>>3878441Split prism always detects a difference, that's its main advantage.
The disadvantage is that you don't always have nice vertical lines to use the split prism in a very precise way - this is partially solved by a diagonal split prism but not entirely.
Also, you can only use a split prism on the centre, so for off centre subjects you have to focus and recompose, which can throw focus off if you have very thin DoF.
>I've read they cannot go beyond f/8 but can pinpoint f/1.8, can they go lower aka f/1.2?That's a different thing.
Another disadvantage of split prism is that half of it goes dark for apertures smaller than f/5.6 so it's rendered useless. I mean this limitation is for the lens's maximum aperture, not the chosen aperture for the exposure.
They have no limitation on how wide of an aperture they can focus though.
There's a compromise to be made here, but you don't get much choice.
A steeper angle in the prisms that makes the split prism, means higher accuracy *but also* darkening at wider aperture than less steep prisms. For instance one split prism might be a bit less accurate but darken at f/5.6 (so unusable for lenses with smaller max apertures), while another might be more accurate but darken at f/4.
>i heard matte screens can do f/1.2 easily but i don't know if it is hard to see a difference from f/2 for example.Yeah matte screens are fine for fast apertures.
There's usually a balance to be struck in how fine you ground a matte screen.
A very finely ground one will be brighter - all things being equal - but it will be less contrasty (=show less the difference) between out of focus and in focus. A coarser screen will be the opposite, i.e. darker but greater contrast between in focus and oof, helping with focusing accuracy.
So for faster lenses a coarse matte screen is better than a fine matte, since you need more accuracy and the lost brightness due to screen is compensated by the increased brightness due to the wider lens aperture.
