UV with a Canon 200mm f2 lens

[JMC]

A couple of years ago I bought an ex demo Canon 200mm f2 lens in a sale. Certainly a lovely lens but it is a bit of a monster. Here's what it looks like on a camera;

 

 

The earlier version of the lens, the 200mm f1.8, was nicknamed 'the eye of Sauron' for good reason.

 

At 2.5kg and 128mm diameter it doesn't really lend itself to front mounted filters but does have a thin slot for filters to be mounted internally (currently with a clear UV filter in there). In visible light I've had some nice wildlife shots with it;

 

 

Given we have a bit of sunlight today, it got me wondering whether it would give anything at all with a UV modified camera. This thing is absolutely not designed for UV - 17 elements in 12 groups, lots of lovely coatings, but this morning I gave it a go.

 

Pictures taken at around 10AM, sunny November morning (small amount of UV but not a lot). Camera was ACS EOS 7D modified for UV with one of their filters. ISO3200, and all taken wide open at f2, focusing done with live view on back of the camera, hand held. Images captured as RAW and then whitebalanced in Darktable. Images despeckled (a lot of noise on them) and sharpened a bit in Photoshop, along with tweaking the curve and auto contrast, before shrinking for sharing. My lovely (long suffering) wife was the victim, er subject.

 

Firstly, autofocus did not work for UV, so manual focus was needed and live view was a huge help for this.

 

 

Well it gives a picture at least. Interesting green cast after white balance. To give you an idea of exposure, this was ISO3200, f2 and 1/50s in fairly bright sunlight.

 

 

My wife wears glasses and these should look dark in UV due to the UV protective coatings on them. They do look slightly dark orange, and there is still some light getting through, so other out of band wavelengths are getting through. Not surprising really given the lens construction.

 

 

This was SPF50 sunscreen on the arm (looked slightly white in visible light). There is definitely absorption of the UV by the sunscreen so some UV is getting through to the camera.

 

The ACS conversion normally gives similar response to the Baader U when I've tested it before, starting UV transmission at around 400nm and a peak transmission around 380nm. There is obviously out band contribution here, but not a huge surprise as the lens will be doing its best to block the UV that the filter would be transmitting coupled with there not being much UV around at the moment. I cannot measure the transmission of this lens as it is too big to fit in my test apparatus, but my guess would be that it is pretty much zero transmission below 375nm or 380nm.

 

For UV work with this it might work better with camera with no internal filter and with something like a Straightedge U filter mounted to the lenses 52mm internal filter holder, if the SEU could be made to fit inside it. At least then the filter would have peak transmission more in line with where the lens is letting a bit of light through (380-400nm).

[Stefano]

Interesting. This shows well what we would expect, these lenses are not exactly meant for UV. Maybe you can see a 365 nm source, like the Nemo torch, but it would be the limit. Surely there is nothing under 350 nm and well above that. If you are looking for nectar guides the 380-400 nm range should show them well.

 

I have a pair of old binoculars. When I read "coated lenses" I knew it was bad news. I tried to look through them in UV, and there was nothing. They block it like polycarbonate. Maybe they can get to 395 nm, but probably not 390 nm. But they work well in IR, and I was able to get some images with a Hoya R72.

[dabateman]

Jonathan,

Shine your Nemo torch through the lens at a wall. See if you can see the light. That is my first test. If your really creative you can photograph the light pre and post lens to see if its darker blue through the lens.

Many lenses will fail the torch through test.

[JMC]

Lens packaged away again for now, but I'll try that David when I next get it out.