Friday, June 17, 2011

[UV, VIS] Cerco 28mm multispectral lens

Today about a CERCO 3.5/28mm multispectral lens, made of fluorite / quartz and a big CAVEAT upfront, as I'm using this lens way outside its defined range, which has been made to serve as a lens for an image intensifier system originally with a 18mm image diameter - I'm using 21.6mm, because that's the sensor diameter of my UV sensitive camera. So what has to be expected is a sharp and contrasty center, but most likely quite some fall-off outside that central area.

Mechanically this lens is very tiny, about half the size of my thumb, c-mount and has a rare front filter thread. Register is quite short due to its defined use for an image intensifier, but meets c-mount lens standards and even focuses from beyond infinity to about 0.5m (so it cannot be used on Nikon bodies, to make it clear).

[click on image to see a larger one]

Transmission is quite good and as expected:

[measurement valid from about 315 - 750nm]

That said, now to the UV and VIS test results. All shots done in sunlight, ISO640, f5.6, focus set in visible light, no focus adjustments done. ReflectionDisc in camera whitebalance, reduced to 1280pix, sharpened for web, contrast enhanced (UV).

1) Rudbeckia fulgida flower close-up (same camera position as in my shots shown in other thread here)



2) Shot down to the city (infinity):



3) Shot of our neighbors house (ca 30m):



Especially from the last two shots it gets quite clear, that this lens was used outside its defined image circle and the loss of sharpness is evident. The close-up shots are much better (as could have been expected due to the much larger projected image circle). The image center is actually quite sharp and contrasty for such a lens and esp. when compared with a Noflexar 35mm, which it certainly beats in resolution and transmission (but the Noflexar 35mm has the benefit to cover a FF sensor). This lens was made for very faint light intensities, so no wonder that it shows some signs of flare if used at high light intensities (although proper shading was used), but I could not detect a prominent hotspot (which its brother, the Cerco 94mm has).

Overall a nicely performing lens for UV-VIS (I haven't tested it for IR) if used inside the specified range, but also quite an expensive one (UV-Nikkor range).

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

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