View Full Version : Two for one special today

09-20-2007, 07:07 AM
I've been busy building things again, as usual this year. We needed a wood shed to keep the firewood dry since the wood shed doesn't have any room for wood. I guess I'll have to rename the former wood shed to "foundry" or something (next year). Right now it's full of assorted crap.

Anyway, I built a simple wood shed using the existing trees for support. It's designed to move with the trees as they sway in the wind. The entire structure is triangular. Only one vertex is actually fastened to a tree, the other two are captured but free to slide as the tree sways. I think it looks sort of like a hang glider. I still need to paint it but it has started raining.


Lest anyone think that this post doesn't have machining content keep reading.

The photo of the wood shed was taken with my Canon Digital Rebel. When I used to use film my camera of choice was my old Mamiya SLR. I have an assortment of nice lenses for it including a couple of telephotos (one zoom) and wide angle lenses. I also have an extensive collection of filters.

The Rebel is a digital SLR with interchangeable lenses. It can accept all the lenses in the Canon EOS series. The Mamiya has the old standard M42 screw on threaded mount, very obsolete now. It so happens that the Rebel can be used in manual mode and when doing so fully resembles a standard film SLR including the ability to take pictures with no film (card) in the camera. It will even take "pictures" with no lens attached, very handy when doing prime focus photography with a telescope.

Which brings me to the point. I want to use my old lenses on the Rebel for several reasons. They are faster than the Canon lenses. They are NOT autofocus and the focus can be preadjusted and taped down at infinity to save trying to focus on dim night sky objects later. This is really important as the biggest problem with most digital SLR's for astrophotography is trying to focus at night, even manually. Both of the Canon lenses I have are zoom so they don't have a stop right at infinity. My non zoom m42 mount lenses can simply be set to infinity and be done with it. This really matters when the lens is wide open. Because the Canon doesn't care about what lens is hanging on the front it can take pictures in manual mode with a totally manual lens attached. Cool.

So, I took some time out from banging nails to make an adapter so I can mount the M42 lenses on the Canon which has a bayonet type mount. The adapter has to be very thin in order to maintain the back distance to the sensor plane.

In this pic is the adapter (item B) shown on both sides. It is made from two pieces, a threaded aluminum ring and the other an acetal outer ring with the proper mounting flange machined in it. The two components are glued together with special cyanoacrylate glue designed to work on all plastics as well as metal. Item A is a thickness spacer to adjust the back distance to achieve critical focus when the lens is set to infinity. This can vary ever so slightly on different lenses hence the adapter shim.


Another view showing the adapter shim better. The shim is adjusted for each lense by machining the outer edge which is where it seats on the camera body.


And, the lens with the adapter in place.


Incidentally, the M42 thread is 42 millimeter by 1 mm pitch. 24 tpi on a non metric lathe is plenty close enough for this purpose as only a few threads are engaged.