Why use old lenses on your brand-new DSLR?
For me, there are two real reasons:
1. Image quality for the money. For the price of one semi-decent new zoom (slow and not very sharp), I can almost outfit myself with a complete set of excellent older prime lenses with wider apertures and higher image quality.
2. Learning. The new lenses are just not designed for manual anything - focus rings tend to be small, aperture rings may not even be present, etc. - the entire lens is expected to be controlled by the camera body. With a manual lens, the learning is a lot more hands-on, more tactile.
Using non-digital lenses on DSLR bodies
Just because the lens does not say it is designed for digital, that does not mean that it cannot be used on a digital body. As a matter of fact, it may be even better than a digital-specific lens because the corners of the image are cropped out by the smaller sensor.
Pentax DSLRs can use any old lens with an M42 or K mount. As a matter of fact, there are adapters from the Leica M39 mount to M42, which allows you to then attach the M42 to K-mount adapter and use the lens on your digital body. Personally, I have never used such a two-stage adapter but I will try when I have a chance (still looking for a cheap Jupiter-6 with the M39 mount).
There is some risk with using older aftermarket lenses. For example, older K-mount Sigmas have been known to get stuck on the body, becoming impossible to remove without the assistance of a camera shop (or a feeler gauge). Use your judgement and proceed with caution. You should be safe if you stick to older Pentax-made lenses and research other models thoroughly.
M42 to K-mount adapters
Unlike teleconverter adapters and some other mount adapters, M42 to K-mount adapters do not cause a loss in image quality because there are no optics involved.
There are two types of M42 to K-mount adapters. One has a lip or flange that goes between the camera body and the lens, effectively moving the lens a couple of millimetres further away from the sensor and causing the lens to be unable to focus to infinity.
The other type is the proper adapter which allows the lens to be seated properly. If you want to see what one looks like, check Pentax web site. There are Pentax-made adapters available on eBay, and there are aftermarket adapters that look the same and work in the same way. Here is a picture.
Modifications to the lens and adapter
The downside of the Pentax and similar adapters is that after you attach it to the lens and attach the lens to the camera, the adapter "clicks" into place and is not removed when you remove the lens. To remove the adapter, you need to depress the springy clip after unscrewing the lens. There is a simple mdification you can make, but at the risk of having the lens detach from the camera and fall when you least expect it: with a small screwdriver, remove the spring clip from the adapter, then screw the lens into the adapter really tight. Attach the lens with the adapter to the camera. Most of the time, the friction will hold the lens in place if you are careful when focusing, changing aperture, etc.
Some people go a bit further and drill a small hole in the lens base itself to allow the normal K-mount pin on the camera to engage and hold the lens in place. I have not done that with any of my lenses yet. Of course, it goes without saying that any modification to your lens or the adapter you are using is done at your own risk. If the adapter is a bit loose, I would not suggest removing that retaining clip!
If you have a K-mount lens, all of this adapter talk is not relevant since you can just mount the lens on the camera.
To use a K-mount lens which does not have an automatic aperture setting, or an M42-mount lens on your Pentax DSLR, you need to enable the -Using Aperture Ring- setting in the Custom menu (refer to your user manual).
Now, for what you can actually expect when using the older lenses, from the most recent to the oldest.
Pentax-F, Pentax-FA, Pentax-FAJ, Takumar-F series: everything will just work. I actually think that they produce better colours than the DA series. There are also some very, very nice prime lenses in those series both for macro and regular photography.
Pentax-A: Everything will just work except, of course, the autofocus.
Tip: Pentax sells a magnifying eyepiece adapter to help with manual focus. Certainly worth buying if you are going to use manual focus lenses.
There are also split-image focusing screens available (like KatzEye) to aid manual focusing, just like the old cameras used to have. I have the eyepiece adapter, have no experience with the KatzEye yet.
SMC Pentax and Pentax-M: no automatic aperture on those lenses. The menu setting I mentioned earlier needs to be enabled, otherwise the lens will remain wide-open and your pictures will probably be overexposed. Thankfully, Pentax DSLRs provide a mechanism for stop-down metering: you set the aperture you want on the lens, press the AE-L button, and the camera sets the correct shutter speed. You can use it as a starting point if you want to make further adjustments.
M42: With the lenses which have the manual and auto settings, set them to manual. The auto setting keeps the aperture open to aid the focusing, then the camera pushes the pin in the back of the lens to close the aperture when you release the shutter. Very nice, except the DSLRs have no mechanism to do it. So, my full sequence is close the aperture to the desired value, press the AE-L value to get the suggested shutter speed. Open the aperture, focus (if you do not open the aperture, the viewfinder will be too dark to focus easily). Close the aperture again, take the shot.
M42: With the lenses which do not have the auto-manual switch and are effectively always at Auto, the lens will always stay wide-open regardless of the aperture ring setting, until the camera pushes the pin in the back of the lens. I believe that you need to modify the lens (can be as simple as taping the pin in the back down).
There are even older designs of M42 lenses which do not have the auto setting or pin. This design is currently mostly found in some excellent Russian lenses available on eBay in M42 mount. With those, there are actually two aperture rings: one that sets the aperture value, and another that actually closes the blades. You set the desired aperture, focus, then turn the other ring to actually close the blades and take the shot. The lenses I have with this system are Mir-1, Mir-20, Jupiter-9 and Helios-40-2. I will probably write another guide on using Russian lenses.
The picture shows three lenses. Crops are also visible above. From left to right:
Karl Gener 35mm. You can see the Auto/manual switch (blue arrow) and the pin which closes the aperture in Auto setting (yellow arrow).
Mir-1B with the kind of adapter which will generally prevent the lens from focusing to infinity (orange arrow).
Helios-40-2 with the "proper" kind of adapter with retaining clip removed.
Using older lenses on a DSLR not only allows you to get excellent image quality on a budget, but also to learn to be a better photographer by forcing you to experiment with different settings and getting immediate feedback. There is a lot of excellent old glass there, lets use it!