Why do split-image and microprism focusing aids darken off-axis or with slower lenses?

Two things contribute to this phenomenon:

• The light collected by split image focusing screens are edge rays collected on the outer areas of the front element of the lens. With lenses that have smaller maximum apertures the focusing screen is trying to find light from an area wider than the front element of the lens. In most situations focusing and metering are performed with the lens at the maximum aperture. Since a lens with a larger aperture collects more light than a lens with a smaller aperture, the viewfinder will be dimmer when using a lens with a smaller maximum aperture, regardless of what aperture setting is used to take the picture.

• The light passed through the focusing screen is fairly directional. Viewing it off axis will cause it to be dimmer. This is especially true of the light coming through the split image portion of a focusing screen found on most film SLR cameras. It should be noted that most DSLRs, especially those with APS-C and other smaller-than-FF-sized sensor designs, are equipped with focusing screens designed to maximize brightness. They do so at the expense of being able to accurately depict depth of field for apertures below about f/2.8 (it varies by camera model).