Homemade Macro Lenses
One easy way to put this to good use is to build a simple macro lens. Macro photography is one area where cheap homemade lenses can compete with expensive commercial ones. Generally, for macro photography, you don’t need or want fast lenses. I find I usually use f/ 16 or higher in order to get an acceptably wide depth of field. Even a simple achromat stopped down to f/ 16 can be pretty sharp. Also, adjustable focus, though certainly nice to have, is not a necessity. Precise focus can be attained by adjusting the distance between the camera and the subject. Finally, extreme close up is not difficult. If you want to get closer, just add another spacer or extension tube.
Here's an example of a homemade macro lens made from the same 75 mm achromat used for the previous shots. The lens is mounted in recycled filter rings and attached to the camera via extension tubes. The location of the diaphragms (shown in the photo) is not critical. They can even be placed in front of the lens, if that works better for your build. (See The Homemade Lens Primer for a more detailed description of the construction of this lens)