Originally posted by Jonathan Mac If focus is not right when manually focusing it's because the viewfinder focusing screen is not in the right place and needs to be shimmed, or the mirror is in mis-aligned. Having said that, the default focusing screens in DSLRs can't usually be relied upon to be accurate below f/4 or so, so unless you're using a very precise split-screen or similar specialised focusing screen then I wouldn't expect to be able to accurately focus with it, especially at f/1.2.
When you add shims, they go between the focusing screen and the prism - i.e. they decrease the distance between the focusing screen and the mirror, which moves the plane of focus forward. If the camera is already front-focusing, adding shims will only make it worse.
I suspect that Pentax shares your view regarding precise focusing using the viewfinder - hence, they no longer refer to "focusing screen", but call it a "fresnel" (theirs has a diffraction grating that provides a little magnification, a-la a fresnel lens). Documentation about cameras on the InterNet now all talk about viewfinders in terms of being able to see what the camera's pointing at, rather than the ability to focus the lens. In fact, there is a large number of so-called external viewfinders, which couldn't possibly help at all.
So my view is that they've done to cameras what's been done to telephones - the addition of new technology apparently designed to interfere with the effective use of the tool. I don't think that people don't talk on the 'phone much anymore not because they have suddenly developed an aversion to speech, but because using a telephone has become WAAAY too complicated and inconvenient. (Part of the "deep state" conspiracy to keep the peasants from being able to inform each other about the evils of politics, no doubt.)
Even at my advanced age and failing eyesight, I can still focus visually much better than any autofocus system I've seen - not necessarily more precisely, but faster and more accurately. With a real camera (i.e., 35mm SLR), since my left hand is already on the focusing ring and my eye to the viewfinder, focusing is done before I even have the thought to do something. With the K-1 for example, I push the button and wait for the lens to respond, and by the time it's found something, it's the wrong something or the proposed subject's already gone. Even with a manual winder, I could have taken three well-focused pictures with a Canon AE-1 in the time it takes to get one picture in focus with the K-1. (Naturally, a small prime lens can be brought into focus much faster than a long telephoto zoom lens. I'm thinking "on average", here.)
One might argue that I can still use the lenses in "manual mode", I can have my hand on the focusing ring, and so forth, but that's all wasted because the viewfinder has been designed merely to show me what the camera's pointing at. Actually attempting to focus the lens using the viewfinder (the main reason I wanted an optical viewfinder, and a big reason why I've got the K-1) is pointless (by design).