Lens Chromatic Aberration in IR Versus Visible Light

Information about how camera lenses perform when photographing in the infrared is quite scarce. Lens designers usually ignore light outside of the visible spectrum in their designs.

I have been analyzing a lens that is well-corrected in visible light to see what happens in a couple of different wavelength regions of infrared. I already looked at what happens with longitudinal chromatic aberration, and next I’m looking at lateral chromatic aberration (CA).

I had a camera converted to infrared by Kolari Vision, which includes an infrared anti-reflection coating on their sensor cover (very helpful for controlling hotspots). This modification passes wavelengths above 590nm, which then includes red/orange plus infrared. This light range lets me create “color infrared” or pure black-and-white images.

I used a separate camera (a Nikon Z8) to take photos in regular visible light using the same lens that I used on the infrared camera.

Lateral CA when using visible-spectrum light

As you can see above, the colors in white light that we perceive can get bent going through a lens by different amounts when CA is present. Infrared (all IR frequencies lumped together) may or may not get similarly split up by the lens. CA typically doesn’t happen in the center of the camera sensor, but it gets progressively worse toward the edge of the sensor.

I frequently put an 850nm infrared filter on my lenses when shooting infrared with my IR camera, so I thought it would be prudent to include an analysis using that portion of the spectrum, too. This filter only passes long-wavelength IR, so the photographs look like black-and-white.

I use the MTFMapper program to look at CA. The author Frans van den Bergh provides a couple of different files that can be printed out for analyzing CA. The standard resolution test charts can be used for also measuring CA.

I printed the resolution test chart using a laser printer. If I had tried using an inkjet printer, then the printout would have been invisible when shooting infrared. The carbon black toner used in laser printers is opaque to infrared, while the ink in inkjet printers is mostly transparent to infrared.

I chose my Sigma 70-200mm f/2.8 Sport zoom for the testing. This lens has a pretty low level of CA, but that ‘low level’ is only advertised for visible light.

A lens with zero CA would have the red, green, and blue light get bent identically, and therefore all light from a subject focuses at the same spot. I focused the lens on the chart center.

Resolution chart corner, no CA correction, 200mm f/2.8

The shot above shows a piece of a resolution chart without any correction for lateral CA. There’s a small amount of visible CA (meridional direction), which gets worse away from the lens center. This shot was done in visible light.

CA range for visible light

The CA measurements for the lens in visible light range from about +2 to +8 microns in red-green sensor pixel shift, and -2.5 to +1.5 microns in blue-green pixel shift.

590nm Infrared 200mm f/2.8

In the test shot above, I switched to 590nm infrared, where ‘color’ doesn’t really have a specific meaning. This camera lets in orange, red, and infrared light. The shot above has been color-balanced to make the target background white; if left alone, the shot would look like it had a deep orange filter over the lens.

For measuring CA, I give the MTFMapper program the raw shot as-is and don’t do any kind of color balancing.

CA range for 590nm infrared

The CA for the 590nm infrared shot drops quite a bit, where now the red-green pixel shift is about 0 to -1.5 microns. The blue-green pixel shift is about 0 to +2 microns.

Next, I put the 850nm infrared filter over the lens…

850nm Infrared 200mm f/2.8

I didn’t need to perform a white balance with this type of deep infrared, since it normally looks like pure black-and-white.

CA range for 850nm infrared

The CA for the 850nm infrared shot drops to near-zero for both red-green and blue-green shift. This is the lowest level of CA I have ever seen.

Summary

Lens behavior in normal light doesn’t guarantee anything when it’s used to shoot in infrared. You’ll have to do that sort of testing yourself. It's interesting that this lens looks better-corrected in infrared than it does in visible light for lateral chromatic aberration.