Friday, January 11, 2013

Quartz lens for reflected UV photography?

Today about using a quartz lens for reflected UV photography. I'm using quite rare Quartz Takumar f3.5 85mm for this and one of the few flowers available, a Phalaenopsis orchid, as well as my "work horse" UV filter, the Baader-U filter. Light source was sunlight and a Nichia UV LED (365nm). The questions was, how the image quality would turn out, knowing that such lenses should only be used for a quite narrow wavelength band (literature states between 10nm and 20nm depending on source), as these lenses are not chromatically corrected. All shots were done stopped down to f8, which is a usual working aperture to get enough DOF and sharpness.

[click on image to see a larger one]
Here some results:
visible light image using UV/IR cut filter:

multispectral image (i.e. unfiltered) and sunlight:

UV image using Baader-U filter and sunlight:

UV image using Baader-U filter and sunlight + 365nm UV LED:

1) There is a substantial focusing difference, here 5mm between the UV (95mm) and the VIS image (100mm between front and camera mount), resulting in a 5% different image size when focused both (UV image is smaller). Distance orchid - camera was 700mm.
2) When white light is being used for visible photography, the 400-750nm band is quite broad, i.e. 350nm, which leads to quite visible CA (around edges), soft image and loss of contrast.
3) When white light is being used for multispectral (i.e. unfiltered ca 300-1100nm) photography, that band is even broader, leading to even softer images and loss of contrast.
4) Also the UV image taken in sunlight using a Baader-U filter (320-395nm i.e. 75nm bandwith) leads to rather soft images, but a bit better as the band is narrower, only 75nm wide.
5) The UV image taken in UV LED light at 365nm (ca 20nm bandwith) using a Baader-U filter leads to the best image, as here only the narrow 20nm bandwith of the UV LED around 365nm plays a role. The image is crisp, sharp (see structures on petals) and has a rather high contrast.

Conclusion is, that only if quasi monochromatic photography is being done (either using narrowband UV filters or narrowband UV light), acceptable results can be expected. For broadband UV photography, chromatically corrected UV lenses are required, such as quartz fluorite lenses.

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

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