Converting a close-up lens into a super telephoto

[Gerald]

The idea
At first sight this seems a crazy idea since a close-up lens is used for macro photography (very close distances), whereas a telephoto lens is frequently used to shoot far subjects. However, Leica used this very same idea in the famous Telyt 400mm F/6.8 and Telyt 560mm F/6.8. These Telyts are basically an achromatic doublet in the end of a long tube.
To understand the idea, remember that with a photographic lens, when the object is in the infinite, the image is at the rear focus. With an achromatic close-up lens, if the object is placed at the front focus, its image goes to infinite. Note that the relation object-image of a close-up lens is the reverse of a telephoto lens!

The experiment
I improvised a setup with two bellows and a few extension rings I had in hand. This way, a 52mm diameter Sigma achromatic lens could be positioned at an adjustable distance around 600mm from the camera sensor plane. The 600mm (0.6m) is the estimated focal length. The FL in meters is equal to the inverse of the lens power (about 1.7 diopters). The achromatic lens was secured by little strips of adhesive tape to the last extension tube. It is important that the lens be mounted in reverse, so the surface that normally would face the object in macro-photography, faces the sensor now. The Pentax bellows has a rack-and-pinion mechanism, which is very handy for focusing. The relative aperture of this homemade telephoto lens is about F/12.


Performance
The picture below is of a communication tower 160m away. The vignetting was caused by the small diameter of the extension tubes employed and the auto-diaphragm mechanism inside the tubes. With extension tubes with larger diameter, the vignetting wouldn't show up.
I think the image quality is pretty decent. There is a little amount of spherical aberration but CA is almost non-existent. The IQ can be improved by reducing the aperture with a perforated cardboard (a kind of Waterhouse diaphragm). I measured an increase of 30% in MFT-50 of the stopped-down when the aperture is reduced to 20mm (F/25). Sadly I don't have any picture of the communication tower for the stopped-down lens.

The following picture is a 100% crop, allowing a better assessment of the resolution. The last picture shows the same cropped area, but from a shot taken with a Sonnar 180mm F/2.8. The Sonnar is a very sharp lens, but its capability to show fine details cannot compete with a lens with FL more than 3X greater.




Practical and final considerations
It became convinced that such a telephoto lens can be practical. Any person with a little of manual skills could construct a super telephoto lens. The lens "barrel" could be made of a length of a plastic PVC tube like those used in hydraulic or electrical installations. If you don't consider the weight of the bellows, the estimated weight of the complete telephoto lens would be something around 200-300g. Possible candidates for the achromatic lens are the Sigma, the Canon 500D (FL=500mm), Elpros, equivalent lenses from Marumi, etc.
Finally, a super telephoto lens made with an achromatic close-up lens is longer but cheaper than a mirror lens. And the bokeh is much better.

[iangreenhalgh1]

Nice idea, but I think a better solution is to use a process lens. They are commonly available on ebay, usually really cheap, they are apochromatic so CA is absent and they have very high resolution, capable of outresolving any sensor.

I use a Wollensak Apo-Raptar 11/540mm that I mounted on a 450mm long tube, I then have a set of bellows on the back of this tube for focusing. It's a bit unwieldy but the results are better than all but the most expensive telephotos. The f11 aperture is a bit of a drawback, but you can't have everything, the lens is capable of 160lp/mm at f11 so as long as shake is avoided, the limiting factor in the sharpness is the camera sensor and the atmospheric conditions.

[ForenSeil]

I was also thinking about using these but never found a proper way to mount them.
Results you got are better than I expected!
Thanks for sharing!

Frauenhofer doublets from cheap telescopes are paritally very good aswell. I have one from an cheap Meade telescope (d=60mm, 900mm, F=14,75) - very good!

@Ian
Unfortuenately not all processsing lenses work well at infinity as tele lens, but if you find one for a good price that's fantastic of course.

[ManualFocus-G]

Very impressive work and a decent result IMO!

[bernhardas]

Edited

[lEo.]

So it means we can use medium format's extension tube instead.

[ForenSeil]

[sammo]

Nice experiment, but not such a crazy idea. Achromatic doublets are used since mid 18th century as long focal lenght lenses without CA. Of course they were used for telescopes first. In my experience from observing with a number of telescopes achromates work nice (read: diffraction limited) to about f/10, for faster optics CA quickly gets unbearable. Simple triplets (one group only) are then ok up to f/6.5 for faster ED glass or flourite is needed. I am talking about focal lengths of 800mm+, of course.