The least expensive systems often employ simple optics. In this case, a projection lens magnifies the object and the image is directed to the screen by one or two mirrors. With two mirrors, the image is correct top to bottom, but still reversed. This second system is often referred to as corrected optics. Simple optic images that are upside down and reversed.
Corrected optics are right top to bottom, but still reversed.
In a simple lens system, changing magnification involves removing a lens from the front of the machine and replacing it with another. Some systems make this more convenient by providing a multiple lens slide or turret. OGP's Focus is a unique corrected optics system, since it has a constant working distance and an internal lens turret. The final image is fully corrected, or in other words, erect and unreversed. Images on machines with fully corrected optics are erect and unreversed.