Again my ignorance what is astigmatism and what does it look like?
That's OK. We're all ignorant at first. Then we ask. Then someone shows us. And then we're not ignorant anymore.Astigmatism
The three images above depict astigmatism: in this case, a differential focus between horizontal and vertical lines. Basically the focal point of the prisms is different to that of the projector lens. Hence you only achieve perfect focus in either vertical or horizontal, but never both.
First, focus the projector on the screen. Place the prism anamorphic in front of the projection lens. Looking at the illustrations (a 4x4x1 pixel checkerboard pattern).LEFT
: The projector lens is focused. When you interpose the prism lens in front of the projector the vertical direction remains focused (as the prisms have no optical power in this direction), but the prisms throw the horizontal direction OUT of focus.MIDDLE
: Leaving the prisms in place, start throwing the projector out of focus significantly. Stop when the checkerboard pattern looks like image #2 (middle). You have been forced to adjust the projector focus because the prisms, by definition, have no adjustment capability. The image in the vertical direction becomes blurred, but the prisms are now projecting clean edges in the horizontal. The projector's out of focus position at this point is, however, perfect for the prisms.RIGHT
Half-way in-between, a "compromise focus". Neither prisms nor projector are in perfect focus, but half-way in between for each. When you move the prism lens out of the way the projector will remain unfocused, because to achieve this "compromise" you have to throw it out of focus to accommodate the optics of the prisms.
A system that, depending on whether you're looking at horizontal or vertical lines, has two (or more) different
perfect focus points is said to exhibit "astigmatism".
Prisms, by their nature exhibit astigmatism. No matter how well made, or finely ground, no matter how perfectly flat their faces are, every prism system exhibits astigmatism. This is why the changeover was made early to cylindrical lenses: there are simply more degrees of design freedom with cylindricals than there are with prisms.
You can add a single corrector lens (a weak cylindrical) to a prism system and remove the astigmatism at one, and only one
focus point. You can also introduce a more complex corrector that is continuously adjustable, so that for any distance the system will be in focus ( after manual adjustment of the correctors, of course). Such a corrector consists of two or more cylindricals in line with the prisms, which begs the question: why mix multiple cylindricals in this manner with prisms? The device would be heavy, expensive and complex optically. This is why the move was made to pure cylindricals back in the 1950s. They are lighter, have more freedom in design, can accomplish both continuous focus, continuous atigmatism correction and color correction at the same time, and are smaller for just about any given anamorphic task.Non-uniform sharpness edge to egde
There is another out-of-focus condition where the edges are blurred but the middle of the picture is sharp (or vice versa). Prisms (and cylindrical systems) also exhibit this kind of aberration. With cylindricals it can be effectively compensated for. It is harder to do this with prisms, because they have fewer degrees of freedom in their design. This is not
astigmatism, but a separate problem.Color Aberration
Thirdly, color aberration can mimic blurriness. If vertical lines are not separated enough to be clearly out of color alignment, they can still be separated enough to look, or mimic blurred. You end up with thicker vertical lines (worse vertical focus) no matter how well made the prism system is. The illustrations above exhibit color aberration (they are a worst case for prisms). Combining two or more types of glasses per prisms (cemented doublets) can fix this, but will not affect astigmatism or edge to edge sharpness.
Uncorrected prism systems exhibit all three of the above sharpness-inhibiting aberrations. Together they reduce sharpness of the image by a substantial amount, easily 50%. Your high definition projector's resolution is not fully taken advantage of in these cases.Geometric Distortion
None of the above will fix geometric distortion which can be defined as equal sized grid squares on the imaging chip being projected at progressively bigger sizes as they reach the edge of the screen. The usual 1:1 point (perfect square grid reproduction) for grid squares in this arrangement is about half way between center and edge of screen. Towards the middle the grid squares are too skinny. Towards the edges they are too fat.