My First Experiences With HDR Imagery

What Is HDR?

From Wikipedia: "In computer graphics and photography, high dynamic range imaging (HDRI) is a set of techniques that allow a far greater dynamic range of exposures (i.e. a large difference between light and dark areas) than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to the deepest shadows."

By taking more than just one photo of a scene, the technique enables you to record a greater range of tonal detail than a given camera could capture in a single photo. You can find more theory about HDR elsewhere on the web, here I'd just like to present the 'how to do it' and a few of my first experiences.

When To Use HDR?

One prerequisite for HDR is, that the scene is static. As this technique requires more shots of a same scene, there shouldn't be much movement - or the flowing water will be blurred, walking people will look like ghosts etc.

In normal conditions, when your camera can capture the whole brightness distribution of a scene, you don't need HDR technique. It is different however, when there are big contrasts, where inevitably a part of your shot will be either overexposed or underexposed. A typical example is taking a photo of a bright summit from a dark valley. In such conditions your camera can not only adapt to the contrast but is because of that high contrast also not able to reproduce the whole range of colours.

Take Several Shots Of A Scene

When decided to do a HDR image, plan first how to take 3 - 5 (or more) pictures of the same scene:

1. Use a tripod. Having with you a small and light tripod is possible also in mountains. Remember - when handling with your camera, there should be no moves! Even with the 'bracketing' function (see below #2) you can never take more completely identical pictures without a tripod.

2. If your camera has a 'bracketing' function, use it. It enables you to take for example 3 consecutive shots, where the first one is properly exposed, the second one is overexposed (you can set the amount, for example 2/3) and the third one is underexposed. If you don't have a 'bracketing' function you will have to change exposition manually - let's say on +1 and on -1, or on +2/3 and on -2/3. Doing it manually, it's worth taking more than 3 pictures - 5 for example.

3. To make sure not to move your camera when releasing the button, use self-timer, for example setting it to 2 seconds. Then release the button and take hands off.

Doing so, you will bring home for example 3 pictures of a scene, like shown below:

Creating HDR Image With Photoshop CS2

You merge your pictures with the command: File -> Automate -> Merge to HDR...

The PS prompts you to choose your set of pictures and to decide whether to take it for granted that they are aligned or to make the program to align them if some movement occured when taking pictures. I found sharpness to be one serious problem of HDR images, so I almost allways use the program to check alignment, even if this procedure takes really a lot of time (many minutes - depending on resolution).

After the program is done with computing on your set of pictures, you can preview the outcome. You can exclude one or more pictures in the set, see if excluding a (bad) picture will improve sharpness etc. Then you confirm the outcome and make the program to compute one single HDR image.

The resulting HDR image will be a 32-bits image (as opposed to the usual 16 or 8-bits). These extra bits are well used to cope with the dynamic range, using floating point numbers for that. But here another problem occurs. Your computer display (or print out) can only show a fixed brightness scale. That's why the HDR image which appears on your screen doesn't look any special at all. The next step will be to convert it into an ordinary 8 or 16-bit image, which can be displayed on a monitor. This process is referred to as tonal mapping.

So, having your original HDR, save it to be able to play with it later. It has only an archiving purpose for now. Don't be surprised if the file is very large - you have to save all those bits.

Converting HDR Image To A Normal Image

Now use a Photoshop command: Image -> Mode -> 8 Bits/Channel... (or 16 Bits/Channel...)



You can choose among 4 methods: Exposure and Gamma offers you to play a little with exposure and gamma, the Highlight Compression and Equalize Histogram method are fixed. But the fourth method, Local Adaptation changes the scene dramatically. You get an unusually bright preview, but as it originally is, it will offer you very vivid, unnatural colours. Play with the outcome. Don't change the curve at all and see the outcome. You can later decrease the saturation and luminance of colours to make your photo more natural. But it will still look very unusual. Below is one of its examples:


The converted image is shown on the left. Then I immediately apply the Image -> Adjustments -> Levels command. The result is in the middle. Then I play a bit more with curves, colour vividness, saturation etc., sharpen the image and get something out of it. The picture on the right looks very unnatural, but I wanted to show the extreme. See how colours are vivid, how there's no black rocks in the cave behind the waterfall and how there's not much overexposed sands. Contrasts were handled even too much, the picture is flat, lacking depth.

On the other hand, if you use the curve and make your conversion more close to a normal scene, it will not look unnatural any more. But still, the contrast of the scene will be handled and the colours preserved. Below is a comparaison between a non-HDR image (on the left) and a HDR image (on the right). The latter still shows vivid colours and a better contrast handling. However tastes might be different.



HDR images look like paintings. They resemble water colour paintings or other graphical techniques. So, why not playing a bit further? You can use your HDRs as a starting point to create some painting-like effects. Below you cane see a simulation of: oil painting, graphic and water colour painting. Have fun!

A Few Examples

Here are a few HDR examples on SP: 1 2 3 4 5

P.S.: A few days ago I also used HDR technique for photographing old dias. I have a tube to put the dia at the end of camera lens. No movement can occur (I can do it without a tripod) and it's easy to make 5 shots of different exposure. I was very satisfied with the result, even if the resolution was lower than obtained by using a scanner. One example is here.

External Links

Sean T. McHugh: http://www.cambridgeincolour.com/tutorials/high-dynamic-range.htm

Images

View My First Experiences With HDR Imagery Image Gallery - 25 Images