They also don't do area of irregular objects well at all. Were your eye is centered in the field will also influence the measurement. The end result is that two different persons will read a reticule differently. This difference may be significant or it may not. If it is you will need to use another system.
Video systems are more accurate and easier to use for very exact measurements.
There are three general types of video measuring systems, video filars, interactive video and fully automatic. Each has its uses and problems and manufacturers make a wide range of each system. All video systems use a video camera, mostly black and white. The camera is connected to a computer or digital system, think of it as a black box. The black box does something and out pops the magic number.