I hope I'm in the right place with this question: I have an Agfa-made industrial lens and because of a weirdly specific number mentioned on its engraving (672 - which might stand for nanometers) I suspect that it might be optimized for near IR. I don't know anything about IR, so I'm unsure if 672 NM even counts as near IR, or if that's still part of the normal visible spectrum...
My question is the following though: Do you know any way to find out if that lens is indeed optimized for that wavelength, without having to invest into a camera conversion or very expensive filters?
The only way to tell is to mount it on your camera and try it out. It might be optimized for 672 nm which is just on the edge of visual. If it doesn't have a red coloration then you will have add a filter. There are some 720nm filters on E*** that aren't very expensive, but not as good as Hoya or B+W. You will have to deal with your camera IR blocking filter too, expect long exposures.
It seems a rather precise wavelength for lens optimisation, but it is the wavelength of SII emission (used in astronomy). However 656nm for Ha is much more commonly used & is close enough that a lens optimised for one will be VERY near optimum for the other.
As @tomm111 mentioned 672 is in the transition between visual & near IR. You should be able to record 672nm with a normal camera. You can get 650nm lasers for ~£2.50 (which again will be close enough) or you could add a #25 or #29 filter & use normal lights to restrict visual wavelengths to roughly 600nm+ (somewhat tighter with the #29, but they are less common). I wouldn't go with a 720nm filter as these have very little transmission at 672nm (The kodak wratten handbook for the 720nm #87 shows 0% at 700nm), while the #29 manages ~90%
To be honest I'm not sure how I'd check a lenses optimisation anyway. Most visual lenses work quite well to at least 1100nm with just variation of the focus distance with wavelength. Outside the visual range focus of Apochromatic lenses often vary more than cheaper achromatic lenses.
I can't remember where I saw it but I do half remember the explanation: To get 2 wavelengths focused the same the focus distance wavelength graph must be a quadratic (U shape), while getting three points the same requires a cubic (S shape). The extra constraint can make the (design irrelevant) tails have steeper slopes.
I know there are some very expensive lenses that are designed to have very minimal chromatic aberration from UV right through to IR, which might mean the explanation is garbage, but use outside of the design situation tends to suffer more as the design requirements are tightened...
Thanks a lot - I'll probably look for one of those lasers mentioned below by @petrochemist - perhaps there will be a difference in how this lens and an unmodified version of that lens capture that...
Thank you very much for the details and advice - that's very helpful. If @xpatUSA is indeed correct and that wavelength is within the usual visible spectrum, I doubt there will be a significant difference, but it might be worth trying, with a laser like you mentioned.
What do I need to consider when I have such a laser... I need a pretty long long exposure as @tomm111 has mentioned, right? So I assume everything else needs to be pretty dark and the lens closed down to f/22?
Many thanks. "Optimized" might be the wrong word, but if it's important enough for the manufacturer to engrave the exact NM onto a lens, there might have been some form of optimization present, I assume. Unless the number means something completely different.
That Coastal Optics 60/4 sounds like a very interesting lens. Would be very curious to hear about your impressions on that as a regular taking lens - if it's not too much of a hassle it would be wonderful if you could share some impressions in the Adapted lens talk part of the forum!
With the laser you wouldn't need the filter, all it's light is at a specific wavelength (and is quite bright). The lasers I've brought to play with are slightly more expensive (~£10) come with interchangeable heads that split the bean into patterns of dots. That might be useful for testing, though that wasn't my intent I can't resist new forms of lighting to experiment with. I've brought red, green & blue/violet versions to play with which also cover the visual spectrum fairly well :)
If I may ask one additional question: I have another (seemingly customized) lens, which has the handwritten inscription UV on its package. Is there a similar way to test for capabilities in terms of UV?
I've heard good things about that lens, but I've never seen it for anything like the amount I'm happy to spend on routine lenses
I guess it's not routine, but I have to pontificate for longer when lenses are no longer pocket money.
In this case IIRC that lens goes for about 5-10x my maximum for lenses, so it will remain a dream for me 😨.
I do have a metal bodied El-Nikkor that Dr Klaus Schmitt recommends for UV. My spectrometer confirms it transmits better than any of my other regular lenses (ie excluding single elements) though I haven't been able to confirm how constant it's focusing is (I'm told it's quite good there) but I think it's less important when shooting tight wavelength groups with a mirrorless camera. I should be able to see the UV focus direct in an EVF.
Thanks a lot - very interesting! I have a soft spot for lenses around 60 mm, but for the most part I got significantly older ones made for enlarging or industrial use originally. Have you ever tried the Schneider Apo-Componon 60 mm f/4? I found that (particularly the Makro-Iris version) a very affordable alternative for macro, even though it hasn't got any special IR or UV capabilites as far as I know.
I think it's the El-Nikor 63 mm f/3.5 that works best for UV. I assume you mean Klaus Schmitt?
