Johan Posted May 12, 2014 Share Posted May 12, 2014 One of the icons of the extreme macro world is the JML 21mm f/3.5. It can deliver really nice photos, and you canget a good 5:1-8:1 out of it or so before diffraction sets in. This is a shot of mine with it a couple of weeksback, a pretty Jewel Wasp. Anyhow I've always wondered why people were landing on my JML page looking for some JML 50 and it turns outthat is a nice UV performer. So I thought I'd see what happens with JML 21mm. I don't have a spectrometer or anything to test UV but I did learn from this thread that sensors don't like excessUV much (the red shot). So I'm hoping red indicates UV transmission, and then I also added a UVIR cut filterto compare. The only remotely UV capable optic I have is El-Nikkor 80 f/5.6, the chrome version, so I also used that asa reference. This is just a cotton bud lit with 365nm torches, one of which is venus filtered. I don't know if this means anything but I'd appreciate it if someone more knowledgable than me might commenton this, or suggest a DIY UV transmission test that would be better. To me this reads like JML21 might be UVcapable to some degree, which would not be all that surprising as we know the 50 isn't too shabby. But if sothat would be rather nice, as it would make one of the nicest extreme macro lenses even nicer! Thank you As another note of interest, a focus shift that I alluded to in my other thread above is actually pretty visible inthe bottom shot, it obviously changes when I cut the UV... Link to comment
nfoto Posted May 13, 2014 Share Posted May 13, 2014 The redness in this case more like represents excess IR. Link to comment
Andrea B. Posted May 13, 2014 Share Posted May 13, 2014 The Jewel Wasp is beautiful! Did you use a filter for the left-hand shot ? I'm not sure what that shot represents ?? Do-It-Yourself UV Transmission Tests:Shoot a known UV-bullseye flower (Dandelion, Sunflower, Rudbeckia). If none are growing wild, you can usually purchase a Sunflower in a grocery store or florist shop.Aim a UV torch through the lens and aim it at a piece of ordinary white printer paper. This kind of paper fluoresces under UV light due to optical whiteners. Look for the round (usually blue) fluorescing spot on the paper. (This is also a good check to ensure there is no internal fluorescence or strong reflectivity from the lens which might contaminate a UV photograph.)Create a UV filter board and photograph it. http://www.savazzi.n...estfilters.html Link to comment
Johan Posted May 13, 2014 Author Share Posted May 13, 2014 Thank you Andrea & Bjorn, I will try the second of those tests tonight!The left hand shots Andrea are just a Nichia torch onto the cotton with no barrier filter. One thing Bjorn I'm unclear about. If the red is excess IR, why does a Wratten 2b block it as per the picson this thread. The curves of that Wratten filter would indicate it passes IR or am I reading that chart wrong :( Anyhow I have a hot mirror on the way, let's see what difference that makes! Link to comment
Johan Posted May 13, 2014 Author Share Posted May 13, 2014 Fwiw, the second of these tests didn't reveal much. Every lens I shone it through shows theblue spot :D I think my understanding (or imagination) is a bit limited here, something I'm not grokking.When we see infrared as red, is it because the thing is hot? I always thought IR meant heat :(Can someone point me to a link that might explain this in layman's terms? Thanks!! Link to comment
Andrea B. Posted May 14, 2014 Share Posted May 14, 2014 The Wikipedia article is as good as any: http://en.wikipedia.org/wiki/Infrared Thermal Infrared is "way out there" at 8000nm or so. We photograph in the non-thermal, near-Infrared wavelengths -- typically between 700-1000nm. The photos above are not fully labeled for easy reference, so it is hard to say why you recorded red. For the bottom left photo above, *IF* you used a 365nm UV-Led with no filtration and a lens with no filtration, then likely you recorded some Visible light leakage from the torch IF you were shooting in a totally dark room. Those torches are not perfect. IF, instead, you were shooting filterless in ambient light, then you likely recorded some IR contamination also. I don't recall what camera you are using, but your blue spot is likely some kind of light leakage. Be sure to cover the viewfinder while photographing in IR or UV. Light can also leak through open ports or through aperture windows (on such lenses as have them). Sometimes there are internal reflections from a lens which causes flare or hot spots when used outside the Visible range. Continue the experiments and soon you will sort out the various variables !! :D Link to comment
Johan Posted May 14, 2014 Author Share Posted May 14, 2014 Thanks Andrea - yes there's a lot to discover, it's interesting stuff and I hope people will put up with my newbie mistakes :D. I have a cap on the viewfinder, the first thing I did because of my ND110 days! The red photos were with 2 365nm torches, the left one had baader u on it. No barrier though. Hohum, onwards and upwards Link to comment
Shane Posted May 14, 2014 Share Posted May 14, 2014 Johan most consumer photography Hot Mirrors do not actually work that well for UV photography. See fig 13 & 14 http://www.beyondvisible.com/BV3-IRcontamination.html Link to comment
Johan Posted May 14, 2014 Author Share Posted May 14, 2014 Rubbing salt in the wound :D. I also read that link - late last night half a day after ordering one!!! Stellar :( Link to comment
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