Color Shift in Out of Focus Areas

20150908 211006 DSCF6041 XT1 canon 50mm f1.8_DCE 20150908 211006 DSCF6041 XT1 canon 50mm f1.8_Crop_DCE

So I was playing around with my Canon 50mm f/1.8 attached to my Fuji X-T1 at the end of a long day (love that crazy narrow focus area and dreamy bokeh at f/1.8) and I notice something interesting.

20150908 214653 DSCF6052 XT1 canon 50mm f1.8 incandescent_DCE 20150908 214653 DSCF6052 XT1 canon 50mm f1.8 incandescent_Crop_DCE

Black text was showing chromatic shift in front of and behind the focus point (at the center of the sensor). (It is not limited to text — you can also see it in the remote control button at the top.) So, of course, I had to do some testing. I designed this text target:

20150908 230038 DSCF6055 XT1 canon 50mm f1.8 incandescent_DCE

Then shot it under different light sources and angles.

20150908 230132 DSCF6056 XT1 canon 50mm f1.8 incandescent_Crop_DCE 20150908 230546 DSCF6058 XT1 canon 50mm f1.8 incandescent_Crop_DCE 20150909 093640 DSCF6060 XT1 canon 50mm f1.8 incandescent office_Crop_DCE 20150909 093707 DSCF6061 XT1 canon 50mm f1.8 incandescent office and fluorescent_Crop_DCE

The foreground text is consistently slightly warmer (redder) and the background text slightly cooler (greener).

The Fuji lens also shows this color shift.

20150908 230725 DSCF6059 XT1 fuji 18-55 at 55 incandescent_Crop_DCE 20150910 200046 DSCF6177 XT1 fuji wb-incan_Crop_DCE

As well as the Canon lens on a Canon 7D.

20150910 193924 IMG_9584 t2i wb-incand_Crop_DCE 20150910 202210 IMG_9589 t2i wb-incand_Crop_DCE

So this effect appears reproducible in multiple cameras and lenses from multiple manufacturers in various lighting situations.

So what is it???

  • Doppler Shift?
  • Chromatic aberration in the lens? (unlikely?)
    • Which usually affects high contrast transitions at the edges of the lens and this is usually one color (purple)
    • This is dead in the middle and different between foreground and background
  • or something else?

I’ve searched all over Google for information, but no luck. Here are some search terms in hopes that someone is also searching Google:

  • Black text looks warmer or cooler, redder and cooler, bluer, greener
  • Out of focus text looks warmer or cooler, redder and cooler, bluer, greener
  • Foreground text warmer color
  • Background text cooler color

If anyone knows, I would love to hear from you.

I know this is probably easily fixed with software*, but I’m trying to do as much in-camera as I can.

*I was a professional graphic designer for years and know Photoshop intimately. I’ve also used a number of photo workflow programs for color correction. I can easily fix this (mostly with just white balance correction), but I will always know that something interesting is underlying the surface, and I want to know why it is there. Call me curious.

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