Saturday, February 8, 2014

[UV] Old Achromatic Quartz Fluorite Lens for Ultraviolet Photography (VII)

This is the time of the year where usually (new) equipment is tested or modified, and being prepared for the forthcoming "UV shooting season". And so today I would like to show more about that quartz fluorite lens from an older spectrometer system I have previously written about a few times.
Today now shots of a decorative winter flower Winter Jasmine - Jasminum nudiflorum in reflected ultraviolet using Baader-U filter (approx. 320-395nm, peak approx. 350nm) as a test target. Lenses used were that adapted and modified f3.2 / 80mm (approx.) Quartz Fluorite Lens system (QF), as well as my CERCO f4.1 / 94mm lens for reference. Light source was a modified Xenon flash. All shots were done at approximately f8 and the difference in focal length has been compensated by adjusting the distance lens - flower. The UV images where whitebalanced using my preset for the CERCO lens to allow for a reliable comparison.

[click on image to see a larger one]

UV image using Baader-U filter and CERCO lens:  

UV image using Baader-U filter and QF lens:  

Diptych of the above images (QF left, Cerco right):  

Diptych of the above images (QF left, Cerco right) - details:  

Measured spectral transmission of that f3.2 / 80mm Quartz Fluorite Lens system (uncoated obviously):  

Both lenses seem to reproduce the specific reflected UV pattern of that Winter Jasmine quite well, and as expected my Cerco lens has an edge over that older Quartz Flourite lens. The QF is about 0.7 stops slower compared to the Cerco, to achieve about the same sharpness level, but the QF overall does quite well for such type of UV photography. So it is one of the very few known lenses which are able to break the 300nm transmission barrier, as even the best suitable lenses for UV I found in years of search (non quartz fluorite ones) stop transmitting around 320nm.

(I have re-evaluted the focal length of the quartz fluorite lens system (QF) and it is about f=80mm and its speed is about f3.2. It seems that the useful aperture settings for this lens are around f8 - f11 to achieve suitably sharp images.)

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