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.

Black and white

I’ve been combing through my archives recently, trying to make some kind of organization and sense out of my photography over the last decade, and I couldn’t help but reflect that, from the very beginning, almost every single photograph I’ve taken for art’s sake was meant to be in black and white. I actually remember when I was first beginning with digital photography: I was looking at a photo, wondering what it’s reason for being was—there was something about it that was important, but I couldn’t put my finger on it—and it struck me that I need only click to convert it to black and white, and it would suddenly make sense.  It was meant to be that way.

…and if I went through previous photos that had something about them, in black and white, they too made sense.

Black and white prints have a singular beauty and richness to them. They also have the heavy weight of tradition behind them. And using traditional black and white film, as I often have, also imparts a special look. But none of that is why my work is in black and white. Those reasons are choices. But I feel that I truly have no choice in the matter. My ideas sing in black and white. They demand it. I don’t have any control over them; I can only acquiesce.

Some artists talk about their vision for a particular photograph, as if they can bend the image to their own will, making it conform to how they would like it to be. I confess that I may not understand what kind of experience they have. I raise up my camera, and the photo takes itself. And the photo is in black and white. There may be colors, but those are just insubstantial threads covering up what wants to show itself. I can choose to keep it covered up, but I can’t choose to make it a color image underneath.

There’s a game I sometimes play with myself. Working with a digital camera, I like to view my images in color at first—they can be so mysterious to me. Why did I take that image? What was I doing? What possessed me? But I press the button to desaturate them, and, however primitive the first rendition is, there it is…and I understand. I feel like a child, amusing myself by turning off the lights just so I can turn them back on and see again.

I have occasionally taken a color photo or two. I do treasure them, but I don’t really know what to do with them. It’s as if a chicken one day lays an egg that hatches a lizard. She loves all her children, but, how should she mother a lizard?

I have some ideas for studio images that could well be in color. I think that the difference between a found scene, out in the world, and one that I bring into existence, is that in the latter case I would start from what can be, and bend what is until the two meet. But all the fine art work in my archives is found—all starting from what is—and it doesn’t need to be bent any more than a chicken needs to become a lizard.

While all this is true of my photographs considered by themselves, collections of photographs are another matter entirely. I do put those together, very deliberately. That’s why combing through my archives, trying to organize and make sense of what I have, has taken so long. But I’ll say more about that process in the future.

Where do the false colors in digital infrared come from? And other questions.

I’ve been wondering why digital infrared images have the colors that they do. That’s a very broad thing to wonder about. So, here are four specific observations that have been on my mind:

  1. Digital infrared images (at 720nm cutoff) are red straight out of the camera
  2. Digital infrared images (at 720nm cutoff) take on interesting differential hues once color-balanced
  3. Digital infrared images (with Wratten 87c / 093) have less R, more B, and less G compared to images with a 720nm cutoff filter
  4. Digital infrared images (with Wratten 87c / 093) are effectively monochromatic

Why? Why? Why? And, why?

I’ve read a lot of discussions about infrared photography and have never come across a thorough answer to all of these questions. But, I stumbled upon an interesting piece of information which may explain three of these four observations. I’m sharing it in case anyone else is also puzzling it over.

Take a look at Figure 2(c) in this paper. It has the responsiveness of one digital camera to different wavelengths of light. (I’ll assume that all cameras are pretty much similar.)

For observation 1. The thing to notice is the responsiveness of R to infrared vs. visible red light near either side of the boundary between infrared and visible light–effectively the same. And the responsiveness of R there far surpasses G and B anywhere in the infrared spectrum, allowing R to dominate the image’s characteristics. This means that

  • if your filter cuts off at 720nm, the whole image without adjustments will be dominated by red
  • if your filter leaks a little visible red light, the whole image without adjustments will also be dominated by red

Whether or not your filter leaks visible light, the result is approximately the same (except when something has a different response to red and infrared).

For observation 2. Notice that R’s responsiveness is decreasing with increases in the wavelength, whereas G and B climb for awhile, and decrease only later. This means approximately that

  • if you color balance so that some particular object is neutral, anything that reflects relatively more low-frequency infrared than that object will have a reddish hue, and anything that reflects relatively more high-frequency infrared than that object will have a non-reddish hue

A filter doesn’t need to leak visible light to produce different hues; a relative preponderance of low-frequency or high-frequency infrared, with a camera with the sensitivities in Figure 2(c), will do it. It only requires R’s sensitivity to move in one direction while G and B’s move differently.

For observation 4. Once you get out to about 850nm, the responsiveness is the same for R, G, and B. So no matter how you color balance with a filter around this cutoff, you change only the overall hue of the image, without differentiation.

Now, I’m left with observation 3. Unfortunately I still can’t explain it! Figure 2(c) seems to predict that, with a deep cutoff filter, which the Wratten 87c / 093 should approximate, the image should be neutral gray. Instead it’s dominated by R, but not as thoroughly as with lesser filters (maybe some light passing below 850nm), with more B than G. Why? Maybe the diminished G is just due to the way that cameras deal with the extra G pixels in the Bayer array. But that’s just a wild, speculative guess.

 

 

Welcome!

To the reader, whoever you are: welcome! I hope to get to know you at some point. And I hope you are interested enough in my work to want to get to know me. I’ll be posting on this blog from time to time about various things; I think it will gradually provide a little insight into who I am, my interests, and so forth, to give some perspective on my photography.