Dark frame subtraction is a noise reduction technique for digital photography. Here's the premise:

For short exposure (less than a couple seconds, let's say), the noise of a digital camera's sensor is acceptable. But as exposure times lengthen beyond that, the noise gets amplified along with the subject of the image.

So if you take an exposure that's 15 seconds long, you'll record 15 seconds'-worth of noise along with the subject matter you're photographing.

In essence, you set the camera to multi-meter, and then spot meter a few different places by pressing AEL. The camera averages these points and determines an exposure.

Point the autofocus target at the shadows, press AEL. Point it at the highlights, press AEL. Do as many of these as you want (up to 8). The camera will come up with an average setting for all these spots. It's sort of like being hand-held while zone metering.

I found this lens at a thrift store for $15.

It's an Adaptall-2 lens, which means that if you have the proper adapter, you can use it with just about any SLR mount out there. This specimen came with what looks like an M42 adaptor, marked 'For Fujica.' The mount-end cap is also marked 'For Fujica.' Maybe it's for Fujica? :-)