Why do high ISO shots under fluorescent lights show a shifting yellow color cast?
Asked 4/2/2014
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I tested a full-frame DSLR on a tripod in a studio scene lit by several fluorescent fixtures. The setup stayed the same for each shot, with aperture fixed at f/5.6 while the camera changed shutter speed as ISO increased from 3200 to 102,400. In the resulting JPEGs, a yellowish area appears at higher ISOs and seems to move position between frames.
Is this kind of shifting yellow cast normal at very high ISO, or does it suggest a sensor problem? Could it be caused by the lighting or by the way the test was done?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
25
I think "several fluorescent fixtures that I use to light my studio" is the key here. I'm guessing that the very high ISOs are accompanied by very short shutter speeds. Fluorescent lights cycle, and there are color variations within the cycle. Repeat your test with incandescent light or sunlight (or a strobe with high-speed sync).
See Do fluorescent lighting and shutter speed create a problem with color cast? for more.
As noted in the comments below, the top-to-bottom change is due to the way focal plane shutters work. It would be physically very difficult to uncover and re-cover the sensor evenly at high speeds, so shutters have two "curtains", and above a certain speed (somewhere around ¹⁄₂₀₀th, depending on your camera) the exposure is actually provided by the gap between these two moving across the sensor quickly — so, the whole frame isn't actually exposed at once. This can be problematic when your subject itself is moving very quickly, most famously in weird, twisted helicopter blades, but you're also seeing it here, as different parts of the frame are lit differently in different parts of the fluorescent bulbs' cycle.
See What is a rolling shutter? When do I have to be aware of it? and for more, and if you are extremely curious about the details follow up with What is the difference in purpose between a focal plane shutter and a leaf shutter on a camera? and What's the difference between vertical and horizontal shutters?
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
12y ago
0
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This is most likely a lighting/shutter interaction, not a bad sensor.
Fluorescent lights do not output steady light or perfectly constant color. They flicker with the AC power cycle, and their color can shift during that cycle. At high ISO, the camera uses much faster shutter speeds, so each frame may capture a different part of that flicker/color cycle.
The position change is also consistent with how a focal-plane shutter works at faster speeds: instead of exposing the whole sensor at once, a moving slit travels across the frame. If the light output or color is changing while that slit moves, different parts of the frame can record different brightness or color, producing a band or cast that appears to move between shots.
So this is a normal effect under fluorescent lighting, especially at high shutter speeds. To confirm, repeat the test in daylight, with incandescent light, or with flash/strobes. If the yellow cast disappears, the sensor is fine.
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