HDR

Learning goal

You will be able to turn a scene with a wider light difference than a single frame can carry into a photo with detail in both its bright and dark areas, by combining frames you shot at different exposures. You will measure the difference between the scene's brightest and darkest areas in stops, and calculate with the bracket range how many frames, and how many stops apart, you need to shoot to cover this difference. You will clean up with deghosting the trails that form in the combined frame because of moving leaves, water or people, and use in moderation tone mapping, which compresses the combined wide range into a range that can be shown on screen or in print. You will notice that local contrast, when overused, leaves halos, a grayed sky and a flattened image, and build a natural-looking HDR deliberately. On different skin tones, you will check how HDR can flatten skin and harden texture in a portrait.

The question

The next morning you go back to the bridge at sunrise. The sun rises behind the bridge; the sky is orange and bright, while under the bridge's arch and the bushes along the stream are still in deep shadow. When you expose for the sky, under the arch and the bushes are a pitch-black void; when you expose for the arch, the sky is pure white, all the clouds gone. No frame on the screen shows what your eye sees at that moment: the eye sees both the orange of the sky and the stones under the arch at the same time.

Your friend from the club puts their camera on the tripod and quickly shoots three frames from the same framing: one dark, one medium, one bright. How do both the sky and the shadows keep their detail? How do three separate frames turn into the single scene the eye sees?

Context

In the dynamic range lesson you saw that a scene's light difference can exceed the range the sensor can record, and that in this case you need either to narrow the range or to combine frames shot at different exposures (Lesson 9.6). You also learned exposure bracketing, shooting the same scene at different exposures one after another (Lesson 9.8). Here we take the two together and turn them into a shooting and combining technique: deciding how many frames to shoot, managing moving subjects, and finishing the combined image in a natural or a deliberate style.

HDR is perhaps one of the most overdone techniques. At one time, photos with bright colors, surrounded by halos, with grayed skies, almost turned into paintings, were identified with HDR. Yet HDR's real strength is showing a scene close to how the eye sees it. This lesson tells both how to build a natural-looking HDR and how to understand where the overdone look comes from; which one to choose is a matter of style.

Simple explanation

How many frames and how many stops are needed?

The first step of an HDR shoot is measuring the scene's light difference. With the camera's spot metering you first meter the scene's brightest area that must be preserved, then its darkest area whose detail you want; the difference between the two readings gives the scene's contrast in stops. If at the same aperture and ISO the sky shows one five-hundredth of a second and under the arch shows one fifteenth of a second, the difference between them is five stops.

The bracket range is the stop difference between the frame at the dark end and the frame at the bright end of a bracketing sequence, and it is chosen wide enough to cover the scene's contrast. Three frames two stops apart add two stops on each side of the middle frame, four stops in total; this is enough for most sunrise or interior scenes. If there is a much wider difference, for example if you are looking from inside a dark church at a bright window, you may need to shoot five or seven frames. The measure matters more than a rule: the darkest frame must hold the detail of the brightest area, and the brightest frame the detail of the darkest area.

During bracketing the aperture is kept fixed and the exposure is changed with the shutter speed; if the aperture changes, the frames' depth of field differs and the combined frame carries blurred areas. Using a tripod keeps the frames aligned; when shooting handheld, the program can align the frames, but the result may be less sharp.

How are moving subjects managed?

Because HDR frames are shot one after another, anything that moves between frames causes problems in the combined photo: leaves swaying in the wind, flowing water, walking people, passing clouds. Because these subjects are in a different place in each frame, they leave half-transparent double images in the combined frame. Deghosting is the process of detecting moving areas during combining and cleaning up these double images by using the pixels of only a single frame in those areas.

Most combining programs offer this process and let you choose which frame is used, automatically or manually. But deghosting isn't always flawless; a tone difference or a line can remain at the edge of the moving area. That is why in a moving scene you need to shoot the frames as fast as possible, use the camera's fast bracketing mode, and check the combined frame by enlarging it over the leaves, the water and the people.

What does tone mapping do?

The combined frame carries a much wider light range than a screen or a print can show. Tone mapping is the process of compressing this wide range into a range that can be shown on screen or in print. It reduces the difference between the brightest area of the sky and the darkest area of the arch, so that both look detailed in the same photo.

Local contrast is increasing the tone differences in small areas of an image independently of the overall tone distribution. When tone mapping compresses the overall range, the photo can go flat; local contrast makes small details pronounced again. Used in moderation, it brings out the texture of the stones and the shape of the clouds. Overused, problems begin: bright halos appear around dark objects in front of a bright sky, the sky loses its natural brightness and turns gray, the shadows are opened too much and the scene flattens, and the colors slip into an artificial saturation.

Natural-looking HDR is a result that, despite the widened range, stays close to what the eye sees in the scene, in which the sky stays brighter than the shadows and halos don't show. For this you need to keep tone mapping moderate, leave the sky bright relative to the ground, not open the shadows completely, and keep local contrast below the point where halos start to appear. An exaggerated, painterly HDR look can be chosen as a style preference; but halos and a grayed sky aren't a preference, they are a processing artifact.

