Open sights have many advantages: they are very common, inexpensive to produce, uncomplicated to use, sturdy, lightweight, resistant to severe environmental conditions, and they do not require batteries. On the other hand, they are not as precise as other forms of sights, and are difficult or impossible to adjust. Open sights also take much more time to use—the buckhorn type is the slowest, patridge, "U" and "V" type notch sights are only a bit quicker; only the express sight is relatively fast. In addition, open sights tend to block out the lower portion of the shooter's field of view by nature, and because of the depth of field limitations of the human eye, do not work as well for shooters with less than perfect vision.