How to Analyze Lens Longitudinal Chromatic Aberration

There are two kinds of chromatic aberration. The aberration most photographers are familiar with is called lateral chromatic aberration, and that’s what causes those purple smears near the edges of the frame (such as around tree branches against the sky). Most photo editors can reduce or eliminate this defect.

This article is about longitudinal chromatic aberration (LoCA), which is caused when a lens focuses different colors of light at different distances along the lens axis. This aberration is mostly noticed by seeing bright out-of-focus subject highlights take on different colors in front of and behind the focus plane. The effects of this aberration are roughly the same all across the camera sensor. Very few photo editors can fix this defect.

LoCA picture thanks to the Wikipedia site

Near (in-focus) light: reddish fringes, far light: greenish fringes

The lights above show some color fringes due to LoCA. It’s not horrible, but you can tell it exists. The picture was shot with my Nikkor 85mm f/1.4 AF-S at f/1.4. This is a crop from a pixel-level magnification.

I used the MTFMapper program by Frans van den Bergh to perform the analysis. He provides files that you can print out, including a couple of special “focus position” charts. To shoot the focus chart, I had to set up my camera at 45 degrees to the chart, with the ‘short’ slanted-line targets nearest the camera. The chart is designed so that the natural perspective distortion will make the slanted targets look like the same height when the short slanted targets are nearer to the camera than the tall slanted targets. Frans provides two styles of focus position charts; here, I chose the chart with a single row of heavy slanted lines.

When photographing a ‘focus position’ chart, it’s not critical that you nail focus in the center of the chart to analyze LoCA. It’s only the relative peak focus between the red, green, and blue channels that’s important.

In the program Settings|Preferences, I set the ‘Threshold’ value to 2.0 and I set the Bayer color channel to red, green, and finally blue in three separate analysis runs of the Focus Position chart. I used raw-format photos of the focus chart. The MTFMapper program doesn’t yet understand my Nikon Z9 ‘HE’ format, since it uses LibRaw. I have to convert them into DNG format using Adobe’s free converter.

Set Threshold and Bayer channel then Open images

For the tests, I used both my Nikkor 85mm f/1.4 AF-S and my TTArtisan 50mm f/0.95 lens on my Nikon Z9 camera. I picked these lenses, because I know that they have noticeable longitudinal chromatic aberration (LoCA). Later, I'll compare them to a lens with very little LoCA.

I photographed the chart at a few different apertures, to see how the aperture would affect the LoCA.

TTArtisan 50mm at f/0.95 R,G,B Bayer channels

The TTArtisan 50mm f/0.95 lens shows the red Bayer channel focusing farther from the camera (left) than the green and blue channels. The plots are shown with R then G then B left-to-right.

The same TTArtisan 50mm stopped down to f/1.4 again shows the red channel focusing farthest from the camera. The green and blue channels are about the same.

TTArtisan 50mm at f/2.8 R,G,B Bayer channels

At f/2.8, the TTArtisan shows the same focus shift pattern as the other apertures. The plots are shown with R then G then B left-to-right. The focus separation between red and green here is 15.7mm.

The focus separation distances depend upon the chart size. If you stick with the same chart, you can directly compare different lenses.

TTArtisan 50mm at f/2.8 red channel close up

When I zoom in on the plot, you can see that the actual focus offset from the vertical dashed line is provided (-21.1mm here). It also includes the peak MTF50 resolution (0.197 cycles per pixel here).

Nikkor 85mm f/1.4 at f/2.8 R,G,B Bayer channels

The Nikkor 85mm f/1.4 lens tests showed very similar behavior to the TTArtisan 50mm lens at the equivalent apertures. The f/2.8 plots above, for example, are a near twin to the TTArtisan plots at f/2.8. The focus separation between red and green here is 17.8mm, versus the TTArtisan’s 15.7mm at the same aperture.

Looking at the red channel focus peak, it’s significantly farther away from the camera than the blue and green channels. This is classic longitudinal chromatic aberration.

The aperture setting has very little effect on the LoCA of either lens.

No channel (luminance), 85mm f/1.4 lens at f/2.8

When no color channel is selected in the Preferences, the plot looks about the same as the ‘Green’ channel does. This is effectively what the camera is using for focus.

Nikkor 24-120 f/4 S at 85mm f/4 Red and Green channels

Shown above is a lens with very little LoCA. The focus separation between red and green here is only 4.8mm.

Many thanks to Frans van den Bergh for his terrific MTFMapper program. It’s very effective for showing how a lens focuses different colors at different distances.