[narual]

If your diopter is off, the image will look blurry, but it won't make you think the camera is focusing somewhere it isn't.

If you're trying to manually focus and the image is blurry in the viewfinder, it may very well do just that... but the complaint in that case would probably be "none of my lenses focus unless I use live view" or something like that.

[Just1MoreDave]

Practice, practice, practice!

I have been practicing (and practicing... and practicing) my manual focusing skills with a SMC-A 50mm f/1.4 and a "cheap" Rikenon 35-70mm f/3.5 - 4.5.

The first thing I did was back off of f/1.4. Now I am between f/2 and f/3.5. My depth of field has increased slightly which subsequently increases my sharpness. I think at f/1.4 I was just "too fast" and it didn't suit my shooting style or my subjects.

That is heresy but I agree. One example for me is my small dogs. A 50-55mm lens works out well for portraits, but at distances where I take these shots, anything below f2.0 has too little depth of field. I would get a new superfast prime, take a test shot of a handy dog, and have one eyelash in focus. At 10-12 feet (50mm) the widest apertures start to work better.

Focusing screens

I have had a Katz-Eye split prism in my *ist DS for a long time. When I bought my K-7, I kept the stock screen for a long time. I can focus with that too, even with the Cosina 55mm f1.2 wide open. That's where practice comes in. The stock screen doesn't show you the correct depth of field for fast lenses. You have to see what's there and estimate how much narrower it will be at your chosen aperture. I got pretty good at it and it's a useful skill. I couldn't do it quickly in challenging conditions. I had to work my eyes a little harder especially at first. The stock screen is OK for slow lenses, metering works fine, and you might like the markings or find it cleaner than a split prism.

I tried a Canon EE-S screen briefly. I cut this down myself and I would never do that again. Expect you will have to shim it to make it perfect, which isn't that hard. The depth of field is perfectly visible. I did not like the way it worked with slower lenses. Because it's ground for fast lenses it gets really dark with slow lenses. You may still need to practice seeing the actual depth of field, because that is the only aid. You can place your target anywhere in the viewfinder, which is great. Metering effect seems small and spot metering is unaffected. It sounds like the current price, installation tools and advice are excellent. I didn't think it matched well with the lenses I have.

I finally went back to a Katz-Eye for the K-7. Yes, the price is ridiculous, and you must get your shims elsewhere. Expect to shim these too. I have never tried the cheaper split-prisms. The aid is in the center only, so focusing off center is a compromise. There's a lot of clutter in the center. Super-slow lenses have prism blackout, where half the prism is black so focusing is hard. (I rarely have trouble with this myself, Katz-Eye is supposed to be better than cheap here.) Spot metering is just unreliable enough to make it useless. (It would be OK if it was consistently wrong but it's off at some apertures and with some lenses.) Still, when this screen works, the precision is fantastic. I can focus on almost a point, not just a range of distances. I have manually focused when it was too dark for the meter. I can repeatedly focus on the same point with the same results, whether it's near or far. I can focus really fast, even in low light, when I'm less confident in AF getting a lock. When the split prism doesn't work, I can sometimes use the microprism collar or rest of the screen too.

I hope that helps a little despite my obvious bias towards KatzEye.

I think no matter what screen you use, a really sharp lens is easier to manually focus than a softer one. I've run through about a dozen 28mm lenses of widely varying quality, and it seems very clear with these lenses. Sharp lenses snap into focus, while with a softie, you can't quite see where the depth of field starts or ends. The sharpness has to be wide open because that's how focusing is done. If two lenses are equal at f5.6, I can still see which one is sharper wide open. You might use a sharper lens at wider apertures too, but you don't have to always shoot wide open to get some advantage out of it. At 50mm, the wide open sharpness varies much less - cheap lenses are still pretty good - but I can see this again with my large 135mm collection.

[Na Horuk]

So, to recap. How to focus with fast lenses, which have thin DoF.

1. CiF aka catch in focus aka focus trapping. Not all lenses allow this, but manual lenses usually do. Its a great tool, just hold the shutter and wait for the subject to get in focus, or twist the focus ring until AF confirm finds it. I think all Pentax DSLRs have this.
2. Focus peaking: In live view, you can turn on focus peaking, which shows roughly which area is in focus. It shows the DoF, so its not always accurate and its not perfect with all lenses (its especially bad with uwa and slow lenses), but it gives a great general idea, its still a great tool. K-30, K-01, and Q have this.
3. Viewfinder loupe. You can adjust diopter and add a magnifying loupe to your viewfinder, essentially making the viewfinder area bigger and focus easier to tell. Third party manufacturers make these, there are many different types, some removable, variable, others static
4. Focus screen: DSLRs can have their stock focus screen replaced by a different one. In the manual days, there were many types of focus screens to help with focusing. Some make the OoF area blurrier, others make it look like a mosaic. Just make sure you get the right one for your camera. People especially recommend the ones from KatzEye, Focus screens.com (their S type), even Jin Finance.
5. AF adjust: All Pentax DSLRs allow AF adjustment. You can use a focus chart and make sure the camera works with your lenses.

Other tips:
6. Practice makes perfect: I hate to say it, but the more you practice with AF and MF and figuring out where the focus really was, the better you get at using your camera and getting it right.
7. AF points are actually much bigger than the AF confirm lights in the viewfinder. The camera might "get focus", just not on the object you wanted it to
8. Stop that lens down, narrower aperture means bigger DoF, which can help you with getting your focus "close enough"
9. Sometimes users think the focus is wrong, but its actually incorrect use of SR (like inputting a wrong focal length number), long shutter speed (motion blur, hand shake blur), poor quality or dirty lenses or cheap filters, high ISO (noise makes things look less sharp)

There were numerous threads on each of these points, so feel free to search the forums for relevant tutorials, recommendations, and discussions. And yes, focusing with fast lenses is a very tricks thing, especially since AF is one of the few things you can't fix in post