Infrared tech talk

There are some lists online about which lenses are likely to work in infrared without making a “hotspot”. Pretty helpful! I want to say a little bit more about my experience with some lenses, beyond the hotspot issue.

I use a converted NEX-7 with a 720nm cutoff filter, and a Fotodiox EF-NEX converter to mount Canon EF lenses to it. Its 1.5x crop sensor and 24 megapixels make for a very exacting test for a lens. All of the lenses I mention in this review are capable of making good prints, regardless of what shows up on the screen @ 100%.

I haven’t gone through all the apertures rigorously with these lenses, I tend to stick with f/5.6 as default on the primes, stopping down for more DOF and opening up for less DOF if necessary.

Canon EF 50mm f/1.8 Mk. I: Sharp everywhere.

Canon EF 100mm f/2: Sharp everywhere, maybe a tad less contrast than the 50mm.

Canon EF 40mm f/2.8: Sharp center, a little chromatic aberration moving towards the edges (chromatic aberration manifests as an unusual kind of blur in black and white images), flares easily.

Canon EF-s 24 f/2.8: Same as the 40mm, but with more vignetting (I assume from the smaller image circle).

Canon EF-s 10-18 f/4.5-5.6 IS STM: Sharp center, significant chromatic aberration outside of the center, flares easily, vignetting at 10mm (fine at 11mm f/8).

Canon EF 24-105 f/4L IS: Low contrast, less resolution than the primes, chromatic aberration towards the edges. No hotspot that I’ve seen up to f/8, in contrast to some people’s lists of good lenses. Don’t recall if it flares.

Sigma 30mm f/2.8: Sharp center, significant chromatic aberration outside of the center, flares easily, sometimes¬† I think it has better contrast compared to the Canon 50mm, but I haven’t tested thoroughly.

Rokinon 8mm f/2.8 Mk. I: Sharp everywhere, corners vignette and color-shift (a sensor characteristic of the NEX-7 with this lens) but looks great, flares easily (ugly-looking flare, too)

The only time I use the Fotodiox’s AF is with the 10-18mm; I sometimes end up too close to my subject and in an awkward position, which makes manual focus hard. It works tolerably well, but I always confirm focus on the viewfinder afterwards, because it occasionally misses. AF simply doesn’t work with some other lenses (e.g. the 100mm).

Overall: all these lenses are good, but some shine more than others. Chromatic aberration is much more of a problem in infrared than in visible light. The 50mm and 100mm are the most free of chromatic aberration in visible light at the center of their big image circles, so stand up to a 24MP 1.5x crop with no compromises. Everything else has some issue or other to be worked around.

The only concrete recommendations I have are to use the Canon 50mm instead of the 40mm, and to pass on the Sigma 30mm unless you really need AF to photograph moving people in infrared (don’t rely on adapted AF). I have read that the Sigma 60mm f/2.8 is very good…but so is the Canon 50mm, so again, I’d pass unless AF is necessary. Other than that, let your decisions be governed by your focal length needs.

About chromatic aberration, I think that a deeper infrared filter, such as the B+W 093, would reduce it. If so, it would transform the 10-18mm into a real powerhouse. Unfortunately, I don’t have one in the right filter size (67mm) to find out. This will have to be left as a loose end.

About the NEX-7: what a great camera for infrared! At ISO 200 or lower, the noise is controlled and attractive (and I usually have exposures like ISO 100, 1/250, f/5.6 anyway), it has excellent features and customizability, and its 24 megapixels are a great boon for prints. And it can be had, used, for very little money.

If you want to remove the infrared filter on your camera but don’t want to do it yourself, there are a lot of different sources to go to that will do it for you. I can recommend Isaac Szabo. He has a good reputation and does quality work. And, if you work with him, you end up supporting an artist and photographer, which is no small thing in today’s world.

Leave a Reply

Your email address will not be published. Required fields are marked *