Rudy Winston
Rudy Winston

Rudy Winston has over 16 years experience with Canon USA's Pro Products team, and has been responsible during that time for training Canon's staff on new products, creating presentations for customers and dealers, numerous writing projects, and providing technical assistance to professional and amateur photographers.

Shooting at different light frequencies

October 01, 2013

At first glance, it seemed the camera had developed a mind of its own. Consecutive shots, taken in a single burst, were showing not only differences in exposure, but overall color as well. Even worse, some seemed to show an obvious transition of color and exposure, from one side of the frame to the other. What the heck was going on?

Two images, same scene, same subjects, same exposure (Manual exposure mode), taken a fraction of a second apart as part of a fast action sequence — yet these unretouched images appear very different. Is something wrong with the camera? Read on...

Many of us have been through this when shooting images with available light at indoor locations. And because it doesn’t happen all the time, it’s easy to assume the first time you encounter it that the problem is something like an erratic shutter in your camera. If you observe this under artificial light, in all probability, it’s not a problem with your camera at all. The problem is with the lighting in the environment you’re in. And nothing short of changing the light fixtures will completely eliminate this problem.

Why is this happening?

The problem is that the lights in the room, office, arena or stadium are flickering and wired together in such a way that their on-off-on cycles are occurring nearly simultaneously. Unlike sunlight or conventional household tungsten illumination, many other types of artificial light don’t illuminate continuously. Instead, fluorescent, mercury-vapor, sodium-vapor and other artificial light sources flicker, usually at twice the frequency of the power supplied by the ballast in the light fixture. Our eyes rarely see artificial lights flickering at 60 or 120 times per second, but what’s happening is the lights are literally turning off and on at that rate. And depending on how the lighting in the venue is wired, if the camera captures a scene at the instant where the flicker is at or near the “off” end of a cycle, the light it captures is a lot less than it would be at its peak.

It’s far beyond our scope to get into the technical reasons why some types of artificial lighting behave this way. But once you’ve encountered this, you’ll want to know how to work around it.

Identifying the problem

The first step is realizing that you may be running into this in the first place. Again, with a properly functioning camera, it’ll never happen in daylight — sunlight is a continuous, non-flickering source of illumination. But I’ve seen this happen under artificial lighting in schoolrooms and offices, and especially in sporting venues — gyms, arenas and so on. Even in camera stores.

Someone who’s not an expert electrician will likely not be able to tell whether a venue is likely to present this problem or not. Normally, you first notice it when playing back the shots you’ve taken, and two or more pictures of the same scene show a visible difference in exposure and/or white balance on your LCD monitor — such as the ice hockey sequence illustrated earlier in this blog post.

Keep in mind that I’m not talking about simple white balance shifts in fluorescent or artificial lighting… what we’re speaking of here starts with shifts in exposure, when two or more shots are taken and appear visually different when compared to each other.

White Balance issues are common under many artificial light sources, but usually appear because of the character of light they give off, not from the on-off flickering of the lights. White Balance issues can often be corrected either in-camera or afterward in the computer, but problems from flickering aren’t as simple to correct.

So even before your actual event begins, if possible, it’s a good idea to shoot a quick sequence, at a fast frames-per-second rate, of some area with light (a wall, the floor, etc.) at the shutter speed, ISO and aperture you intend to work at and play them back on your camera’s LCD monitor. Your histogram display — including the R-G-B histogram — can be a big asset here, alerting you to unexpected changes occurring from one frame to the next. Obviously, no two histograms are likely to be perfectly identical, even under the best of circumstances. But the histogram can back-up any deviation you may notice with your naked eye as you scroll through a few quick images of the same scene, composed the same way. A continuous sequence of five or six quick images should appear virtually identical, if taken in a burst with no changes in composition or camera settings. If they do, you’re probably good to go. If exposure and overall color seem different, the problem most likely is the lighting in the venue you’re shooting in.

Lighting is one gremlin; fast shutter speeds are the other

One reason this problem of erratic, frame-to-frame changes doesn’t always appear may be the shutter speed you’re using for your pictures. The faster the shutter speed is, the more likely you are to encounter this problem in certain artificially lit environments.

