A7R problem or a common lighting problem with digital camera?

Hi all,

In the attached photos, my A7R has done something strange.

I was playing around with the camera, in the environment of fluroscent light and white subject, I see a brown zone. This happens when aperture is large and ISO is low. as I start to adjust the focus, the brown zone area shifts up and down to a different location.

However, If I smaller aperture and higher ISO, the brown zone starts to go away at F4. I have tried on 3 lenses (2 MF RF and FE55/1.8). same






Problems with A7R? first photo




Problems with A7R?2nd photo

thank you for your time!

Warm regards,

The light in a fluorescent tube fluctuates back and forth at 1/60 second, so (like television and computer monitors) if you shoot at a higher shutter speed than that you'll catch the light mid-cycle and get weird color fluctuations in your shot. Science!

Beauty of stop motion photography. You are catching the fluorescent light cycle as indicated.

Fluorescent lights flicker at twice the electrical frequency of 50 or 60 Hz (so they flicker at 100 or 120 Hz).

Now we look at shutter speed, 1/50 or 1/60 would catch two cycles of the fluorescent, and the shutter would be entirely open for most of that time. This averages out the white balance shift from the lights over time.

Take 1/160 of a second, this is less than one cycle of the fluorescent light, we are only capturing a partial cycle. The shutter speed of 1/160 of a second is special as well since it is the max sync speed of A7r, fastest speed where shutter is all the way open for part of the exposure, faster than this and you get a moving slit down the face of the sensor.

So at 1/160 the shutter is open all at once, but a good percentage of the exposure is taken with the shutter partway open or closed. This means you will start seeing a shifting white balance as the fluorescent light flickers. Each part of the sensor captures a different part of the light's cycle.

You really notice this around 1/160 and faster since that is the max sync speed of the camera. The top half of the sensor is exposed during one part of the light cycle and the bottom half during another.

Tungsten lights don't have this issue as they have very stable white balance, they are hot elements which don't change temperature much during different parts of the cycle.

Eric

I get this strongly with the NEX-5N when photographing a lightbox that uses fluorescent tubes.

It is likely to happen on any digital camera, and likely on any film camera with a focal plane shutter.