Originally posted by alain Gillot on a pentax istD S [ 6.1 MP ] LIKE NEW , the focusing screen sucks , i use this camera with T mount tele lenses [ birds ] it work great in automatic function , but focusing is difficult due to the poor quality on the screen . can this be replace with a smooth one .the existing one is made with circular grooves . a 800mm lens f11 is toucy on the focusing .thank you .
The circular grooves you refer to are the ridges of the screen's Fresnel lens. They serve to improve screen brightness and are not an indication of poor quality or design. Almost all modern screens use some form of Fresnel lens. The Fresnel "lines" are generally not visible when using faster lenses typical of general shooting but become very obvious at narrower apertures. Replacement screens do exist that do not have a Fresnel, but those will be quite dim with your lens. Screens are available that have split-image and microprism focus aides, but neither will work well at apertures narrower than about f/5.6-f/8. I would normally suggest using the focus confirmation feature of your camera, but it too is essentially inoperative at apertures narrower than about f/8.
Steve