If there are people in the scene, HDR needs care. People move between frames, and ghost trails can remain on faces. Tone mapping compresses faces too: dark skin can slip toward a gray, flat tone as the shadows are opened, and light skin can look pale as the highlights are held down. Local contrast, on the other hand, hardens skin's texture, pores and lines. That is why in an HDR that includes a portrait, taking the faces from a single well-exposed frame and combining them with a mask, or, instead of HDR, narrowing the scene's range during the shoot with a reflector or fill flash, often gives a better result. In every case you need to enlarge the faces and check the skin's color and texture.

Visual evidence

At the bridge, with spot metering, you meter the bright orange of the sky and the stones under the arch; at the same aperture the difference between them is five stops. You put the camera on the tripod, fix the aperture at f/8 and shoot three frames two stops apart: one five-hundredth of a second, one hundred twenty-fifth of a second and one thirtieth of a second. The dark frame holds the clouds of the sky, and the bright frame the stones under the arch.

When you combine them, the leaves of the bushes along the stream look doubled because they moved in the wind; when you turn on the deghosting option and choose the middle frame, the leaves are single and sharp. In your first tone mapping you have opened local contrast too much: along the edge where the arch's stones meet the sky there is a thin, white halo, and the sky has turned gray. You pull local contrast back and brighten the sky a little; the halo disappears, and the sky stays brighter than the arch's stones. The dark skin of an old woman taking a morning walk over the bridge had leaned toward gray in the first attempt; when you take her face from the middle frame and combine it with a mask, her skin returns to its own warm brown.

Common mistakes

Summary

An HDR shoot starts by metering the scene's brightest and darkest areas; the bracket range is chosen wide enough to cover this difference, three frames two stops apart add four stops for most scenes, and the aperture is kept fixed during bracketing. Leaves, water and people moving between frames leave double images; deghosting uses a single frame in these areas. Tone mapping compresses the wide range into a range that can be shown, and local contrast brings detail back, but too much of it leaves halos, a grayed sky and a flattened scene; natural-looking HDR keeps the sky bright relative to the ground, and in a portrait, the color and texture of skin is preserved by taking faces from a single frame or narrowing the range during the shoot.

Mini assignment

In this assignment you will work on a tripod in a scene with both a bright sky and deep shadows.

1. With spot metering, meter the scene's brightest and darkest areas and write the difference in stops.
2. Keeping the aperture fixed, shoot three frames two stops apart; in a scene where the difference is very large, try five frames.
3. Combine the frames; get two results, first with deghosting off, then on, and compare the moving areas by enlarging them.
4. Finish the same combination once with natural and once with exaggerated tone mapping; mark where halos appear. If there is someone in the frame, enlarge their face and check the skin's color and texture.

Check yourself

• Can you measure a scene's contrast and decide how many frames, and how many stops apart, you need to shoot?
• Were you able to notice when deghosting is needed and where it can leave flaws?
• Did you check how tone mapping affected the skin in the frame by enlarging the faces?

Glossary

Mini quiz

1. If at the same aperture the sky shows one five-hundredth of a second and under the arch shows one fifteenth of a second, how many stops is the scene's contrast?

• A) 5 stops
• B) 2 stops
• C) 3 stops
• D) 8 stops

2. Why is the aperture kept fixed during HDR bracketing?

• A) To preserve the colors
• B) To shorten the shutter speed
• C) So that the frames' depth of field stays the same and no blurred areas form in the combined photo
• D) To lower the ISO

3. How do you fix leaves swaying in the wind looking doubled in a combined HDR?

• A) By raising local contrast
• B) By turning tone mapping off
• C) By converting the frames to black and white
• D) By turning on deghosting

4. In an HDR portrait, a dark-skinned person's face has slipped toward a gray, flat tone with tone mapping. Which is the most fitting fix?

• A) Raising local contrast even more
• B) Taking the face from a single well-exposed frame and combining it with a mask
• C) Darkening the sky
• D) Narrowing the bracket range

Answers

1. A — From one five-hundredth of a second to one fifteenth there are five full stops.
2. C — If the aperture changes, the depth of field changes and the frames don't sit exactly on top of each other.
3. D — Deghosting uses the pixels of a single frame in moving areas.
4. B — A face taken from a single frame isn't affected by tone mapping's compression and keeps its skin.

Summary card

• Meter the scene's brightest and darkest areas; write the difference in stops.
• Three frames two stops apart are enough for most scenes; keep the aperture fixed.
• Turn on deghosting for moving subjects and check by enlarging.
• Keep tone mapping moderate; halos and a gray sky are processing artifacts.
• In a portrait, take the face from a single frame or narrow the range during the shoot; check skin by enlarging.

Bridge to the next lesson

The HDR photo of the bridge was liked at the club, but your friend wanted something else: to show the whole valley seen from the bridge, from the hill on the right to the minaret on the left, in a single photo. Even the widest lens they had barely took in half the valley; a wider lens, on the other hand, bent the edges and curved the horizon. How do you shoot a landscape that will not fit in one frame? The next lesson tells about building a wide panorama by overlapping and combining frames shot side by side.