Exposure with a drone

Learning goal

You will be able to get a workable file from a drone's small camera, even in a bright scene with harsh light. You will know that a small sensor's RAW file isn't limitless either, how far you can open its shadows, and why a blown white doesn't come back. While bracketing with a drone hanging in the air, you will take into account the small shift between frames, and prevent in flight the things that spoil the merge. On a drone whose aperture is usually fixed, you will fit an ND filter for a longer shutter speed, but see that the limit of this shutter speed is set not by a tripod but by the wind and the drone's ability to hold still. You will learn to take the skin of a person who looks small in the frame from the cleanest frame instead of rescuing it from deep shadow.

The question

Opening the frames from the salt lake flight on the computer, your friend is disappointed. The bright parts of the salt are a flat white, the shadows of the dikes at the edges of the pans black spots. They try opening the shadows; the face of their cyclist friend standing in the shadow of a salt heap appears, but her skin is full of colored speckles. On their big camera the same operation would give a clean result. The drone's camera is small, and they can't change that. How do you get the best result from a small sensor?

Context

In the file formats lesson you met the file format that keeps all the information the sensor recorded, leaving the camera's processing decisions for later, that is, RAW (Lesson 3.8). You also used automatic bracketing, which shoots the same scene one after another at different exposures, in high-contrast scenes (Lesson 9.8). And in the long exposure lesson you fitted the ND filter, which reduces light without changing its color and allows a longer shutter speed (Lesson 13.3).

On a drone, all of these work a little differently. The camera is small, often working with a sensor close to a phone camera's; its aperture is fixed on many models; and the camera stands not on a tripod but in the air, inside the wind. This lesson resets three tools you know for these three conditions.

Simple explanation

How much does RAW rescue on a small sensor?

Recovery latitude is how far you can go in a RAW file, opening the shadows or pulling down the bright areas, before noise, color speckles and banding appear. A small sensor's pixels are small and each collects less light; that is why the signal in the shadows is weak and noise appears there first. A shadow that opens easily in a big camera's RAW file breaks down much earlier in a drone file.

Blown whites, on the other hand, don't come back on any sensor; there is no detail recorded in those pixels. That is why on a drone the single-frame rule is simple: protect the bright areas, look at the highlight warning on the screen, lower the exposure if needed, and when opening the shadows later, don't forget the latitude is small. If the scene's contrast exceeds this latitude, instead of trying to rescue it in a single frame, you need to shoot several frames.

A white salt flat also fools the metering. The drone's metering tries to pull the scene to an average gray and darkens the pure white salt a little. This protects the bright areas but also darkens the shadows more than needed and wastes part of the small latitude. Raising the exposure up to the point just before the highlight warning starts blinking on the brightest parts of the salt increases the light recorded in the shadows and reduces the need to open them later.

Why does bracketing in the air drift?

Bracket drift is a drone hanging in the air slightly changing position and direction between frames during bracketing. On a tripod three or five frames are shot from the same point; in the air, though, the wind pushes the drone a little between each frame, and the drone corrects its position. The merging software tries to stack the frames on top of each other, but if the drift is large, double lines stay at the edges, and ghosts in moving people and water.

The ways to reduce the drift are in flight. Waiting for the moment the wind is calm is the first step. Height helps too: the drone's same few centimeters of drift take up a smaller share in a frame shot from higher up showing a wider area. Choosing a mode that shoots the bracket fast shortens the time between frames. If someone in the frame is moving, taking them from a single frame in the merge prevents the ghost.

How long can the shutter speed get in the air?

A hover exposure is an exposure made from a drone standing in the air that can be longer than fractions of a second. On a drone with a fixed aperture, in bright daylight, the almost only way to lengthen the shutter speed is an ND filter: it is used to soften water flowing from a channel into the salt pans, or to turn waves on a beach into mist. But the tripod's place is taken by the gimbal and the drone's own balance; in calm air a short long exposure can come out sharp, while in wind the same setting blurs the whole frame.

An ND filter also has an effect on color. Cheap filters especially add a slight warm or cool tone to the frame, and this tone shows most in skin. Once the filter is fitted, shooting a test frame with a gray surface or a familiar skin tone in it helps set the white balance correctly later. If there is a small person in the frame, taking their skin from the brighter frame in the bracket, instead of opening it from deep shadow, protects the skin from speckles on a small sensor.

