dome is based on, ( r=cd )
All thas is needed from housing manufacturer is ( r ), ( fe ), ( Ff ), and ( ed ). All known from manufacturer or easily measured.
We want the distance from dome surface to entrance pupil, ( Pd), to be the same as the distance of the center of the dome to the surface, ( cd ). ( cd=dome radius ), which is ½ the diameter of the dome ONLY in hemispherical dome. Also( c ) is in the plane of the back of the dome only in the case of hemispherical dome.
From the Diagram:
The Camera flange to dome surface distance is ( Fd = Ff + fe +ed )
The camera flange-pupil distance may be computed using the film plane datum is ( FP = LP- LF ).
The Pupil to dome surface is ( Pd = Fd - FP )
If the dome is in the wrong position, rays do not pass through the dome to the lens perpendicular to the dome. This causes refraction, and thus chromatic aberration and blurring...the effect is most noticable at the edges of the dome, where the angle is furthest from perpendicular.
Also, larger dome have more gently curved image planes and require lower diopters,if any, for focusing; this is a lesser concern with fisheye lenses than with rectilinear lens.
And this image is to show that sometime a small dome can be a part of a larger sphere, yet it will retain it's original radius, the important thing to remember is that altought your dome might look small and non spherical, always consult with the manufacturer to find out its true diameter if it is unknown.
The 12-24 DX entrance pupil is given at 1.5-2 mm focal length increments by Wisniewski ... these use image/film plane as reference (rather than lens flange, which would make more sense to me, since it's a lens attribute...but it's probably easier to measure in some bench and panorama setups).
12-24 DX entrance pupil relative to image plane
Focal Length, mm
..........Entrance Pupil, mm
The entrance pupil does move to and fro along the lens axis, but only slightly in the "center" of the zoom range, 15-20mm. I don't think you'd ever be able to detect 1 mm of "error" in pupil-dome center placement in images, especially on a 75-100 mm radius (6-8 in diam) port. But a port optimized for his center range might be far enough from optimal at the extremes (12, 24) for the most discerning to detect. Of course, if Pd is not near cd (in the above diagrams), that probably will be detectable as softness and CA -- and here the error could be cm, not mm. But I dunno...just guessing. Will try to make some test shots when I have a waterproof thingy with a covered hole in front to put my D70 in.
As far as I know, housing manufacturers do not usually provide the needed length/radius data, and I don't have a D70 housing yet to measure them. Ike? Jean? Others?
Oh, and all the distances/locations in the text with Jean's diagrams are measured along the lens axis (especially d!).
Illustration by Jean Bruneau a.k.a Viz'art
Text by Chris White a.k.a CeeDave