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.
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.
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).
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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