Looking at Lenses in the Infra-Red

 

The week's column by Ctein

It's time to talk lenses for my IR-converted Olympus Pen. Unless a lens has been specifically designed to work well in the infrared, there's no way to tell how will perform based upon its visible-light performance. A lens that is good in the visible can turn out to be pretty crappy outside of its design range.

All optical properties of a lens are wavelength-dependent. The refractive index of the glass changes with wavelength (the rate at which it changes is called "dispersion"). This quality is so basic to optical glass that Newton felt compelled to develop his reflecting telescope, because he was certain the problem was unsolvable. Happily, he turned out to be wrong. By combining several different elements and lots of different glasses, you can build modern optics which exhibit good corrections for all aberrations over the entire visible spectrum.

Once you get outside the design range of the lens, things can go completely to hell. Let's take longitudinal chromatic aberration, the tendency for a lens to bring light of different wavelengths into focus at different distances. This effect is somewhere between small and negligible for a decent lens in the visible spectrum, but that red dot that you see on the barrels of so many manual focus lenses? That's the infrared focus spot, for the benefit of folks who were using infrared film. You'd focus the lens normally and then rotate the focus ring a bit to move the indicated distance from the normal marker to the red dot.

Happily, with sensor-based focusing like in my IR-Pen, this is not a problem. The same's true of lateral chromatic aberration. In the deep IR, I'm doing essentially monochromatic photography. Problems aren't limited, though, to just the two chromatic aberrations. All aberration corrections in a lens are achieved by appropriate combinations of lens elements with certain refractive indices and dispersions. The corrections are always an approximation of perfection, and designers don't worry about them outside the design range of the lens. A lens that's well corrected for spherical aberration in the visible may be very poor in the infrared.

Another problem that can pop up is hot-spotting. If the anti-reflection coatings on the lens elements don't work terribly well in the infrared, and many of them don't, a lot more light bounces around in the barrel of the lens and makes its way through to the sensor. The result is an image-veiling glow. This is correctable in Photoshop, but it is annoying and inconvenient.

The very first lens I looked at was my Panasonic Lumix G 20mm ƒ/1.7. I'm not desperately in love with this lens, as some folks are, but it's a very good performer. I find the field of view is also a nice match for the IR "look." Unfortunately, the lens is poor in the infrared; there's substantial smearing and fuzziness towards the edges of the frame that's just barely under control at ƒ/5 (figure 1). I don't think it looks entirely acceptable until ƒ/8. Oh well, at least there's no hot-spotting.

Fig. 1. The Panasonic 20mm is a poor performer in the IR, as these center (left) and corner (right) sections at ƒ/5 at 100% show.

OK, not an auspicious start. How about my Olympus 45mm ƒ/1.8? A totally different story. This lens performs very well in the infrared, even wide open. That's most unusual. Wide-open I think I may see a bit of curvature of field, but there's not a lot, and there's almost no smearing or blurring at the corners. At ƒ/3.2, a stop and a half down, the image quality is exquisite (figure 2). The uniformity's excellent with corner quality almost indistinguishable from the center (figure 3). There's no hot spotting at any aperture.

Fig. 2. The Olympus 45mm is excellent in the IR. This is an ƒ/3.2 full frame from the camera...

Fig. 3. ...And these are 100% sections from the center (left) and corner (right).

On to the Olympus 12mm ƒ/2. Not one of my favorite lenses, but it is what it is. Is what it is good enough for infrared? Yup. It's pretty decent by ƒ/2.8 and very good at ƒ/4 and on down. Yes, there's a bit of smearing in the corners, same as in visible light, which is why it ain't one of my favorites. But it's not worse than in the visible, which makes it very good by IR standards. And, again, no hot-spotting.

How about my 85mm ƒ/1.4 Rokinon manual lens? It continues to impress. It's soft/smeary in the corners wide open and at ƒ/2–2.8. I could use it wide in a pinch, but I would rather not. It's improving as it stops down, though, and by ƒ/4 it's looking mighty good everywhere.

So, three of my four primes hold up well in the IR. What about my zooms? My 14–42mm Olympus kit zoom (first generation) is a surprise win. At the 14mm setting there's smearing in the corners wide open, but stopping down one stop cleans up most of it. At 20mm, it's definitely superior to the Panasonic 20mm ƒ/1.7. Wide open, at ƒ/4, it's close to as good as the 20mm at ƒ/8. Stopped down to ƒ/5.6 it's superior. At longer focal lengths wide open is OK but not great. Stopping down a stop makes things a lot better and is probably the optimum aperture across the board.

The Panasonic Lumix G Vario 45–200 ƒ/4–ƒ/5.6 also turned out to be OK in the IR. At 45mm, it's nowhere in the same league as the 45mm Olympus, but if it were already on the camera I'd feel comfortable using it. At 72mm, it's a bit off in the corners wide open (ƒ/4.3) but not too bad; ƒ/5.6 improves things. At 120mm and onward, the image is uniform but soft wide open; it's very decent half a stop to a stop down. There seems to be a lot of light loss wide-open with this lens, at all focal lengths—a good half stop at least—so there's not much reason to use it wide-open anyway. And, still, no pronounced hot-spotting.

So, are hot spots a thing of the past? Nope, I'm just lucky in my choice of lenses. I played with my friend DDB's Olympus 60mm ƒ/2.8 macro a bit, and while the sharpness and uniformity were acceptable (though not spectacular), the hot-spotting was severe (figure 4).

