First of all, why would we want to put old lenses on our cameras? Well, for a start, they can be cheap. Here's a Kalimar 28mm lens I picked up on a certain global auction site for a few bucks. It is made of metal, not plastic, has a nice action and decent enough optics. Elsewhere in the land of manual focus lenses you will find glass that equals and even out-performs today's auto-focus models. So long as you are careful to avoid buying fungus-infested or scratched glass, you might be in for a nice picture-taking surprise.
This Kalimar looks like it has the same K-mount as Pentax and compatible lenses. In fact that is how it was advertised, and you can't really blame the seller. But there is a difference that might cause the lens to get locked to your camera body, though you have to know where to look to find it.
The first clue is on the aperture ring, where we observe the lettering "KR". This tells us this is a Ricoh lens made to be compatible with the Pentax K-mount. And indeed it was... until the age of automatic focus created an unfortunate coincidence. (You might also find other distinctive lettering. A list of offenders is at the bottom of this article.)
Turn the lens over to examine the culprit.
Note the pin at the bottom left of the mount. It's further around the circumference from several electrical contacts. This pin was designed to communicate aperture information to Ricoh bodies. It has no such function on a Pentax body.
Here's the coincidence: this pin sits exactly where there is a depression on Pentax bodies, including all digital Pentax SLRs, for an auto-focus contact. If you mount this lens on such a body the pin will go into the auto-focus slot where it does not belong. And it might never come out, resulting in a lens permanently attached to your body. I doubt we want that. Unless it's a very, very good lens. Which it isn't.
So how big is the risk? Actually, not that great. This lens went on and off the camera several times until I read of the problem. A second KR lens, a Sears 135mm, also worked on the camera perfectly... and came off perfectly as well.
Even if you do get one stuck, don't reach immediately for the hacksaw. Put the body in AF mode to help pop out the pin. Then use a small metal shim (or a feeler gauge, or even a metal tape measure) and slip this between the body and lens near the pin. With some patience you should be able to pop up the pin. Cutting your lens to pieces is dramatic but not really necessary.
Still, there is a way to remove all Ricoh risk... simply get rid of the offending pin. Some have suggested taping or gluing it down, but the idea of stray bonding agents around the auto-focus insert is not a reassuring one. Others have apparently sanded or ground down the pin enough to remove the risk, but as it is sprung inside its hole that is rather difficult to do.
Let's just get rid of the thing entirely. All you need is a Phillips #0 screwdriver.
But not this screwdriver. This is a typical "jeweler's screwdriver" that might come in an inexpensive electronics kit, or is sold to adjust your eyeglasses. These are cheap and often badly made. In any case they offer almost no grip. Avoid.
Instead purchase a screwdriver like this one. It has a much larger grip so you can apply far more torque without ripping all the flesh off your hand. It has a better head that won't get frayed or bent. And it's only about 5 euros for something you can use time and time again. Like, say, to fix your eyeglasses. To avoid damaging the screw get the correct size: a Phillips size 0 (that's zero).
Some cleaning tools might help in case you get dust onto your lens. Of course you should never touch any glass, or you will need a liquid cleaning solution to remove grease. For lighter work a blower is best. You might want to use a lens cleaning cloth to dislodge slightly more persistent particles. But start with a clean environment free of wind or heating system air circulation and you'll be fine.
You may also want a vice to hold the lens but I did not bother.
With your tools at the ready it's time to begin. There are four easy steps.
Step 1: Four screws hold the lens mount onto the lens proper. These can be removed with a good amount of force, but be careful not to strip the heads. Having the correct screwdriver is 90% of the battle. Let me tell you how difficult it was before I discovered this! Before fully removing the screws note how the mount fits the lens so you can put things back together. Store the screws safely; you don't want to lose them.
Step 2: After the screws are out the lens mount pops off. Turn it upside down and have a look at where the pin is held in place.
Step 3: A small metal flap is held by two screws. This flap then holds the pin in place, acting as a spring. These screws need to be removed so the pin can fall out. In this case there was so much goopy plastic glue that I found it impossible to remove one of the screws. Nonetheless it was easy to pry up the metal flap while turning the mount upside down. The pin simply fell out. Gravity -- it's a useful tool.
Step 4: Re-attach the metal flap using the two little screws, invert the lens mount, and attach it to the lens with the four longer screws. The result is pictured here. The lens is exactly as it was before, but with a hole where the pin used to sit.
Job done. Have a beer. Then shoot some photos.
Similar instructions are available from SelrahCharleS. After my own documented adventures I thought it might be handy to have complementary info here.
Here are the lens markings to look for:
* "P/K-A R-P/K"
Here are known problem lenses:
* Tokina 80-200
* various Kalimar lenses like the 28mm above
* Sigma zoom-beta II 60-200mm multicoated and Sigma zoom-master 35-70mm multicoated
* Sears 135mm f/2.8 Macro, Sears 60-300mm, Sears 70-210mm
* Adaptall mounts marked "K/R"
* and possibly Chinon, Albinar, Soligor and Kiron lenses made for Ricoh
In addition there are other old manual lenses incompatible for a different reason. They have an large black flange that prevents mounting on today's K-mount cameras (though older bodies worked fine). The solution here is similar: take out three screws so the flange can be removed and discarded. Like in this example.
Of course not being able to mount a lens is a lot less annoying than not being able to dismount it. Nonetheless here are some troublesome Vivitars:
* Vivitar 70-210mm Macro Zoom
* Vivitar Series 1 90-180mm
* Vivitar 28mm f/3.5
Please don't let this scare you off trying older lenses with your Pentax. The Vivitar Series 1 105mm macro f/2.5 is possibly the best macro lens ever made. Its price has shot up on the used market with the increased popularity of Pentax DSLRs, now topping $400. The Cosina 100mm f/3.5 macro (branded as Vivitar and others) is a wonderful low-cost low-weight plastic alternative. I have taken hundreds of great shots with this lens, that cost me about a hundred bucks for a mint copy.
For more information and a discussion of the k-mount see the Mark Roberts site.