High Dynamic Range

I recently wrote a post about exposure and dynamic range in a scene. Well, sometimes, you can’t hold all the detail that exists in a scene in one picture, simply because the range of tones in that scene is wider than the range that the camera sensor is capable of registering.

To dive into that a little bit, a camera sensor is capable of registering a range of tones, in fact it registers 256 tones from black to white. Those are shades of grey on a digital sensor they’re registered as bits of information, as 0s and 1s, that a computer then interprets and shows on a screen. We often refer to the range of tones in a scene as contrast, and it is determined by the light that hits the scenes and the difference between the light areas and the dark areas.

Consider the photos below as an example:

Consider the contrast here in this scene

Consider the contrast here in this scene

This is the image as taken with no edits made. I took it because it made me laugh: whoever erased the VEHI from the sign left text that simply says “Police bums”. I want to draw your eyes here to the contrast in the scene. The bright area that is lit is well detailed because the exposure is good for the highlights. We still see detail in the shadows behind the signs, so detail is retained in the whole of the scene, despite the contrast being quite strong.

When I take an exposure, I am thinking about not letting the lit areas go to white, or the shadow area go to black. The reason this is important is because the tone and colour information is lost when something registers as pure white or pure black.

For example, the grey of the brick in the sunlight is a different grey to the brick in the shade, and the camera registers them as different values. If I overexposed the lit brick where it registered only as white, all the detail would be lost. The same is true if I underexposed and the shadow detail is lost.

If you’re interested, and you use editing software, the “best” way to shoot is to overexpose the image until you almost have blown highlights, then correct the exposure in post – this gives you an incorrect exposure in camera but gives you the greatest detail and lowest noise in the image.

So I was faced with the following scene, and this was the first exposure I created:

Correctly exposed picture

Correctly exposed picture

So this is the scene – the light is fading slightly and the sun is going down. I wanted to keep some of the colours in the sky, which looked gorgeous to my eye, but because I’ve had to overexpose the sky to maintain some detail in the shadows of the trees, neither the shadows or the lit areas are correctly exposed, and there’s nothing I can do about it in a single frame. (Technically, this isn’t true, but it’s the subject of another blog post).

So I want to be able to recover all that gorgeous detail: what can I do? Photoshop and other software allows you to merge two or more exposure into one, where more detail is retained. The technique is simple, in a scene, shoot a variety of exposure, depending on what you want to correctly exposed. I took three images. This was the first, I took a second one that was underexposed but had the sky correctly exposed, and a third image that was overexposed but was correctly exposed for the trees. I took that into my HDR software which is Photomatix Pro (there is a trial version which is free to use but adds a watermark to the image) and the result is below:

Now that's how I remember the sky.

Now that’s how I remember the sky.

As you can see, the scene has come alive! The greens of the trees are as I saw them and the rich oranges and blues in the sky are beautifully recreated. Overall, I think the saturation is a bit much for this image, but the technique is nicely demonstrated.

The sharp eyed among you will have spotted the ghost on the left, a gentleman who was in all three of the exposure and who I haven’t “deghosted” out, but I could spend some more time on that if I wanted. I’d also clone out the red traffic light on the right as it is distracting.

What HDR allows you to do is amplify the dynamic range of your sensor and reduce the contrast in the scene. It’s great for landscape and travel photos like this, and I like to use it to show what my eye could see rather than what the camera can capture.

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