The Minimal, Normal, Aggressive, optimizations are used depending how many objects are used in the scene, few objects = Minimal, lots and lots of objects = Aggressive. This basically doesn't change the quality of the render, but helps speed it up. Ray's per pixel helps with the quality of the render, it determines how many rays are sent into every pixel in the ray tracing process. More rays= longer render times. I seldom go over 64 even with large resolution images. 48 bit dithering helps smooth out any color layering, like you can get with moon rings. Where you can actually see the color change from one pixel to the next, instead of it being blurred together. This image is a good example where 48 bit dithering needed to be used. http://stevesartgallery.bryce-alive.net/gallery8/wolfnight/pic19.html Maximum ray depth determines how many transparent or refection will be penetrated in the ray trace process. Like if you were to place two mirrors angled so they reflected into one another. How many times would you want the reflections to go from one mirror to the other. Or if you where to stack 15 transparent objects on top of one another, you would need the ray depth set to 15, for the last object to be penetrated by the rays.

Stephen Ray