Home made telephoto lens for the CP950/990.

First published August 30, 2000.   Last updated Feb. 11, 2001

Attention Jack Austin! Yes, you Jack! You emailed me with a question about the light stop, but your return addresses are spam blocked or non existant. I'd like to answer your question; send again with a correct address! -Bryan


Using optics from an old pair of broken binoculars that someone gave to me, I made a 7X teleconverter for the CP990/950 with 50mm aperture. At the end of the article, I will give some construction tips if you would like to make your own lens. I'm calling this the "Hardware-Eye" because it uses hardware store parts.

 The lens has the following advantages...

* Cheap! Total cost was about $10, assuming that you can find a broken pair of binoculars and already have a step up ring for your camera.

* Lightweight. Because I used lightweight PVC tubing and because I didn't use the prisims from the binoculars, the entire lens is fairly light.

* 21X magnification when used with the 3X zoom, or about 770mm focal length.

* Unlike the approach of attaching a smaller 7X monocular, the 50mm objective has enough light gathering power to allow you to use fairly fast exposures, up to 1/1000 second in daylight.


The finished lens. Overall length is less than 9 inches.


Back end of the lens. The lens uses a 28mm to 37mm step up ring from CKC Power which attaches to a 37mm to T thread adapter. With a little adjustment of a pice of PVC pipe, the T-adapter thread fits.


Looking into the front of the lens, out through the eyepiece. Note that the interior of the tube is painted flat black to dull reflections.


The lens split into two parts. The back half is press-fit for cleaning and adjustment. This shows several things, first, the optics used were just the objective from the binocular, and the eyepiece. The eyepiece is press-fit into a slotted ring which is cut from PVC pipe. This in turn fits into a piece of PVC pipe that is again fit into a slotted ring of PVC. The three set screws allow the exact position of the eyepiece to be adjusted. Focusing is accomplished by sliding the eyepiece in and out. The lens is fix focused at infinity. Not shown is a paper field stop that I added later.

Note also that I threaded in a tripod mount screw, there is a base for my tripod head attached.

Sample images.

(Note, "Hardware eye" images have been inverted but have had no other processing). These images were taken before I added a paper field stop to reduce stray light.


Wide angle shot of a scene.


3X built in telephoto


3X telephoto with 2X TC-E2 converter lens


"Hardware-Eye view

Click image to see the original (big!)

(the original is sharper than this reduced version)


3X telephoto of a flag (it is in there, trust me)


View through the "Hardware-Eye"

Click image for the original (big!)

Click HERE for additional (better!) samples, and samples with the light stop in place.

Last updated September 1, 2000


First of all, if you are going to build a lens like this, be aware before you start that the image will be upside down! That is because I left out the prisims which in a binocular invert the image so that it appears right side up. The images above were inverted to appear right side up. By leaving the prisims out you get rid of a lot of weight, you can have a straight through design and you elimiate four reflections that will rob the image of contrast and quality. It takes a bit of getting used to the lens, but it is not as much trouble to work as a lens that inverts left to right. I find it mildly annoying, but it is the price you pay to have a lens for $10.

The quality of the images produced is fair. The lens has some chromatic aberration, the field is not entirely flat, and there is some loss of contrast. I may not have everything adjusted optimally yet, it is very sensitive to small adjustments. I would say that the shots taken through this lens are the equal of some of the add on lenses such as the Eagle Optizoom. The images are NOT as good as produced by the Nikon TC-E2 teleconverter, but they get you a LOT closer than using the teleconverter.

The extra light captured by the 50mm objective as compared to one of the smaller monoculars is great. That allows a faster exposure, which allows hand held shooting in some cases. For even faster shots, you can zoom out a little. The lens does not vignette until you are zoomed out about half way.

With this particular lens, the closest focus is about 30 feet at full zoom on the camera, which gives about 21X overall. If you place the camera in the macro mode sweet spot, it will focus down to about 6 feet away, but the lens is "only" about 15X at that setting.

I've been having best results with center weight metering, and with the camera set to macro focus. I usually hold the lens with my left hand, the camera with my right, and shoot with BSS.

Construction notes:

I can't provide detailed step by step plans for constructing a lens like this because each set of optics that you might be able to obtain will be different. All I can really do is give some construction tips, it will be up to you to mess around with pipes and fittings in the hardware store to make it work! Fortunately, an 8 foot piece of PVC pipe is about $2, so you can buy a few to play with. Don't neglect the couplings available too!

* Paint the interior of the lens flat black to adsorb reflections.

* I just press fit all parts, that allows taking the lens apart and minor adjustments. Resist cementing parts unless you have to, it will just make trouble later.

* I would recommend the set screw approach to mounting the eyepiece, it allows for adjustment.

* Adjusting the eyepiece position is critical. You want three things, first, the eyepiece has to be square to the objective, or you will have color aberration. Second, you need to have the camera lens as close to the eye lens on the eyepiece as possible without touching. This prevents vignetting, and allows you to zoom out farther. It also cuts off the sides of the field which are not as flat. Third, the eyepiece has to be set to infinity focus or beyond. If you set the focus too close, the camera will not be able to focus "farther" than infinity to compensate. In my design, the eyepiece can be tilted and moved back and forth.

* The attachment of the adapter ring is dodgy, I will admit. I turned down a piece of oversized PVC pipe until I could screw on the T adapter ring and let it cut its own threads. While it is on really tight, I would not trust the camera not to drop off some day, and I keep the strap wrapped around the tripod.

* I later added a black paper field stop/ light baffle inside the tube to reduce stray light and improve contrast. This helped the images quite a bit with no real loss of light. There was quite a bit of reflection off the inside of the PVC tube, even though it had been painted flat black.

What is a field stop? It is there to keep stray light from entering the eyepiece. Think of it as a hole in a disk between the eyepiece and lens that blocks the eyepiece from "seeing" the sides of the tube.  I just cut one out of black construction paper and taped it inside the tube about an inch from the eyepiece. I cut the hole in the paper by trial and error; here is how to do it... point your lens at something bright (OK, NOT the Sun),  and hold the eyepiece a few inches from your eye. You will note that the eyepiece can "see" the sides of the tube. This light from the tube sides is reflected at a low angle, so even if you painted your tube flat black there will still be some, and it reduces the contrast of your final image. You want to cut a round hole (does not have to be perfect) in  a piece of paper (paper is good enough) and get it fixed inside the tube to stop this; the eyepiece should only be able to see the lens. In a "real" telescope, this would be done by ray tracing the design, and the stops would be put in precisely; we don't really care. It will not hurt if your stop is a little bit too small either, that will cut off the eyepiece from seeing some of the light from the very outer edge of the lens, which is actually where most of your chromatic aberration comes from (lens has its greatest curvature there). If your optics show a lot of color fringing, your might want to do this on purpose! Obviously, if you make it TOO small you will start to loose significant amounts of light, you just have to play around with your particular set until it looks good.