On February 3, 2019 at 11:59, Ernie Gilman said...
Ah. I had no idea you were talking about whether the lens was built in a way that allows mounting masking to it. I was asking about fuzzy edges, which is an optical thing. Sorry I missed your point.
Not into photography, eh? How do lens filters mount? Usually, it has a threaded ring that mates with the front of the lens- that's why I asked about asking the manufacturer if they offer a masking element. WRT an opening cut into a lens cap is that you may have an umbra and penumbra, which will make it look like there's a problem when there really isn't- it's just one of the phenomena that occur when an object is placed in front of a light source.
You didn't say you were not just talking about the optics.
When someone asks about masking, the brand IS important because somewhere, someone offers a product for achieving that and since all lenses aren't the same, it would need to be brand-specific.
No. The radius of almost every projected image I've ever seen is infinity. That is, the surface onto which the image is projected is flat. Like a screen, for instance. In this case the radius is about seven feet. The shape of the space is cylindrical, with the center line being vertical.
Infinity? Maybe for long throw, but a 7' radius isn't going to work with that type of lens. The image is focused on a plane at some distance from the lens and just like capturing an image, the lens has a limited depth of field. Because of this, a projector that's near its inner limit of useful distance to the screen may be in focus at either the edge or center, but not at the other when projecting its image onto a flat surface. The whole reason curved screens are used is so the viewers will be able to see things in focus when they turn their head or rotate, without their eyes needing to
refocus as they turn. The problem- if they're not at the correct locus for the curve, it won't be in focus as it should be and if the lens isn't right for the curve, ditto.
How far from the screen is the projector to be mounted?
Remember- the elements of a lens aren't flat, which means they naturally distort the image to some extent. They have corrective elements, but those only work when the lens assembly can focus at a specific range of distances- anything inside or outside of that and spherical aberration occurs. Also, the colors don't have the same wavelength, so it's possible that the same kind of rainbow effect may be seen that's visible with DLP TVs- the colors don't focus on the same plane.
