HDR : High Dynamic Range explained so anyone can understand

There are dozens of articles and blogs on HDR on the Web, so why another one? We decided to write up HDR for you because each new item seems to be more complicated than the next. And those of you without the latest Photoshop or flashy dSLR often seem left out of the loop. So here goes.

What is dynamic range?

Let's start with dynamic range. Compare the sound from your MP3 player head-phones: the difference between quiet which you can hear and the loudest is not very much. Not when compared to, say, a hi-fi system with big speakers which can go from whisper quiet to loud enough to make you unpopular with the neighbours. The dynamic range of the MP3 player is said to be narrow or low. The dynamic range of the big hi-fi system is said to be wide, or high.

So, a scene with a high dynamic range is one with a big difference between the lightest or brightest part and the darkest or shadow part. But look at this shot of a tree lit by a single lamp: is the scene high dynamic range?

No - not high dynamic range - the darkness is very deep, but we don't care because there's no detail at all. And the brightest part appears bright only relative to the darkness. That's the difference between high-contrast and high dynamic range: in high contrast tones simply look clearly separated, high dynamic range depends on actual difference between measured brightness.

Now, is this scene, of Sydney Harbour high dynamic range?

Sorry, it is and it's not. Why not? Surely the difference between the girl's black back-pack and the white of the Opera House domes is enormous? Sure it is; but all the details we want have been recorded comfortably in a single shot. So for our purposes we do not need any HDR techniques applied.

What is high dynamic range, then?

When people say something is 'high dynamic range' what they actually mean these days is: "the scene's range of brightness is too large to record all the details we want, given the camera gear we have."

This shot, taken in Udaipur, India, is pretty high range: it's straight into the sun which has burnt out much of the nice blue sky and there are deep shadows where we'd like to see some details.
So the problem is, how do with get an image which both holds the blue sky colour and at the same time holds details in the shadows?

Enter HDR

Cue HDR: this is a bunch of techniques that enables you to capture a wider range of brightness in the scene than can be recorded by your camera. This can sound a bit of a contradiction.

The key is that you cheat: you make different exposures to favour the brightest, mid and darkest parts of the scene. Then combine them.

By the way, this is not new. In my dark-room days, we were known (if rarely) to shoot a landscape at two exposures, then print the foreground part with the heavy negative, swap negatives and print the sky. But, boy, was it an awkward, fiddly thing to do.

Another old trick: when film scanners weren't very good, in the early 1990s, I squeezed decent results out of them by making two scans at two different exposure settings, then blended them using Photoshop's Layer Blend Options control.

First one bright scan to get the shadow details knowing we will lose highlight details. Then we make a darker scan to bring those highlight into range, allowing the shadows to go south.
That's the idea: if the machine can't get everything done in one go, you split the job into manageable parts.

HDR today

Today, we have progressed from exposure blending to more sophisticated versions of blending - often calling themselves HDR, together with a technique called tone mapping. Put them together and we have very powerful ways to image both the dark parts and the light parts of the scene, overcoming part of the dynamic range limitations of our cameras.

Like all techniques, HDR can give stunning results. And it can give questionable results that look, to some eyes, artificial or too painterly. But each to their taste.

But silly claims like "HDR increases the dynamic range of your photographs" are not helpful. HDR does nothing of the sort. What it does is to squeeze (compress) the dynamic range in the scene so that it fits into what your photograph can show.

A simple example

Here is a simple example, shot on assignment in Auckland, New Zealand, for a hotel group. Although I had some lights, I did not have enough to balance the fierce light coming from the Auckland sun. So I made one exposure to get the lace curtains with some colour - it looks very dark because most of the setting was lit by room lights. Then I made an exposure which gave me good exposures in the shadows, without making them too light. This is actually near the correct exposure, but it's clear we've lost details in the curtain and some of the glass-ware. Plus the normal exposure, whose density lies between the other two.

The HDR combined shot gives a good balance of all the features without too much artificiality.

It's not all HDR

HDR images have a certainly look: the shadows are not only full of detail they usually have more colour than you'd expect. And skies, with clouds, are richly coloured, sometimes unnaturally dark. In fact, many look like this:

(It's the Observatory at Jaipur, India.) But, actually, this was created with the Highlight/Shadow command in Photoshop. What about this one, then?

Nope; not HDR either. This is shown to give an example of the artifacts which accompanies the Shadow/Highlight command applied with inappropriate Radius settings, leaving haloes around density boundaries.

True HDR images combine two or more images which are identical apart from being exposed at different levels - the bracketing exposures. As the images must line up to blend cleanly, you do not want any change in the aim of the camera between shots. Even if software can align the images, turning the camera slightly introduces parallax errors which prevent good alignment.

OK, so how do you do it?

In summary, you make three bracketing exposures on aperture priority as quickly and steadily as possible. Then combine them using HDR software. It's that simple.

Here's the easy version, set out step-by-step. Purists will throw up their hands in horror with some details; let them. Once you've got some experience you can try the refined stuff.

1. Set ISO 200, or ISO 400 if you have a modern camera
2. Set exposure mode to Aperture Priority (exposure time changes)
3. Set the camera to make series exposures - as rapid as possible: 3 shots per second or better is good
4. Choose your scene: it should be bright and with key things static - moving clouds and leaves on distant trees are OK; (but flying pigeons are not).
5. If your camera or lens has image stabilization, make sure it's on.
6. If you have the sun to one side or behind you, set the camera to bracket exposures by 1 stop (that is, it makes three exposures: one over-exposed, one OK, one under-exposed).
7. If the sun is in front or there are very bright areas e.g. white buildings in full sun, set the camera to bracket exposures by 2 stops.
8. Try to find something to lean on, or rest your camera on. Compose your shot and check it's in focus. If you can't find something to rest the camera on, and have only an LCD screen on your camera then: frame up, hold the camera steady and bring your face against the camera to steady it.
9. Now breathe out gently, and before you take the in-breath, squeeeeeeeze the shutter button gently, and hold down until all three exposures click off.
10. Open the images in software such as Photomatix (not expensive) or Photoshop if you have it, and HDR your images. The web sites and on-line help will step you through the process. Be prepared to be amazed, but don't overdo it (or know when you are).
    

See? no tripod, no elaborate calculations, nothing raw. With this technique, you may even be able to capture action that is fairly static like the tourist photographing in Sydney (though the water may look weird).

If you want super-refined results you'd shoot at lower sensitivities, in raw, make seven bracketing exposures (phew!), and yes, use a tripod. That alone counts out over 90% of people who enjoy photography.

There are many sites giving details about how to work with the images, settings to use, and so on. These include:

www.hdrsoft.com (developers of Photomatix)
www.luminous-landscape.com
www.outbackphoto.com
www.popphoto.com/howto/3038

And look at zillions of HDR shots on Flickr - over 8000 HDR groups and counting! E.g.
www.flickr.com/groups/hdr
www.flickr.com/groups/myfirsthdr

Techies will know I've left out lots - and I've been terribly good and haven't insisted that we talk about scene luminance range. But if you want to know more, like what is tone mapping, why 256 levels is enough to record any dynamic range (actually just two will do), drop us a line in the comments.

Posted on October 22, 2007 06:54 in Photography Techniques
Tags: hdr, high dynamic range, tone mapping
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