Originally Posted by Drexler
Thanks for your answer and I don't doubt that you're right! However, there is something that doesn't make sense to me. It might be that I have misunderstood something. If I write my thoughts about it, maybe you can make sense if it?
Projectors have an optimum focus at a fixed distance from the lens (the distance depending on the focus setting of course). The plane of focus is therefore curved. However, since the depth of field is quite large you are still able to get excellent focus across the entire image also on a flat screen.
When an anamorphic lens is introduced the depth of field is greatly reduced and you might need a curved screen to allow for perfect focus. This is getting worse the shorter the throw is.
Not 100% sure where that came from. The depth of field should not change. The need for a curved screen is not to correct focus, it is to correct pincusion. Pincushion has concave top and bottom edges and looks like a bow tie (very crude I know).Given the light beam height is taller at the edges than in the centre, curving the screen (bringing the edges out) means that those portions of the screen are now closer to the projector, hence the light beam height is reduced at those sections as well. The curved lines are not straight.
If this is true I don't understand how you can get perfect focus across a flat surface by adjusting the front element of the lens. I.e. you're adjusting the focal length of the horizontal dimension, but you can't have two different focal lengths at the same time (screen edge and center). Hence a curved screen is needed to bring the whole image in focus.
Good optics are complex using many elements. An anamorphic lens is much simpler in design. You start by making sure that the projector is in focus. You add the lens and make the astigmatism adjustment. This changes the distance between the front and rear lenses and compensates for the fact that there are differenent focal distances from the projector to the screen (due to the curve). As you make the adjustment, you will see H and V lines come in and out of focus. This focus shift is not as radical as rocking the projectors primary lens. One you get the H and V lines in focus across the screen, the entire image will maintain focus across the screen, however it all starts with the projector's primary lens.
So, either there's quite a good depth of field even with the HE-lens in place, or I have gotten something wrong with my understanding of optics.
The distance between the centre and the edges of my curved screen would be less than 75mm (3") across 2400mm (8 feet). You can see the curve when looking across the screen from one edge, however from the seating position, the only real tell tale is the horizontal masking has a slight bow when the lights are on.
So the distance between the centre and the edges of the 1.78:1 width are not that great as to affect focus from the projector's primary. What you see with the lens removed is barreling where the ends of the image are shorter than the centre. The image is still in focus at this point, it just has convex top and bottom edges. If the image was out at the edges, then an anamorphic lens could not refocus that.