Taking photos of fast moving objects such as planes, sports cars, or birds with tele lenses is a challenging task for several reasons:

Facing these problems, I had the idea to use a reflex sight—sometimes also called red dot sight—commonly used on hand guns as a viewfinder for my camera. Actually, shooters face a very similar problem when shooting at fast moving targets. It is very hard to sight a target with an optical sight with great magnification (and therefore small field of view). The reflex sight on the other hand has a nice property making it particularly well-suited for such situations with fast moving objects at short distance: You can aim with both eyes open. One eye is looking through the sight, which has a magnification of one, i.e., no magnification at all. The other eye looks at the target. That alone would not be very useful, if you did not have a marker that shows where your gun (or camera) is pointing at. To this end, a red (or green) dot is projected onto the target. For those who are interested in technical details, I refer to Wikipedia. Here it should be sufficient to say that the point is not created by illumiating the target as would be the case if you used a laser pointer, i.e., the target does not know that someone is aiming at it—it is certainly no good idea to blind a pilot at an airshow with the laser pointer on your camera! The red dot is rather created within the sight and projected to infinity so you can focus a distant object and at the same time see a sharp red dot on the target. The same principle is used by head-up displays and some cameras to display information overlaid over the picture.

Building the Prototype

After the idea, there comes the design and implementation. So we need a sight and a fixture to mount it on the camera.

Buying a reflex sight is the easy part. There are many available. I looked for one at a reasonable price and finally bought the sight shown in this photo for 30 Euros (about 43$). This sight is sold under many different brands, and I guess they are all manufactured by the same company. The body is made of aluminium and looks quite durable. (Actually, for sights there is always the discussion whether it can stand the acceleration of a firing gun, but the flapping of an SLR mirror is not really comparable to the recoil of a rifle, although the seller claims he has tested this sight with 7.92x57mm Mauser cartridges). The brightness of the dot can be adjusted in 11 levels. The brightest level is also visible against the bright sky as long as you do not look towards the sun. Brightness really matters if you want to take photos of objects like planes and birds.

Adapter Hotshoe adapter with Weaver rail and mounted sight (click on image to enlarge)

Besides the sight itself and of course the camera, we need a mount to attach the sight to the camera. The good news first. There are standards for sight mounts. Now the bad news. There are different ones and none is supported by cameras, so I decided to build a mount on my own.

The different standards for sight mounts are 11 mm rails, 21 mm Picatinny rails, and 21 mm Weaver rails. 11 mm rails are usually used by smaller guns such as airguns. Since 11 mm are really small rails, it is harder to mount them onto the camera. Therefore, I decided to use a 21 mm rail. The major difference between Picatinny and Weaver are the grooves on the rail, in particular their distance. These grooves are useful to keep the sight at its position also for guns with heavy recoil—although we do not care about the recoil of our camera as said before. To keep it short: Both Weaver and Picatinny rails are OK, at least for the sight I bought. Its mount only uses one groove, and therefore the distance between the grooves does not really matter. In case you are interested: Picatinny equipment should also fit on Weaver rails (and Picatinny rails of course); Weaver equipment might not fit on Picatinny rails. So if you want to buy a rail, Weaver rails seem to be the safest choice. Instead of buying a rail, I built one from a 6 mm aluminium plate. The official measures for Picatinny rails are available here, so you only need a saw, a file, a drill, and a caliper ... and some time.

Now that we have the rail, we only need to fit it onto the camera. The best position in my opinion is the hotshoe of the camera. I searched almost the whole WWW for a generic hotshoe adapter that could be used to screw the rail on it ... and I found nothing. There are lots of hotshoes to be mounted on tripods, but not the male counterpart. Again time for saw, file, drill, and caliper. The self-made adapter shown on this photo is again made of aluminium. I got the measures for it by measuring the adapter of several flashes. Since aluminium is an electric conductor, it should not get into contact with the contacts of the flash shoe. Therefore, the bottom of the adapter has a 0.2 mm groove in the middle. With a standard 5 mm screw, I fixed the rail to the adapter, and in turn the red dot sight with its Weaver mount to the self-made rail.

Final Result

Photos of the completed prototype can be seen below. In this photo, the red dot appears to be bigger than it really is. This photo is focused on the camera rather than infinity. As said above, the red dot is projected to infinity to let you see the distant object and the red dot sharp at the same time. Therefore, the red dot is blurred in this photo.

A more realistic situation is shown in this photo. This picture was taken with 50 mm focal length, which pretty much reflects the scene as seen by the eye (the scene observed by the photographer is actually even more similar to the view you see with your eyes with a red dot on the target, but this is hard to capture with a photo). The field of view of the photographer corresponds to the complete picture, not only the part viewed through the sight! Remember that you aim with both eyes open.

Camera with installed red dot sight Camera with installed red dot sight (click on image to enlarge)
Camera with installed red dot sight Camera with installed red dot sight (click on image to enlarge)
View from the back side View from back side. The red dot seen through the sight is not sharp in this photo since the viewer is not focussing infinity. Therefore, it appears bigger than it actually is. (click on image to enlarge)
View through sight View through the sight. The red dot in the middle of the sight can be seen better in the large version of this picture. (click on image to enlarge)

Conclusion and Future Work

Overall, I am quite satisfied with the result. It needs some training to keep your eyes open during shooting. But when you are used to it you certainly get a better and faster overview of the whole scene.

For static scenes without fast moving objects, I still prefer the view through my SLR viewfinder where you can see the picture you actually get. In particular, the view through the sight does not give you an impression of the clipping of the picture with respect to the zoom level. However, this is not problem for the situations it was designed for. With fast moving objects, adjusting the zoom while shooting is hardly possible. So you have to select it in advance anyway.

The only major improvement that I would desire is a better fixture to the hotshoe. Currently, the adapter might slip from the shoe if the camera is pointed to the sky. The only forces holding it is the spring within the hotshoe and the friction between shoe and adapter. However, the whole device weights more than 200 g, which is comparable to a heavier flash device. To prevent it from slipping out, I put my thumb on it as shown in this photo. Actually, this is not really inconvenient since your thumb has to rest somewhere there anyway. But it would be more save to have a nut pulling the adapter against the shoe. This should not be to hard to do, and if I have some time, I will present an improved design.