Lens Resolution: Red Green Blue Comparison

Lenses don’t focus all colors of light the same way. When a lens gets its focus calibrated on a particular camera, there are a lot of compromises taking place.

If a lens has longitudinal (axial) chromatic aberrations, then it focuses different colors of light at different distances from the sensor. I made an article on the topic here:

When you perform lens focus calibration, there’s a good chance that the calibration is biased toward a particular color. Where you might notice this bias more is when you’re shooting under different lighting conditions. Shots might look a bit fuzzy, even when you’re certain that you nailed calibration.

I wanted to show you a way to measure the magnitude of the color/focus error on resolution. I’m using my usual resolution measurement software: MTFMapper.

I picked my little nifty-fifty for these tests (Nikkor 50mm f/1.8 AF-D). This is a cheap, but sharp lens. It’s not without its share of flaws, however. One of those flaws is axial chromatic aberration.

Normal MTF50 resolution plot

The MTF50 resolution plot above shows what “all” sensor Bayer color channels combined provide. The peak resolution is about 34 lp/mm. This lens is showing its age in many ways, particularly in corner resolution. To be fair, the lens was shot wide open; it looks much better when stopped down. Give this lens credit, though; it's dirt cheap.

Now, let’s explore what happens when we isolate sensor color channels.

Sensor channel color selection in MTFMapper

For the following MTF50 resolution plots, I changed from the default “none” Bayer channel selection to explore what red, green, and blue will produce.

Red channel MTF50

Green channel MTF50

Blue channel MTF50

All of the plots shown above come from the same single photograph. The peak resolution ranges from 30 lp/mm for the blue channel to 34 lp/mm for red, and finally 37 lp/mm for green.

If you were to photograph something with a lot of blue in it, you might end up thinking that you slightly missed focus. The resolution drops 19 percent from what the green channel sees.

Summary

It’s unfortunate that nobody seems to mention axial chromatic aberration specifications or measurements when reviewing lenses. This lens issue can have a very large effect on resolution. As always, life is complicated.