Katz Eye Split prism

I have a KatzEye on my K20, and it's a great improvement for manual focussing. It doesn't affect autofocus, but as andrewk says, it affects spot metering. Centre-weighted and 'green' metering seem about the same.

One thing you should be aware of when fitting a new focussing screen for accurate manual focussing is that you may also need to fit a new focussing screen "shim" as well. The "shim" is a little washer type thing that ensures that the focussing screen is positioned at the correct distance from the lens so that when the subject is in focus on the screen, it will also be in focus on the sensor. Each camera has a shim fitted together with the focussing screen and accurate calibration of the focussing screen is achieved by fitting the correct thickness of shim. The correct thickness varies depending on manufacturing variations in structure that holds the focussing screen in position.

It appears that Pentax's quality control on the calibration of the factory fitted focussing screen is not as good as it might be and it seems not uncommon for the factory fitted focussing screen shim to be the wrong thickness. They seem to assume that you will be relying on auto focus and that accurate calibration of the focussing screen is not important. You can check this by seeing whether focussing manually and focussing using auto focus give the same result.

Note - by manual focussing I mean manual focussing through the viewfinder using the focussing screen only, ignore the green focus confirm light, that's part of the auto focus system. Its also worth checking whether there is a difference between manual focussing via live view and the auto focus system. That will tell you if the auto focus system is calibrated correctly. If the auto focus system is not calibrated correctly, sort that out first, then move on to calibrating the focussing screen.

I had this problem with my K5. When I first got it, I had a lot of mis-focussed photos when using manual focus through the viewfinder, but not when I was using live view magnification for manual focussing or auto focus. When I replaced the factory fitted focussing screen on my K5 with a Katzeye screen the problem did not go away. However, sets of different thickness shims are available from Pentax UK Service and replacing the factory fitted shim with a slightly different thickness shim sorted out the problem nicely. Now manual focussing via the viewfinder, live view and auto focus all line up nicely.

It might be worth adding, that I did not have this problem when I fitted a new focussing screen to my K20D - maybe I was unlucky with the K5 or perhaps Pentax's quality control standards have got worse over recent years?