Monday, March 19, 2012

[UV] Focal Reducer or Telecompressor for Ultraviolet Photography

You may remember the recent contribution about using older quartz fluorite parts for taking UV images. It turned out to be a f4/84mm lens.

Here now about a modification of that to change the focal length, specifically using a focal reducer to widen the field of view which also means reducing the focal length. I'm using a focal reducer method, also known as telecompressor in astronomy and it is the opposite of a (negative) Barlow lens. Here a positive (quartz in that case) element is used.

Here some results. I'm showing full format shots here.

[click on image to see a larger one]

without focal reducer:

focal reducer at work:

I measured he reduction factor to about 0.41x and still, there is hardly any focus shift (no adjustments done between shots). In theory the speed of the overall system would change from f4 to f1.6 and the focal length from 84mm to 34mm - we have created a wide angle f1.6/34mm achromatic UV lens now.

Cropped image of focal reducer at work:

BUT, there are downsides which have to be mentioned: the whole projected image of the quartz/fluorite lens system is reduced in size, so the outer, less sharp and well defined image parts will now be visible, which before was cut off by the sensor size and mechanical restraints. If the image projected is not large enough, the vignetting will appear and only part of the sensor will be filled. So this method requires an image size that is at least 2.4x (1/reduction factor) larger before using that focal reducer. And another downside: the back focal distance will also be reduced, so this only works well, if before the system had enough working distance between lens and sensor as in this case here.

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

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