Problems with graduated discolouring on DSLR

[McShaman]

I was doing some photography for an event a few days ago. My camera (Nikon D800) was performing as normal. When the sun started going down I was shooting in a school hall under fluorescent lights. I switched my cameras white balance to Cool white fluorescent and continued shooting and started getting some strange results.

As you can see from the following photos, in a series of shots from a quick successive burst some of the photos have a dark green graduated discolouration. I thought it could be the fluorescent lights changing colour faster than my eye could detect however I don't think this is the case for the following reasons:

1. I don't think fluoresents change colour
2. You will notice the direction of the discolouration changes with the direction of camera. i.e. In the landscape shots the direction is from the roof to the floor and in the portrait shots it is from left to right.



This suggests a camera issue to me. As you can see it looks like the camera has not metered correctly and thus the shot is darker... However that does not explain the discolouration and obviously makes correcting it in post very hard.



Here is another interesting sequence... The first shot meters correctly the second doesn't but you can see the discolouration in the lights above and then the next one its gone.

What could be causing this? Should I contact Nikon?

I was shooting in quick bursts of around about 3 to 5 shots in continuos high speed mode with my aperture fully open at 2.8 (AF-S NIKKOR 70-200mm f/2.8G ED VR II). As mentioned before the white point is set to Cool white fluorescent, metering is set to spot and ISO 4000.

 

[480sparky]

What was your shutter speed?

You may be picking up the result of lighting that is on a 50 or 60Htz cycle.

 

[McShaman]

Properly metered photos are between 1/125 and 1/160 and the under exposed shots are between 1/500 and 1/2500.

So are you saying that some lights change colour really quickly that our eye can't see it? If so where could I find out more about this?

 

[480sparky]

Properly metered photos are between 1/125 and 1/160 and the under exposed shots are between 1/500 and 1/2500.

So are you saying that some lights change colour really quickly that our eye can't see it? ....

Lights on an AC system cycle between full brightness and some level of dimming either 100 times per second (on a 50Hz system) or 120 times (on 60Hz systems). It's too fast for human eyes to detect, but a fast enough shutter speed can catch the lights in their dimmed cycle

.............If so where could I find out more about this?

Here's another thread on the same subject.

 

[McShaman]

Sounds like it could possibly be the problem... Although if you look at the image I caught of the lights, they don't actually look dimer but a different colour. And what is also interesting is that all of them in a row are the same.

 

[480sparky]

Sounds like it could possibly be the problem... Although if you look at the image I caught of the lights, they don't actually look dimer but a different colour. And what is also interesting is that all of them in a row are the same.

It a function of the cycling. When you turn out a normal incandescent lamp, it goes from yellow to orange to red as it goes out. It's so fast you can't see it, though.

 

[Gavjenks]

Yes I think it is almost certainly the lights. That's just how the room looks at 1/200th of a second.

The green blue gradient aligns with your camera because your shutter goes from top to bottom (or bottom to top) in your camera. And you were holding it sideways to take portrait oriented photos. At 1/500th it's a sliver being exposed at once, moving across the frame. So as the lights vacillate between colors, you're picking up different parts of the cycle.

The faster your shutter gets, the less overlap there is between different parts of the scene being exposed at the same time and the more pronounced the gradient.

Underexposure is probably because the brightness is also cycling, and it metered correctly for the top of the light pulse, but then the shutter happened to trip at the bottom of the pulse and can't detect or change to correct things in mid-photo.



Solutions:
1) Use a flash instead of ambient.
2) Use slow shutter speeds around 100 or less, as slow as you can with your ISO and aperture limits. This will fit multiple cycles into the photo, and thus leave it almost perfectly even looking.

 

[McShaman]

Great! Thanks for the explanations guys. It is all making sense to me now. This is what I love about photography... Always something new to learn about

 

[bratkinson]

That's why I hate shooting under florescent lights without a flash to 'override' them.

I routinely take 3 or 4 shots, of a group, for example, as fast as I can press the shutter button. Every one of them has a different hue/color/white balance to them. Fortunately, the 'secret fix' is in Lightroom. Once I find/set the white balance I want on one of them, I simply copy those settings (SYNC) to the other pictures I've shot under those lights. Of course, shooting even a different angle where some sunlight is streaming into the room requires I find a good WB all over again. And this Sunday evening, I'm back to those florescent lights again. I'm getting pretty good at fixing this in LR these days <self pat on the back>.

Alternatively, I sometimes use a flash to 'override' the florescent lights, but I still use the same WB setting methodology.