What is HDR? Everything you ever wanted to know about high dynamic range images
So what is HDR photography all about? In short, HDR techniques allow you to take pictures of high-contrast scenes and preserve all that important shadow and highlight detail. But it comes with a lot of jargon. Here we answer all the common questions about HDR images.
What does HDR stand for?
HDR stands for high dynamic range; and the abbreviation is often used in a longer form, HDRI – high dynamic range imaging. HDR is a form of photography that enables you to create a picture with a greater dynamic range than is usually possible.
To understand what it is and to appreciate its use, you first need to have a grasp of what dynamic range is all about.
So explain dynamic range to me
Dynamic range is a measure of the range of different light levels – from the darkest black to the brightest white – that can be recorded or displayed by a device. It defines the amount of contrast you can capture or show without losing detail at the extremes.
The dynamic range that can be captured with your SLR is greater than can be displayed on your monitor.
Why is this important?
Some scenes contain too much contrast for us to capture successfully with our cameras. We avoid taking pictures in the midday sunshine as our cameras can’t cope with the full range of light levels. Low-light scenes are another common problem area – we can expose successfully for the shadows, but not for the brightly-lit areas or vice versa.
Exposure for midtones
Are there ways around this?
Digital imaging has made it easier to resolve because we can see the result immediately and take remedial action. We can also use flash to help reduce contrast on a sunny day and use a graduated ND filter to balance the brightness difference between the sky and landscape.
What’s more, there are processing tricks we can use in Photoshop, particularly if we shoot in raw, that enable us to get information from the darkest and lightest parts of our pictures.
So where does HDR come in?
HDR enables us to shoehorn a greater range of brightness into an image in a way that a straightforward picture can’t achieve. A true HDR image is created from several shots of the same scene taken with slightly different exposures.
Each exposure captures part of the full tonal range. They are then combined into a single image with software. The trouble is that these true HDR images are hard to see…
What do you mean?
A true HDR image contains a far greater range of tones – too many, in fact, to be displayed on a normal computer monitor, or printed out on paper.
They are typically stored as 32-bit files – allowing 4.3 billion shades in each colour channel. By comparison, a standard JPEG image allows 256 (8-bit) shades per channel, and a raw file 4,000 (12-bit) to 16,000 (16-bit) shades per channel.
So what do you do with these very large files?
The next stage in most HDR images is tone mapping. Here the program uses the 32-bit HDR image to create an image with a contrast range that can be shown in print or on a monitor.
Each tonal value is remapped onto a scale that creates an image in which you can see detail in the brightest highlights and the darkest shadows, and without any clipping in these extreme areas of brightness. It’s this tonal mapping that creates the controversy with HDR.
Why the controversy?
Tonal mapping brightens shadows and darkens highlights, which slightly flattens the contrast of an image and increases edge definition.
But many HDR enthusiasts use the software to go further, creating an image in which all the detail can be seen clearly, but which no longer looks realistic. The effect is similar to that used in ‘hyper-real’ styles of painting. Some people like it, some don’t.
Exposure for shadows
What sort of software do I need?
There are lots of HDR programs available – including some free ones. The best known is Photomatix Pro, but the latest version of Photoshop (CS5) has a built-in HDR facility.
HDR programs usually have a range of sliders to help you control the tone-mapping effect to your own taste.
How do I take pictures in preparation for HDR effects?
Essentially, the process is the same as that used for bracketing. The number of shots you need is largely dependent on the actual tonal range of the scene you’re shooting. The more contrast, the more shots you need.
Three is the usual starting point, but you may need to take as many as nine, each with a one-to-two stop difference. Some SLRs have an AEB (automatic exposure bracketing) function, which will enable you to do this without too much fuss.
Exposure for highlights
What other settings should I use?
Your sequence of exposures should be as similar in content to each other as possible (although obviously, the brightness will vary). Any changes caused by movement can create a ghosting effect that the software will struggle with.
Set the focus manually, use a tripod, and set the exposure to aperture priority (so the depth of field remains constant). Set the camera to the fastest continuous drive setting available.
Is there an easier way?
Creating HDR images involves some effort at the time of shooting and processing but it’s relatively straightforward. However, there are easier ways.
A number of programs offer false HDR effects that can create realistic-looking HDR images from just a single picture. Similarly, a number of DSLRs and compacts now have built-in automatic HDR facilities that will take the sequence of pictures for you and compile them into your tone-mapped image in the camera itself.
How to blend two photos for perfect exposure
Recover clipped highlight detail: how to rescue your over-exposed photos
Truthful Tone-mapping: a quick guide to realistic HDR in Photomatix Pro
20 tips for faster photo editing
on Saturday, March 2nd, 2013 at 2:00 am under Photography Tips.
Tags: DSLR tips, HDR