see :
focusingscreen.com - PentaxForums.com
--and
Will a different focusing screen help? - PentaxForums.com
here's some other out-of-context quips I've posted earlier on the same subject:
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The problem with this kind of discussion is that people tend to assume autofocus in all situations. I had a similar problem and I found it difficult to communicate the idea of actual manual focus, i.e., focusing by peeking through the viewfinder and rotating the focusing ring on the lens without any appeal to the camera's intelligence. After more research, I discovered that this matches the assumptions of the DSLR makers. Pentax doesn't even use the phrase, "focusing screen" for that part that makes the scene visible in the viewfinder. It's called a "fresnel" because it's got a bit of magnification to make the viewfinder image bigger, but there's no suggestion anywhere that you could use the viewfinder to focus the image. I found one place where they said that the purpose of the viewfinder is to generally know what the camera's pointed at.
I've stopped trying, it's practically impossible for me, with or without glasses. My solution was to "go with the flow" and make sure all my lenses are properly "calibrated" and use AF all the time, imprecise and slow as it is.
I don't understand why people want a system that's harder to use, slower, and fuzzier than what you can do with a good split-image focusing screen and manual focus. To me the only real advantage to the DSLR lies in the elimination of film-related inconvenience and expense. I can take a couple hundred pictures without having to change rolls, I can see what I've done right quick, and I don't have to mess with chemicals.
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Having checked fairly recently, I'd say that FocusingScreen.com is the only option. But I don't recommend it. There are no split image focusing screens made for Pentax. What FocusingScreen sells are Canon screens that are pretty close to the correct dimensions, cut down to fit (in two of the three dimensions - the Z axis may vary wildly from what will fit in the camera). I put one in a K-50 that worked pretty well, and I got one for the K-1 which was a disaster - could not be used to focus at all because it was too thick. Even where it goes well, the installation is too much trouble, and their "shims" (needed to adjust for front/back focus) are really flimsy having been made of thin plastic. The ones I got were bent up (and could not be straightened).
I've also learned since then that Pentax cameras' light-sensing meter is directly fed by that central point in the screen which will be distorted by the use of the split-image screen. Pentax doesn't even refer to the thing as a "focusing screen", by the way, and it appears to me that they have no intention of your really being able to do manual focusing using the viewfinder. They refer to the thing as a "fresnel", because it isn't "ground glass" (they're all plastic anymore), it's actually got a graduated diffraction grating that has a magnifying effect, presumably acting as a Fresnel lens. ("Fray-nell", named for the French guy who invented it - a stepped lens with multiple sections, having the same effect as a larger conventional convex or concave lens, but with a flatter aspect.)
This is my very best favorite pet peeve about Pentax cameras - I've always been a manual focus guy, and I don't like it that they've made it so hard to do, especially now that my eyesight is beginning to go. I feel like I've been tricked, and I don't like it. But, oh, well, I'm learning to use "autofocus", which by my lights will never be as good as manual focus can be, but then it will work well enough for the average snapshot taker, and I guess that's good enough for modern marketing. (Grumble, grumble.)
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"In that case you could exchange the plane focus screen with a spilt screen. I havn't done it myself but I think I have read it is possible."
I have tried it. In the K-1, it was a disaster. The screen I got from "FocusingScreen.com" was a cut-down Canon screen that is smaller in width and length but thicker than the original. Just plain wrong size. Attempting to use the split-screen focusing aid was what got me to worrying about this (with concomitant testing and analysis) in the first place.
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"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).
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