Visual evidence

The next morning your friend returns to the same salt pans and, at an hour when the wind is calm, takes the drone higher and shoots a fast five-frame bracket. In the merged frame the texture of the crystals shows in the bright parts of the salt, the earth can be picked out in the shadows of the dikes; there are no double lines at the edges.

Their cyclist friend is standing in the shadow of the salt heap again. This time your friend takes her image not from the merge but from the brightest frame in the bracket. Her dark skin stays warm and detailed, without speckles; placed side by side with the first version they opened from the shadow in a single frame, the difference shows clearly.

In the afternoon they try a hover exposure with an ND filter over the channel carrying water to the pans. In the first try the wind had picked up, and the frame is blurred. When the wind dies down, the same setting gives a sharp frame: the edges of the channel crisp, the water like soft silk. In the first test frame with the filter, their friend's skin looks slightly cool; your friend corrects the white balance according to that test frame.

Common mistakes

Summary

On a small sensor the recovery latitude is narrow: the bright areas are protected, bracketing takes over if the scene's contrast exceeds the latitude, and bracket drift is reduced with calm air, a fast mode and height. The limit of a hover exposure is set not by the ND filter but by the wind, and the skin of a person in the frame is taken from the cleanest frame of the bracket instead of being opened from shadow.

Mini assignment

In this assignment you will see a small sensor's limit with your own files; if you don't have a drone, you can apply the same steps with your phone.

1. Shoot a scene with bright and shadowed areas as RAW in a single frame, protecting the bright areas; open the shadows step by step in editing and note where noise begins.
2. Shoot the same scene with a three- or five-frame bracket and merge it; compare it with the single frame, and look at whether there is drift at the edges.
3. If there is someone in the frame, with their agreement, prepare their image once by opening the shadow from a single frame, and once by taking it from the bright frame in the bracket, and compare the skin.

Check yourself

• Do you know how far you can open the shadows on your own camera or drone before noise starts?
• Is there a double line or a ghost in the merged frame, and if so, why did it form?
• Is the skin of the person in the frame clean and in its own color?

Glossary

Mini quiz

1. Why do shadows break down earlier on a small sensor?

• A) Because small pixels collect less light, so the signal in the shadows is weak
• B) Because a small sensor can't record RAW
• C) Because shadows always blow out
• D) Because the drone's aperture is open

2. How is bracket drift reduced?

• A) By increasing the number of bracket frames
• B) By shooting at the moment the wind is strongest
• C) By waiting for the calm moment, choosing a fast bracket mode and climbing higher if needed
• D) By removing the ND filter

3. What most sets the limit of a hover exposure?

• A) The sensor's resolution
• B) The wind and the drone's ability to hold still
• C) The card's speed
• D) The color of the propellers

4. How does the skin of a dark-skinned person standing in the shadow of a salt heap stay clean?

• A) By opening the shadow all the way from a single frame
• B) By raising the saturation
• C) By turning the frame black and white
• D) By taking their image from the brighter frame in the bracket

Answers

1. A — In small pixels that receive little light, the shadow signal is close to noise.
2. C — As the time and movement between frames decrease, the frames stack better.
3. B — There is no tripod; how much the drone moves in the air decides the sharpness.
4. D — In the brighter frame the skin was recorded with enough light and doesn't need opening.

Summary card

• On a small sensor the recovery latitude is narrow; noise starts first in the shadow.
• A blown white doesn't come back in RAW either; protect the bright areas.
• Bracket drift: a calm moment, a fast mode, height.
• A hover exposure's limit is the wind; an ND filter only opens the door.
• Don't open skin from shadow; take it from the bracket's clean frame.

Bridge to the next lesson

While waiting for the hover exposure over the channel, your friend is caught by the image they see on the screen as they glide the drone along a shore: the salt pans pass slowly beneath it, then a dike enters the picture. Instead of a still frame they want to record this movement itself, but their first tries are jerky and hurried. The next lesson takes up how slow, smooth movement is built when shooting video with a drone.