Balancing Exposure for Video, Thinking in Stops

In this article, we’ll look at a technique to quickly balancing exposure. When you’re working on a production, you’re always in a rush to setup and shoot. So it’s great when you find a way to work faster, without compromising quality. And the key to this is to think of all the exposure settings in terms of stops.

Let’s imagine you’ve used a grey card to set a good exposure and you’re happy with the shot. You’re about to go for a take, when the director says they would like a more shallow look. So, you open the aperture by 2 stops to decrease the depth of field.

If you want to know more about setting exposure with a grey card, watch this video:
Don’t ETTR, Use a Grey Card

But now the picture is over-exposed. To compensate you need to use a different tool to lower the exposure by 2 stops. You could get out the grey card to reset the exposure, but it would be quicker if you knew instantly which ND filter would cut two stops, or how you could do the same with another exposure tool.

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The Key to Balancing Exposure, Thinking in Stops

The aperture of your lens is measured in stops. You’ll either see them printed on the aperture ring or displayed on your monitor. If you’re using photography lenses, these are f-stops. If you’re fortunate enough to be using cine lenses, it will be T-stops.

Unlike middle grey, a stop doesn’t represent an absolute brightness level. It’s relative, which means it measures a change in brightness. So we can say we’ve made the picture brighter by 2 stops or we’ve made it darker by 1 stop.

When we increase the brightness of the image by one stop, the brightness is doubled. And when we decrease the brightness of the image by one stop, the brightness is halved. These might sound like big changes, but our eyes are logarithmic, so halving or doubling the brightness don’t seem like a big jump. You can also use fractions of a stop if you need to.

Aperture

These are the whole stops you’ll find on the aperture control. The difference between 1.4 and 2 is one stop. The difference between 2 and 2.8 is one stop and so on. It’s useful to remember what the whole stop numbers are. But at first glance, there doesn’t appear to be a pattern, which makes them more difficult to memorise.

Balancing exposure - stop numbers

However if we split the numbers, you can see two alternating patterns. In both rows the numbers double as they increase. The only blip is where the doubling of 5.6 is rounded to 11.

Balancing exposure - alternating stop numbers

Using ISO to Balance Exposure

So, let’s see how we can think of ISO in term of stops. Let’s imagine your camera’s native ISO is 800 and that’s your starting point. We’ve seen that changing the exposure by one stop either doubles or halves the brightness. We can translate this directly to ISO units. When you double the ISO number, the exposure is increased by 1 stop. And when you half the ISO, the exposure is decreased by 1 stop. When you’re balance exposure, this is easy to remember.

So, if you want to decrease the brightness by 1 stop then you halve the ISO to 400. And if you want to decrease it by 2 stops, set it to 200. Returning to 800. If you want to increase the brightness by 1 stop, then you double the ISO to 1600. If you want to increase brightness by 2 stops, set it to 3200.

Balance Exposure with the Frame Rate

When balancing exposure with the frame rate, you can also think in terms of doubling and halving. If you increase the camera’s frame rate to create a slow motion effect, you’ll also be making the picture darker. Let’s image your current frame rate is 24 frames per second. If you double it to 48, you’re reducing the brightness by 1 stop. If you double it again to 96, you’re reducing exposure by 2 stops.

It’s unlikely you’ll set the frame rate lower than 24, but if you halve it to 12 frames per second, then you’ve increasing brightness by 1 stop.

Balancing Exposure with Shutter Speed or Shutter Angle

If you want to change the amount of motion blur in your picture, you’ll need to change the shutter settings. This also changes the brightness, so you need to take this into consideration when balancing exposure. If you’re shooting at 24 frames per second, the default shutter speed is a forty eighth of a second. So we’ll use that as a starting point.

If you double the shutter speed to a ninety-sixth of a second, the exposure is reduced by 1 stop. Halving the shutter speed to a twenty fourth of a second increases exposure by 1 stop. If your camera uses shutter angles, when thinking in stops, it’s a case of halving and doubling.

The default angle is 180º. If you halve the shutter angle to 90º, the exposure is cut by one stop. If you double the shutter angle to 360º, the exposure is increased by 1 stop. Because the shutter angle can’t go beyond 360º, the brightness can only be increased by one stop above 180º.

Balancing Exposure with Built-in and External ND Filters

Built-in ND Filters

ND filters are used to reduce the amount of light reaching the camera sensor. If your camera has built-in ND filters, these might be shown as ND1, ND2 and so on. This isn’t particularly useful, because it doesn’t tell you how much light is being cut. But somewhere in the camera settings, they’ll be displayed as fractions.

The internal ND on a camera, usually increases in 2 stop increments. ND1 cuts the light to a quarter of its original brightness, which is 2 stops. If you select ND2 the light is cut to a sixteenth, which is 4 stops. Some cameras have adjustable ND filters, so you need to remember how you’re set them up.

External ND Filters

Even if your camera has built-in ND filters, external NDs can be a life-saver when it comes to balancing exposure. If you’re using external ND filters, they are rated by their density as decimal numbers. A filter with a density of 0.3 cuts exposure by one stop. If you double the density to 0.6, the brightness is cut by 2 stops. So when you increase the density by point three, you’re decreasing the brightness by 1 stop each time.

If you’re using a graduated filter, these density ratings still apply, but they refer to the darkest part of the filter.

ND Filters and ND Grads

There are two main types of ND filters. Standard NDs that have the same density all over. And graduated filters, where the density decreases across the filter.

Using Lighting to Balance Exposure

Just like the other methods of balancing exposure, we can change light levels by one or more stops. With a point light source you can use the inverse square law. This means the light levels change in a predictable way depending on the distance between the light and the subject. This is more accurate with a point light source, rather than a broader panel light.

Let’s imagine the light is 2 metres (6.5 feet) from the subject. If you halve that distance so the light is 1 metre (3 feet) from the subject, the brightness level is quadrupled. So it’s an increase of 2 stops. But if you double the distance instead, so the light is 4 metres (13 feet) from the subject, the brightness level is quartered. So it’s decreased by 2 stops.

For accuracy, I recommend you measure the distance between the light and the subject. With a larger light source, you might want to use an incident light meter. When you dim a light, the dimmer isn’t measured in stops. So again, a light meter will be handy.

Balancing Exposure with Nets and Diffusion

Nets and diffusion also reduce the amount of light reaching the subject. And these also can be measured in stops. If netting has a green edge, it’s known as a single and cuts the light by half a stop. If it has a red edge, it’s known as a double and cuts the light by one stop.

Diffusion material such as artificial silk cuts between 1 and 2 stops. Coloured gels also cut light, but it varies depending on the colour and the density.

Summary

The purpose of the technique is to save you time during production. When you’re balancing exposure, think in terms of stops. It makes it easier to compensate for any changes you make. To make it even easier, I’ve created a cheat sheet. Click the link in the description to download it. All the info’s on a single page, so it’s easy to bring it up on your phone.

If you own your camera, you can stick labels on the body with info on exposure setting. If you’re using glass ND filters, it’s good practice to stick a piece of masking take on the matte box and write the strength of the filter you’re using.

From now on, I hope you’re going to be thinking of all your exposure tools in terms of stops. Let me know if you think it’s a useful technique and whether you’ll going to be using it in your next project.

If you’ve got any questions or suggestions please comment.

Go Further

Download the Balancing Exposure Cheat Sheet!

If you found this useful, check out the rest of my fantastic Camera Training videos:

Return to Cinematography Training page


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