Like the ill-fated charge of the light brigade, screw-mount lenses have succumbed to the twist of bayonets– the method most digital SLR cameras now use for interchangeable lenses.

But from the 1950’s well into the 1970’s they were used on many popular SLRs and today, hundreds of thousands of them, maybe even a million or more, are still around– many gathering dust, to be sure, but a myriad of others available to be bought for a song.

Cruising eBay one day, I was surprised to see hundreds of these lenses being offered at auction, most going for between US $30 to US $100. I was one of only two bidders competing for a Super-Multi-Coated Pentax, 50mm Takumar f/1.4 screw-mount lens. The other guy dropped out early and I nailed it for only $40.

In less than a week, my lens arrived in near-mint condition and I then had to figure out how to use it on several cameras I had available. Obviously, I was going to need some adapters and, for those, the man to go to was Stephen Gandy, "Mr. Adapter." If there’s a way to adapt almost any lens to work with any camera, he knows how to do it. His web site is just chock-full of information on mixed marriages between lenses and camera bodies and his adapters, while pricey, are precision-made, not like a lot of flimsy, badly-machined stuff floating around that can cause you a lot of grief.

Pentax had previously sent me a screw-mount adapter for their *ist D so I was set there. That left a Canon 20D and an Olympus E-300. Gandy sent me adapters for both of those. He also sent me an adapter that would allow me to use some of my bayonet-mount Nikon lenses with the Olympus E-300 (I told you he knows his stuff). The only digital camera that’s tough to fit with an adapter for screw- mount lenses is the Nikon. Adapters are available, but they won’t allow the lens to focus to infinity. Depending on the kind of shooting you do, that may not be a problem. But if you want the full range, Gandy can get you an adapter that has a lens element in it which will do the trick.

There are over 40 models of about two dozen different camera brands that used M42 screw-mount lenses (the "42" being the designation for the thread size) and a bunch of third party manufacturers also made M42 lenses. Screw-mount lenses come as "primes" (a single focal length) and zooms. Some of the primes have apertures as large as f/1.2, simply great for shooting under relatively low light conditions, such as in sports arenas. Imagine being able to stop action at 1/500th of a second or faster at a basketball or hockey game by shooting wide open (or nearly so) at a moderately high ISO from any seat in the house without worrying if your flash will carry (which, of course, it won’t).

Or how about using that same large aperture to selectively focus on flowers, pets or people. The 50mm Pentax Takumar lens I bought has a 35mm equivalent of 75mm to 100mm, depending on which dSLR it’s used on– a perfect focal length for portraits. The picture of the cat was taken using it on an Olympus E-300, from about three feet away at f/4, with the camera’s built-in flash. There are some little inconveniences you’ll have to put up with when you use these lenses, but nothing more difficult than the routine photographers have gone through for many years.

First, you have to focus manually because the lenses are not linked to your camera’s autofocus system. And you have to physically stop down the lens by turning a ring to your desired aperture, unless you’re shooting wide open. Most screw-mount lenses have click stops so you don’t even have to remove your eye from the viewfinder to look at the aperture ring when stopping down, you just have to know how to count. On my Takumar lens, there’s one click between f/1.4 and f/2.0 and then two clicks between all the other lens openings until you get to f/11 where one additional click takes you to f/16.

The up side is that your camera’s metering system can usually calculate the correct exposure automatically. Here’s the drill. Set your shooting mode to Aperture Priority. Then open the lens to its widest aperture. Focus. Then stop down to whatever aperture you want to shoot at. Fire! If the distance to your subject changes significantly, open wide, and focus again. If not, just keep shooting. It quickly becomes second nature.

If you want to use a particular shutter speed, switch to Manual exposure and select a speed. Then shoot a couple of test shots at different apertures to find the correct one (here’s where checking the camera’s histogram can be invaluable) and you’re set to go. With some lenses, the camera’s light metering system may consistently over or under expose. If that’s the case, just adjust your exposure compensation to take care of it and leave it at that setting while you’re using that particular lens. If you’re into nostalgia, you can dust off that old light meter and set your exposure manually.

Using half-century-old M42 screw-mount lenses with modern digital cameras is a fun trip because they’re a real bargain and produce excellent images. And there’s something delightfully perverse about using a forty dollar lens on a thousand dollar camera. Sure, you have to go through a little song and dance to get good results, but it can make you think about what you’re doing and take you back to your photographic roots.