The Olympus 60mm ƒ/2.8 macro lens has a serious hot-spot problem in the IR.

And that's the state of lenses in Ctein's IR kit.

©2013 by Ctein, all rights reserved

Ctein, who is coming up on his 300th column(!), is TOP's de facto Technical Editor. His columns appear on most Wednesdays.

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Comments

[Josef]

Ctein,

Does the relatively poor dynamic range of the Olympus E-P1 cause much trouble when it comes to infrared photography?

I shot briefly with an E-P2 a few years ago and was never very happy with the sensor's ability to capture high contrast scenes without blowing out highlights or blocking up shadows that showed lots of noise when trying to bring them back a little.

I ask because infrared images often seem pretty "hot" to me, lots of delicate and bright whites. The last image in this blog post illustrates the problem well with what appear to be blown-out highlights in the grass.

[Oskar Ojala]

Good summary of the challenges. Your list does not actually surprise me :-) My experience has been that it's easy to find tele lenses that perform well, while wide angles are a different story. I remember how my old Nikkor 85/1.8 was razor sharp, while getting a wide that holds corners well was a big challenge, especially in wide band photography. Now that I've optimized for a very compact IR kit, I'm finding problems with evenness of illumination in superwides and strong IR filters, my RG780 giving me very dark corners. Such is life.

But while I read this post, the foremost thought I had was that it's all relative. Slightly blurred corners? one could crop or compose differently; a nuisance for sure, but workable. Similarly, the serious hot-spot of the 60/2.8 doesn't look nearly as bad as I got from a good and not complicated normal lens. But hot-spots depend a lot on subject matter. Sure changing the way one shoots due to lens deficiencies is a nuisance, but sometimes it's easiest to just live with certain limitations.

[Robin Whalley]

I use a converted GX1 and have similarly found some lenses not to perform well. The Panasonic 14-45 is poor at the wide end but much better at 45mm. The Olympus 9-18 is pretty impressive although also suffers a little at the wide end. MyOlympus 45mm is also very good as is the Panasonic 45-200.

Something else I have noticed is some of the UV filters I have used have degraded the image or caused hot spots. I have now switched to using clear glass protectors.

Great to find someone else also using Micro 43 Infrared. Thanks for sharing your findings.

[ctein]

Dear Josef,

Not, not at all a problem. Please read last week's column. Also, there are no blown out highlights in Figure 4.

pax / Ctein

[Andrea B.]

Thanks for the useful info about which of your m4/3 lenses are IR-capable. I shall add a link to this column (and the last) in our IR reference Sticky.
It's nice to see you shooting IR. "-)

[Will]

Ctein,

For the uninitiated, could you please explain "hot-spotting?" What is it, what should I be looking for in the example, and what causes it, optically?

[DavidB]

Ctein, please keep up the IR posts, it great to read more on this.
I am not too surprised by the Olympus lenses doing well. I think the new coatings allow IR through, and its rumored that Olympus uses the IR to assist for rapid contrast focusing. So the camera and lenses I would expect to do well in IR now.
I must say you eyes must be different than mine. I don't see a severe hotspot in figure 4, actually I don't think I see any hotspot. The ones I have found/seen over the years are blatant. As in a bright sun now magically appears in your shot. The circle of light was never there before.

[ben ng]

hi there Ctein,
I just received my IR-converted EP2 a week ago, got it modified by a Canadian store in Quebec. I have been using it with my Olympus zooms, 9-18mm and the telephoto 40-150mm with great results. I suppose when I say "great results", I am so accustomed to to working on images in photoshop that it is normal for me to spend time with a single image, just like in the past when spotting archival silver prints.
Anyway, it's a great change from my Fujifilm IS-1, factory-made as full-spectrum IR, which had the most annoying lens-flare.....
IR isn't for everyone, but there is a beautiful creative dimension if we put in the work...

[Ctein]

Dear Will and DavidB,

Well there are two kinds of hot spotting, although both are caused by the inability of the lens design and anti-reflection coatings to suppress stray infrared light.

The kind I'm describing is an overall diffuse glow, where the center of the field is suffused with veiling light. That's what you see in figure four. The dark tree trunks and shaded grass there aren't brighter because of any real-world lighting effects. They should look as dark as the similar objects at the periphery. Clearly they don't! It'll happen to a greater or lesser degree with any scene, no bright lights in the frame required.

Because anti-reflection coatings often don't work well in the IR, you can also get flare and ghosts, just like you would from pointing any lens at a very bright light source, but much much worse in the IR. That's a more normal problem, and you deal with it in the IR the same as you would in the visible-- "Doctor, it hurts when I do that." "We'll, then don't do that!"

pax / Ctein

[MikeB]

The 12-50 "kit" lens that I bought with the EM5 is also surprisingly good with no hotspots. I've used it on a converted GF1

[Tim]

I'm running into this exact issue trying to find good m4/3 lenses for IR. I have the 14-42mm zoom and while I am satisfied with it, I would like to have something a little more compact for travel (like the 20mm or the 14mm). I found some people saying the 20mm was a fine IR performer, but here you are proving them wrong, so I'm not certain what to think about the 14mm (which the same people said was good as well). I'm starting to think the reviews merely look for hotspots. Sigh, I wish there were more m4/3 IR shooters so we could have more reliable reviews.