Setting exposure for video recording

Sunday 28 January 2024, 14:07 by Nando Harmsen | 3,679 views | 6 comments

This article was translated from Dutch with the help of AI. Read the original.

You know that good exposure is achieved by choosing the right combination of shutter speed, aperture and ISO. You have some freedom when doing this. Video is different, because the choice of these settings determines what your video will look like.

It is simple for photography. You choose an ISO value and an aperture, and the required shutter speed for a good exposure follows from that. Or you choose an ISO value and shutter speed, from which the required aperture follows.

Usually, you can also use the PSAM settings for video. It is still always better to film video in M mode.
You have plenty of freedom to choose a fast shutter speed, slow shutter speed, large depth of field or small depth of field. This is different for videography, because the shutter speed determines the appearance of the moving image.

First things first: framerate

First of all, it is important to know something about the framerate. Video consists of a large number of images per second shown quickly one after another. This creates the illusion of a moving image.

The more images are shown per second, the smoother the image becomes. Modern cameras can record up to 50 images per second in PAL format, or 60 images per second in NTSC format. Since we use PAL format in Europe, I will ignore the framerates for NTSC format for convenience.

Choose the framerate of your video according to your own preference. This is often indicated as 50p, 25p and so on. The other number, with the M after it, is the recording quality. The higher the number, the more storage space is needed.
You may know that some cameras can also record 100 images per second, and sometimes even 200. This is intended for slow motion, which is then displayed at 50 images per second, or even 25 images per second.

Although 50 images per second looks beautiful, it can be less pleasant to watch. It is considered too clinical, although this is of course also a matter of taste.

For a cinematic image that shows beautiful, smooth movement, there should be motion blur. This means that for this type of image it is better to choose a framerate of 25 images per second.

The 180-degree shutter

To show smooth movement in video, the shutter speed is extremely important. It does not matter whether you use 25, 50 or even 100 images per second. You need to match the shutter speed to the framerate.

The rule of thumb is: the shutter speed should be 2x the framerate. This is 1/50 sec for 25 images per second, and 1/100 sec for 50 images per second. This corresponds to a 180-degree shutter.

This 180-degree shutter is a term from analogue film. The rotating circular shutter used there was half open, at 180 degrees.

In the past, the shutter rotated, with the opening determining how long each frame of the moving film strip was exposed.
You could close it further, giving you, for example, a 90-degree shutter, which corresponds to 1/100 sec at 25 images per second. Or you could use a 270-degree shutter, which is the same as 1/25 sec at 25 images per second.

You understand that a slower shutter speed creates motion blur. The same applies to the images you create for video. But by showing these images one after another, the movement becomes very natural. This is the case with 50 images per second. But it is even more natural with 25 images per second.

A fixed exposure time

All this means that you have no room to adjust the shutter speed when recording video. You choose it based on the framerate. Of course, you are not required to always choose 2x the framerate. You choose the shutter speed based on how smooth you want the movement to look.

The shutter speed is especially important for video with a lot of movement. Choose the shutter speed based on the framerate and how naturally the movement should appear in the image.
Since the shutter speed has then been chosen and fixed, only the aperture and ISO remain for controlling the exposure. But suppose you want to use a particular depth of field. Then only the ISO value remains for setting the correct exposure.

The need for an ND filter

Bear in mind that when you want to film with a higher dynamic range, the so-called LOG setting, the minimum ISO value is often ISO 400 or even ISO 800. When there is a lot of light, it may then be necessary to stop the aperture down completely.

A variable ND filter is essential for video. This one is from Haida and also has a built-in polariser.
As a result, it becomes impossible to use the desired depth of field. The aperture is needed to achieve the correct exposure. The only solution is then a variable ND filter.

A variable ND filter allows you to control the amount of light passing through the lens. This gives you an extra way to set the exposure. With an ND filter, it is then possible to achieve the desired depth of field at the minimum selectable ISO value and the shutter speed matched to the framerate.

I was filming on location in sunny weather. I used the CLOG3 setting with ISO 800 as the minimum selectable value. To avoid having to use f/22 at 1/50 sec, 2x the framerate, the variable ND filter was the way to control the exposure.

Choose the framerate deliberately

I mentioned 2x the framerate as the ideal shutter speed, the 180-degree shutter. This produces smooth footage that is pleasant to watch. You can of course deviate from this, but do so for a deliberate reason.

More movement from making the shutter speed equal to the framerate gives the video image a very different feel. The same applies to a shutter speed that is, for example, 4x the framerate.

However, avoid an automatic exposure setting such as aperture priority. Otherwise, there is a good chance that the video image will no longer be pleasant to watch because of constantly changing shutter speeds.

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