Prior to the development of SLR, all cameras with viewfinders had two optical light paths: one path through the lens to the film, and another path positioned above (TLR or "twin-lens reflex") or to the side (rangefinder). Because the viewfinder and the film lens cannot share the same optical path, the viewing lens is aimed to intersect with the film lens at a fixed point somewhere in front of the camera. This is not problematic for pictures taken at a middle or longer distance, but parallax causes framing errors in close-up shots. Moreover, focusing the lens of a fast reflex camera when it is opened to wider apertures (such as in low light or while using low-speed film) is not easy.