Turn an Old SLR Lens Into a Telescope

Film is dead. At least that's what they tell us. I'll confess I haven't shot a picture with a film camera in years; digital has just gotten too convenient. I don't miss the hours - and expense - I used to spend in a darkroom getting the photo just right.

Still, I have a wonderful, full-manual, Minolta film SLR with several expensive lenses just gathering dust, and while, yes, I could adapt them to a modern digital SLR, I can't justify another camera right now.

There's not too much difference between a telephoto lens and a telescope; one focuses on film (Or a digital sensor), the other focuses on an eyepiece. I have a really nice 500mm Minolta mirror (catadioptric) lens that's very compact and was very expensive when I bought it new a couple of decades ago - I think it was during the Reagan administration... All the reviews say it's a super piece of optics, and it'd be a great shame to not put it to some use.

Why not turn it into a telescope?? (Or should that be Frankenscope?)

There's an old song by Johnny Cash called "One piece at a Time," where the hero tries to assemble a car from parts snuck out of the factory over the course of several decades (Note: I don't condone stealing parts from your employer, but the song is funny!):

The transmission was a '53
And the motor turned out to be a '73

And when we tried to put in the bolts all the holes were gone.

So we drilled it out so that it would fit

And with a little bit of help from an A-daptor kit

We had that engine runnin' just like a song.

Somehow, I needed to adapt the eyepieces I had, with a smooth 1 1/4 inch barrel, to the bayonet mount of the Minolta lens. I don't think anybody has ever (commercially) made such an animal! My first try was laboriously concocted from an extra rear lens cap, a telescope focuser mount, and a lot of cutting, grinding, blood, profanity, and epoxy. It worked okay, but one issue you run into with camera lenses is focusing distance. I could focus the lens to infinity, but just barely, and I wanted to see if I could put an erecting prism on it as well as an eyepiece, so images would be right-side up. This would necessarily lengthen the optical path between the eyepiece and the lens rear element.

A camera lens is designed to use it's own focusing ring to focus on a specific plane - the plane of the film or sensor, usually only around an inch from the back of the lens. If the eyepiece is too far away from the rear of the lens, the lens will focus on very close objects, but cannot focus to infinity, making it pretty much useless as a telescope. The trick then becomes to get the front of the eyepiece as close to the rear of the lens as possible. I certainly didn't want to cut up any of my eyepieces to make them work on this project; I have other telescopes I want to use them on.

Fortunately, this particular lens, having a very large front aperture (which would make filters prohibitively expensive), has a provision for attaching filters to the back of the lens - in fact, it comes with a so-called "normal" filter already attached, ensuring that the optical path always has the same amount of glass in it. The lens is designed to have a filter there all the time.

Since I would be using the lens for a different purpose than designed, I decided to remove the "normal" filter (It just unscrews) and see if that gave me more room. With the filter removed, I could mount an un-modified eyepiece about a half inch closer to the the rear of the lens - In fact, the barrel of the eyepiece slid smoothly over the rearmost element of the lens without touching the glass.

When this lens was made, around 35 years ago, technology available today didn't exist. Even optics have improved a great deal; you can get cameras with 20X - or even 40X - zoom lenses, making SLRs somewhat less desirable for anyone but a professional (or very serious amateur) photographer, since one of their main selling points is interchangeable lenses. But I digress. One thing that has recently become available is affordable 3D printing.

While hobby-grade 3D printing still leaves a lot to be desired in many ways, it is capable of making a lot of very useful things. Having recently bought a 3D printer, right now, to me, every problem I come across looks like it can be solved with... You guessed it, 3D printing!

While 3D printing isn't ideal for many things for a variety of reasons, it did work well for this project. I designed an adapter that would hold the front of the eyepiece right up against the lens, with a knurled thumbscrew to hold the eyepiece in. The adapter has an inner recess for a 4-40 T-nut (Metric would work fine, too, if that's what you've got), because I didn't have any small threaded inserts, which would have been preferable.

The T-nut is installed by positioning it and heating with a soldering iron or similar tool to seat it into the softened plastic. The knurled knob has a recess in the bottom for a nut, so a long screw can be threaded into the nut. The nut is held on the screw with a small dab of super glue, making the assembly permanent.

After printing, I test-fitted the adapter, and it fits quite well on the lens bayonet, while the eyepiece is a snug fit in the other end. I can now focus the lens to infinity with a lot of room left over. Success!

For those without a 3D printer, companies such as Shapeways will print something like this for around $10, and it will be better and stronger than something printed at home.

Sadly, the erecting mirror/prism did not work; it lengthened the optical path too much. But I still have a nice telescope as long as I'm okay with upside-down images.

More precisely, You Can't Hold This in Your Hand and get very good results. The magnification of a telescope can be found by dividing the focal length of the scope (lens) by the focal length of the eyepiece. The lens is 500mm, the lowest power eyepiece I own is 25mm. 500/25=20 power at the lowest. 20X is right at the edge of what's feasible to hand-hold; the image just shakes too much to be very useful. If I was to use my 9mm eyepiece, the magnification would be over 50X, impossible to hand-hold in most cases.

So, before I'm done with this project, I need to find a way to mount this lens to a tripod.

Some of the Really Big monster lenses you see sports photographers using have tripod sockets on them, since the lens often weighs six times as much as the camera, besides being as long as a baseball bat! In that case, you're hanging the camera on the lens instead of the lens on the camera. No such luck here; the lens is only about 5 1/2 inches long and weighs about a pound and a half. To further complicate matters, only the knurled ring at the very back of the lens - intended as a grip for attaching to and detaching from the camera - is a fixed part of the lens. All the rest is a focusing ring, which has to turn freely. If I'm ever to mount a prism, or even a finder scope, on this, I must mount the lens by this rear ring only.

What's the easiest way to solidly mount a round thing? Fish a four-inch hose clamp out of the junk pile! It needed to be reshaped a bit so it could mount a round thing to a flat thing: the 3D printed mounting cradle.

I designed the cradle with notches for the hose clamp and a groove in the rear cradle to match the knurling on the lens. There is also a front cradle, which is not attached to the lens and is only there to take some of the weight off the lens and mount. The focusing ring rotates freely in this cradle.

The .stl file also has a recess for a 1/4"-20 nut that fits most camera tripods. A bit of super glue holds the nut in. After some massaging of the hose clamp into shape, everything fits and works very well.

While this project mounts a specific lens, the adapter can be used with most any Minolta bayonet mount lens and any 1 1/4" eyepiece. If you do this with a different lens, of course you're on your own as far as mounting is concerned.

I tried this adapter on my 75-205mm zoom lens, and while it worked on that also, a longer adapter would be advisable because the eyepiece barrel hits the rear element of the lens and could damage it.

This idea will work, with varying degrees of success, on just about any 35mm camera lens, as long as it can be manually focused. It's just a matter of adapting a rear lens cap for that brand of lens. Without a 3D printer, drilling a centered hole in a rear lens cap and finding an eyepiece mounting tube is really all that's necessary. The best part is none of this is irreversible, so the lens can still be used as intended!

Bring those orphaned lenses back to life!