A fast speed, like 1/500th or 1/1000th of a second, is far more likely to capture the instant the room or arena lighting is in the midst of an “off” part of a cycle than a modest speed of 1/60th or so. Obviously, if shooting sports or action pictures, we don’t have the luxury of dropping shutter speeds to 1/60th or 1/30th of a second. But if you’re shooting controlled subjects and you can do this, it’s one way around the problem of light variations from shot to shot caused by the flicker of artificial lights.

Are there any workarounds if I do discover this is happening?

I wish I could tell you to activate a custom function and it would clear this problem up, but unfortunately, that’s not the case. If you see this happening, the lights in the venue are flashing on and off at a probable rate of 60 to 120 times per second and as mentioned previously, if a shot is taken at a faster shutter speed (like 1/1000th of a second) and it happens to catch the “off” instant of the on-off cycle, the camera’s imaging sensor sees a much darker scene at that moment and captures a darker image. If you’re at 6 fps with a camera like an EOS 5D Mark III, the next shot you take 1/6th of a second later may happen to catch a peak moment of brightness and appear OK.

There’s no way that in-camera automatic exposure, or Auto White Balance, can react to such rapid changes in lighting and shift how each image is recorded. So if you’re forced to work at fast shutter speeds — as you might be for a sporting event — a simple change of exposure mode won’t eliminate this issue.

There are a few things you can do in-camera to mitigate this at least a little bit. The biggest safety net comes later, however, during post-processing at your computer. In-camera workarounds include:

  • Shoot at slower shutter speeds: Not always possible or practical, but if you can safely back speeds off to about 1/60th or even a little slower, that will usually allow each frame to capture the entire on-off cycle of the artificial lights in the venue you’re in — tending to make each shot appear consistent in exposure and white balance.
  • Over-expose slightly: The keyword here is “slightly.” By giving perhaps 1/3 stop or so more exposure than you normally would, you provide at least a slight cushion for any shots that appear darker due to unavoidable flickering of the room or venue lighting. Obviously, don’t over-expose so much that important highlights begin to wash out!
  • Shoot RAW: This is the big one. Even if you normally use a JPEG workflow, you need the full flexibility in post-processing to make adjustments to scene brightness and white balance to bring any of the darker images that you shoot back to “normal.” RAW images will save the day here. JPEGs will probably have you tearing your hair out because of the more limited range of computer adjustment that JPEG files will tolerate.
  • Avoid available light and shoot with flash: Again, not always a realistic option. But if you are close enough to your subjects that using a Speedlite is possible — even if not your first choice — flash-only exposures may be far more consistent and far easier to work with than available light shots in a venue where you’ve seen erratic shot-to-shot results when shooting with available light only. Slow-sync flash pictures would work here because at their inherently slower shutter speeds, artificial indoor lighting will tend to appear more natural from one shot to the next.
How can I be sure it’s not a camera problem?

There’s an easy way to tell if sudden frame-to-frame shifts in exposure and/or white balance are the result of flickering artificial lights or evidence of some sort of in-camera problem (a shutter that’s dragging, problems with a lens’ aperture stopping-down consistently, a sensor that’s not recording properly for some reason, and so on). Step outside in daylight (it can be overcast, but be sure there’s no artificial light immediately nearby that’s contributing to exposure) and shoot a fast sequence of shots at a fast shutter speed, such as 1/1000th or 1/2000th of a second. Use high ISOs if you need to and shoot at your camera’s fastest continuous fps rate. Shoot something as evenly toned and preferably relatively light in overall tone/color, as possible.

Now, play your images back. Even if you encountered something like the two ice hockey images earlier in this article when shooting indoors, with a healthy camera, your outdoor shots will be fine. This is clear evidence that lighting caused the problem when shooting indoors, not the camera.

The bottom line

There’s no single way to guarantee a location shooter won’t encounter this, but it’s easy to shoot a few quick test shots before an event starts to see if it’s likely to happen. Don’t trust your eyes when you just look at the scene itself; shoot test shots at the speed/aperture/ISO you plan to work at. Many modern venues will never exhibit this problem, part of the reason it’s a rare occurrence for most photographers (again, it’ll never happen in sunlight, or under tungsten illumination). But it’s not unheard of, for instance, for a press shooter who normally photographs professional or major college sports to suddenly encounter this if he or she photographs an event at a smaller high school venue.

Understand that if you do see suddenly inconsistent exposure or white balance, you can’t eliminate it with a single camera setting, but there are ways to work around it and get the results you need. Learn from the experience and take even better pictures as a result!

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