http://www.makethephoto.com/articles/how-to-use-a-classic-manual-lens-on-your-dslr-camera
Along with the mount, each lens is designed to have the back of the lens at a very specific distance from the camera’s internal image sensor. This is called the “flange focal distance” of FFD for short. If this distance is wrong, the optics of the camera will be incorrect and the camera won’t be able to focus at all distances.
Instead, the most old common lens you will find for cheap is called an “M42 mount” lens. These style of lenses were made starting in the 1940s and were popular through the early 1980s.
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