My Friend Flicker

Q I’ve been experimenting with shooting time-lapse. My success rate has been pretty good, but there has been a lot of trial and error (emphasis on the errors). There have been some obvious issues that I’ve fixed, like not having my camera as secure as I’d like and making sure something isn’t going to come in and block the shot (two hours of my life that I’d like to get back). But there’s one issue that hasn’t been fixed: Exposure. I’m not talking about trying to change exposure over time, like with a sunrise or sunset; I’m trying to get plain, middle-of-the-day exposure right. I understand light levels change outside, but I’ve had problems even inside. The exposure isn’t consistent. I’m left with flicker. What’s a solution?
P.K.
Via email

A Flicker in time-lapse shots is pretty common. If you aren’t using manual exposure, your flicker could be caused by tiny light level changes in the scene where the camera is metering.

Even if you use spot metering, the area that’s being measured is still rather large and can induce minute exposure changes. If you use any of the camera’s other automatic metering modes, the area measured is even larger, and averages or more sophisticated algorithms are used to calculate “proper” exposure. If, as I assume, you’re using manual exposure, then the flicker issue is all about aperture.

When your camera goes through the process of capturing an image, one of the steps is to close down the diaphragm in the lens—the aperture—in order to control the amount of light passing through to the image sensor when the shutter opens.

Why isn’t the aperture already at the right aperture setting? Why, if you set it for ƒ/11, isn’t it at ƒ/11? If the aperture in the lens already was stopped down to ƒ/11, it would make the image in the viewfinder or the LCD very dim. That makes composition, and especially focus, very hard to do.

Don’t believe me? Find the depth-of-field preview button on your camera and press it while looking through the viewfinder with an aperture of ƒ/11 (or smaller) and tell me what you see, or what you can’t see. The image will be pretty dark.

Instead of stopping down to the preset aperture, the camera is designed to always have the lens be wide open until you take the shot. Unfortunately, closing down to that specific aperture isn’t done with the accuracy needed for time-lapse. Arguably, the tolerance is acceptable for regular still photography—you never noticed it before.

Although you might not notice a variance of aperture from shot to shot with stills, that’s not the case with time-lapse. Minute changes in exposure across hundreds or thousands of images show up as flicker. And the smaller the aperture, the more obvious the flicker will be.

There are software solutions to correct this issue in post, but there are also things you can do to prevent it in the first place. First, if you shoot wide open, the camera shouldn’t have to adjust the aperture. Second, you could use a manual-aperture lens. Depending on your make of camera, this might involve scouring around for used lenses or getting a lens adapter that will allow you to use another brand of lens.

Lastly, there’s one technique that, done carefully, will allow you to use your current setup. But I repeat: It has to be done with absolute care or it could damage your camera.

Remember that depth-of-field preview button I talked about? You can use that to stop down the lens and keep it stopped down for the duration of the time-lapse. The only extra equipment you’ll need is scissors and paper.

Before you get out into the field with your setup, take off your lens and look at the electrical contacts on the body of the camera that are used to control the lens. Now, take the scissors and cut a small piece of paper that will just cover those contacts without blocking the image area.

I suggest using scissors because simply ripping the paper will expose the tiny threads of the paper pulp and might create dust. Some people use tape instead of paper, but the chance for leaving adhesive residue on the contacts makes me stay away from this approach. In a pinch, however, I’ve used a portion of a Sticky Note.

Here’s what to do in the field: First determine what your aperture should be, set the camera to manual-exposure control, and then set your aperture. Speaking of manual, everything should be in manual—ISO, focus and white balance should be manual or on a preset.

Since the next step involves taking off your lens, make sure you’re in as controlled an environment as possible. Dirt on the sensor can be just as bad as flicker. If I’m outside, once I’ve determined exposure, I’ll take my camera inside or into a car.

Now, press and hold the depth-of-field preview button to force the camera to set the lens aperture. Confirm this by noticing that the image dims in the viewfinder. With the depth-of-field preview button still depressed, press the lens release button and take off the lens.

By removing the lens with the depth-of-field preview pressed, you’ve mechanically set the lens to that aperture. To keep it that way, gently place the paper you prepared at the beginning of this process over the camera body contacts and carefully mount the lens.

If done correctly, you should now see that dimmed image in the viewfinder without having to press the depth-of-field preview button. If it doesn’t look darker, take off the lens and try it again.

Of course, with a darker image, it will be trickier to set composition and focus. But with live view or just taking sample shots, you should be able to overcome that issue.

When you’re through, remember to remove the paper right away. If you don’t, you might forget it’s there. Then, when you remove the lens the next time, you might forget to use the right technique and the paper may fall into the camera. And what’s the right technique? Let gravity do its thing: Position the camera with the lens facing down and remove the lens. The paper should fall right out.

MAKING THE GRADE

Last column, I answered a question from someone who was turning over his project to a colorist for color grading, but was unsure of how to prepare for the process. I got through making sure the colorist had footage in good order to work with. But there are two more steps before the fun begins.

As I mentioned last time, I’m not a colorist. Over the years, however, I’ve had the opportunity to talk with several about the process and what people can do to make for successful grades once you’ve verified that the colorist has the right files to work with. You should also make sure they have the right information about the footage. If it’s not obvious from the file format, you need to be clear about how the project was shot. That includes the camera or cameras used and the modes the cameras were in during capture.

Were you working with a specific camera profile, shooting RAW or Log, or something else? If Log, which Log? Some cameras have multiple options for Log. Were you using a camera that uses an image-processing or picture profile instead of a full camera profile?

If you used a viewing LUT on set, what was that LUT? Even better, bring it along. By giving the colorist the LUT, you’re not saying, “This is what I want” (although you could); instead, you’re telling the colorist what you saw when you shot. This will give them an idea of expectations.

Finally, you need to talk about what the next step is. That next step will vary with the project, with the director and with the colorist, but it’s critical that the two of you be clear about how to proceed.

For example, the colorist might want to start with an overall discussion about your film, maybe highlighting key ideas that you had in mind where color might have the greatest impact for the narrative. And then the colorist might want you to go away for a while so they can immerse themselves in your project.

This might result in a first pass at a grade where you come in and collaborate on the final pass. With this method, you gain the interpretation of someone who’s looking at things with a fresh eye.

On the other hand, if there’s a lot to do with a limited amount of time or a tight schedule, the colorist might want you there at every step of the way, coming to quick agreements on the look of scenes.

Those are just two examples of how the process might proceed. What’s most important—as colorists have told me—is that this conversation is the key to a great result and a great experience.

To have your video production technical questions answered, send an email to [email protected]

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