uncoated lens best for Infra Red!!

[chippy]

as the title implies..is it true?

i dont think so but would like to be proved or shown different. I am not that technical from a precision lens designer point of view but i do love the old lenses ( i have many from Petzvals to whatever) and wish they could be labeled as better for infrared but are they?

I notice of late that the rumours 'use uncoated 1930's lenses for infared' are spreading through the net, like chinese whispers it is turning into fact. based on what??? i realise that the really early lens wernt corrected for the red so much but i dont see how that makes them more suitable for IR, nor do i see how lenses (such as Tessar, Scopar Xenon or Xenar) from the 30s that are uncoated makes them more suitable for IR ...how so?? more suitable for UV i think but IR?🙄 in fact i am have the idea that modern lenses are probably better suited/corrected for IR

not that i have shot much IR lately and dont really have the inclination to do so just to dispell any possible myths but the question is, is there any truth to it...i cant see it as i think the coating only serves in some cases to eliminate UV light (the oposite end of the spectrum to IR) .

what opinion have you? i would like to be proved wrong..happy as a ......

 

[charjohncarter]

Junk science: it is a big problem. I hadn't heard this rumor, but so I would like to see real evidence, if possible. I shoot some IR and I haven't noticed lenses make much difference. The IR exposure is the real mystery. I hope someone else jumps into this discussion.

 

[Al Kaplan]

There may be some truth to it. After all, glass itself is a pretty good filter of ultra-violet light so lenses designed to use with UV are ground from quartz. Some lens coatings might be reflecting or absorbing IR rather than transmitting it.

Years ago I shot a lot of Ektachrome Infrared, which was E-2 process back then. I used to use an orange filter and exposure was very much hit or miss. Lots of bracketing was required.

 

[sevo]

Hardly the coatings themselves. But lenses of the pre-coated age generally have less glass - the gold standard back then was the four-lens, three element Tessar - and glass by itself is a IR attenuator or can have non-linear properties outside the visible range.

Besides, old lenses were calculated with little more than adding machines to help, so that even few spectral points were quite an effort - the resulting nice and predictable designs will usually also behaved gently beyond the invisible range. New ones are computer calculated thoughout to much tighter specs and are much more complex, so that they can have worse behaviour beyond their specified range unless the manufacturer explicitly takes care of that.

Sevo

 

[Sonnar Brian]

Older Leitz lenses are better corrected for Chromatic Dispersion than most others. Look for the Infrared Index to be close to the visible focus mark. On a Summicron, it's at about the F2 DOF mark. On most lenses, it is between F4 and F8. The closer the IR mark is to the "regular index", the better the lens will be for Infrared. The wavelenghts will not disperse and soften the focus as much.

Now, my BEST LENS for Infrared is uncoated. It is a Calcium Fluorite optic, an "Ultra-Achromat". It is uncoated as the spectral range covered by the lens is far greater than the coating could handle.

BUT: for most Infrared applications, the spectrum covered is from visible to about 1.1uM (11,000 angstroms). The Coatings are not going to hurt anything. I've used a Canon 50/0.95 TV lens (same optic as the RF lens) with an Infrared Sensor that went out to 2uM. It worked great.

 

[Sonnar2]

Brian, I use this "thumb rule" for chromatic aberrations as well. Obviously on the "blue side" of the spectrum the lens can behave different.

In this graph Infrared is on the left, UV light at the right. In the middle light visible to human eye. The aberration curve zeroes 3x - what is usually called a APOCHROMATIC lens.
(from a patent writing of A.W.TRONNIER for Voigtllaender, Color-Skopar)

 

[Sonnar Brian]

I did not realize that the Color-Skopar is an Apochromat. No wonder that Vitessa T does so well.

Apochromats, or "APO" lenses, are very good for Infrared and typically do not require refocussing.

We had some novel optics in an old lab, including Quartz and Salt lenses. They were made for UV work and for Infrared work. The Pentax 85/4.5 is an "Ultra-Achromat", and zeroes at four wavelenghts. This CFL lens transmitted from UV through to Midwave Infrared.

 

[Sonnar2]

Probably it was a marketing issue why Voigtlander didn't claimed the apochramtic correction of the Color-Skopar in public, because nobody else would have bought the more expensive, famous "soft sharpness" Heliar. This is what Tronnier's son (who is in his eighties now) suspected in conversation to me. Since the business board of Voigtlaender and Zeiss Ikon did some other ill decisions in the 1950's and 1960's, I tend to believe it...

Not sure about the wavelengths of Infrared and UV light, the above correction graph suggests that a lens corrected on the one end of light spectrum does not need to work well on the other. Sadly, lens makers don't offer diagrams for chromatic aberrations very often.

 

[chippy]

Not sure about the wavelengths of Infrared and UV light, the above correction graph suggests that a lens corrected on the one end of light spectrum does not need to work well on the other. Sadly, lens makers don't offer diagrams for chromatic aberrations very often.

thats pretty much how i understood it to be. in general, UV focuses ahead of the film plane and IR behind ..so a highly corrected lens may incorporate the IR spectrum but not the UV as well, the same applies in reverse for a specific UV designed lens not being usable for IR afaik

Brian, you are far more familiar with Leitz lens than i am, but i thought i read somewhere at some time that Leitz dont mark their lenses with the IR scale (probably more a recent thing?).

only from memory so i may be wrong, but more or less because they dont think the scales are of much value, because depending on the sensitivity spectrum of the film used and the varying tones of red colour filters that can be placed on the lens, the infra red scale markings would need to be in a different place each time...so they just omit putting the marks on the lens at all..

either way, the question of whether a non coated lens is better for IR photography, given the information so far seems to be a myth

 

[Sonnar Brian]

My older Leitz lenses have the IR index on them, and it is usually located at the F2 DOF mark. The IR shift is much less than most other lenses. so: A 5cm f2 Summicron can be used wide-open with most IR for good results without refocussing, most other lenses need to be at F4~Ff5.6.