Hi,
Welcome to the Pentax Forums. Manual focus with fast lenses is a bit of an challenge with most dSLR cameras regardless of brand. This is true even when using the AF system to provide focus confirmation. I shoot with a K10D and feel your pain. There are multiple factors at work here:
- The AF system has poor precision when used with faster glass. This is true regardless of AF mode or how the system is used (AF, focus confirm, or catch-in-focus*).
- The area for any of the AF focus points in the viewfinder is fairly large with obvious side-effects for three-dimensional subjects when DOF is shallow.
- The stock focus screen is poorly designed for straight manual focus
Any of the above points can be explained in great detail, but for now please take all three as being true.
What you can do:
- When using AF confirm, switch to the center focus point. Doing so will give you more control over what you intend to be in focus.
- Shoot at narrower apertures to increase depth-of-field
- Practice your manual focus technique using simple subjects and a tripod
- Consider upgrading to an aftermarket focus screen designed for manual focus. I use a screen with combined split-image/microprism focus aids. Other users have found that various fine-matte screens work well.
Regarding your Pentax-M 50/1.4:
- It is a very good lens with solid build and sound optical design
- As with all f/1.4 lenses, DOF is very shallow when shooting wide-open
- It is somewhat soft wide open
Steve
* Catch-in-focus (CIF) is an undocumented feature of your camera where the shutter will not release in AF-S mode until the AF system has acquired a focus lock. As noted above, it involves pressing the shutter release and moving the focus ring, subject, or camera until focus is acquired and the shutter releases. It is sort of fun, but I have not found it suitable for fine focus.