Don't worry about small amounts of internal dust, minor scratches (sometimes referred to as innocent-sounding "cleaning marks" in online descriptions) or scuffs on the lens barrel. Finding a previous owner's Social Security number scratched into the side of the lens may be disconcerting, but it won't affect sharpness or anything else.
Some Of My Favorites
On Micro 4/3rds and Sony NEX bodies, a 50mm ƒ/1.4 becomes a truly spectacular 100mm portrait lens. The Asahi Pentax Super Takumar, in its M42 threaded iteration, has a handy top-mounted cam that makes it easy to close the aperture to the preselected ƒ-stop after focusing. Bargain 35mm ƒ/2.8 lenses become very useful 70mm ƒ/2.8s, good for portrait and general use, and 100mm ƒ/2.8 lenses are transformed into dramatic 200mm ƒ/2.8 telephotos.
If you're looking for a fast, medium-long telephoto, pick up a 135mm ƒ/2.8 (270mm equivalent). Once you get accustomed to the manual focus and exposure process, you'll find this combination ideal for birds and other wildlife.
Adapting Lenses Attaching the adapter to the lens is straightforward. Removing it can be another matter altogether. The better adapters have a sturdy button-type lever that lowers the locking pin that holds the adapter onto the lens. Cheaper adapters use a bent piece of steel instead. If you plan to move the adapter from lens to lens frequently, that flat metal tab can be bothersome and sometimes even painful to operate. The button-type lever is much more comfortable.
In a nutshell, all operations are manual. Focus is manual, but many digital cameras provide focus confirmation. The Sony NEX-6 and Pentax K-01, for instance, offer focus peaking. When a subject is in focus, the borders between highlight and shadow areas shimmer to indicate that correct focus has been achieved. In some instances, this assisted manual focus may yield better results than autofocus, depending on the subject and lighting conditions.
Correct exposure is set either manually or by using Aperture Priority. While these options may seem cumbersome today, they were de rigueur back when these old lenses were brand-new. In either case, it will be necessary to focus with the lens wide open and stop it down to meter and before releasing the shutter. The process takes time and, if you've never done it, a bit of practice.