Thursday, December 9, 2010

Ultraslim helicoid allows infinity focus

This is about an ultra-slim, deeply recessed helicoid, which solves the problem of allowing further away focus with some lenses (mostly enlarger lenses) suitable for UV photography. I have developed a working prototype now. [If you need to find out how to determine the length of such a helicoid, please read that first]

It allows lenses up to about 50mm diameter to be deeply seated into that mount (including that later plastic EL-Nikkors if not thicker than 50mm!) and if their rear lens element is not thicker than 38mm in diameter. Some lenses may not clear the mirror, that has not been fully tested out yet (and I don't have all possible lenses to do so).

The effective thickness flange-to-flange (= optical path length) of that helicoid is 5.4mm (minimum extension) resp. 10.3mm (maximum extension) now.

The achievable VIS focus ranges are (subject to change), sensor-to-object (not front of lens!):
1) EL-Nikkor 3.5/63mm: 90cm to infinity
2) EL-Nikkor 2.8/50mm: ca. 32cm - 50cm
3) EL-Nikkor 4/40mm: ca. 25cm - 30cm
4) UV-Rodagon 5.6/60mm: 65cm - infinity
5) UV-Planar 4/60mm: 40cm - 60cm
6) Micro-Nikkor 5/70mm: 120cm - infinity
7) Focotar 4.5/50mm: 33cm - 45cm
8) ... more to follow ...

Infinity focus cannot be adjusted exactly to infinity for all lenses, some go beyond, some do not reach infinity at all, depending on lens design and esp. focal length.

minimum extension:

maximum extension:

Just to make things clear: closer focus is always possible using suitable normal helicoids or extension tubes. That was not the issue here. It was about getting such lenses as close as possible to the camera sensor to allow for further away or even infinity focus!

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site