White balance shift--anyone noticed?

[Jared]

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Is it just me? Or does the color cast on some lenses change as you stop them down?

 

Here is an example of a shot taken with a 75mm Summicron Apochromat wide open and at f/5.6. The white balance was set manually in Lightroom using the exact same temperature and tint settings for both. The images were taken only a couple seconds apart, so I know the lighting didn't shift. Both images are cropped to, roughly, the central ⅔ of the original image.

 

The image taken at f/5.6 has much stronger magenta tones than the image taken at f/2. I see the same thing with my 50mm Summilux Asphere (though to a lesser extent--mostly the Summilux is just a bit warmer than my other lenses at all apertures).

 

Has anyone else noticed a shifting color cast with aperture? It's not a big deal for my photography since I tend to leave white balance set to automatic in the camera and adjust in software after the fact using RAW files only, but it surprised me nevertheless.

 

I don't recall seeing this mentioned in this or other forums. If it makes a difference, RAW conversion was done using Lightroom 4.4 using the embedded profile (my preference over the Adobe Standard profile which, on average, produces less consistent results). No sharpening or any other adjustments aside from cropping to the frame size and applying the same white balance settings to both images.

 

- Jared

 

p.s. Obviously these aren't intended to be interesting photographs--I'm just trying to learn the characteristics of my new 75mm lens and was shooting a flat subject to eliminate the effects of depth of field--was trying to get an idea of field curvature, optimum f/ stop, etc. and was surprised to see a difference in color.

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[StephenPatterson]

Does your room perhaps have some of those neat little fluorescent light bulbs? This looks more like your light source cycling to me. Try slowing down your shutter speed to less than 1/60th sec. as US A/C power cycles at 60Hz (I believe Europe is 50Hz).

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[jaapv]

I see a difference in exposure rather than WB.

[Jared]

Does your room perhaps have some of those neat little fluorescent light bulbs? This looks more like your light source cycling to me. Try slowing down your shutter speed to less than 1/60th sec. as US A/C power cycles at 60Hz (I believe Europe is 50Hz).

 

I think you may be right about the light source. I was using a halogen bulb not a fluorescent light, so was not expecting a shift since it should just act as a blackbody radiator, but when I duplicated the test this morning using daylight there was no shift in color balance.

 

As Jaap noted, there is also a slight difference in exposure (about 0.2 stops), but that was not what I was referring to. In my posted examples there is definitely more magenta in the f/5.6 image, even if I correct for the difference in exposure and vignetting wide open. Looks like it was somehow due to the lighting, though, not the lens or the camera.

 

All, please ignore my previous post.

 

Thanks for the responses.

 

- Jared

Edited February 26, 2014 by Jared

[Overgaard]

Well, the light is actually different in the room. Look at the frame's upper left corner. If you put both in Photoshop, you can see an overall change of the light, booth direction and intensity.

 

Apart from that it would not be completely unlikely that a lens would behave differently )slightly though) on different apertures.