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Dynaminc Range and HDR Photography

What is Dynamic Range?

Dynamic range by definition means the latitude or range of tones that you can see within an image (also known as Luminance), running from a pure white to a jet black.

Dynamic range is applicable to colour and black and white images as it referrers to the shadows, mid tones and highlights contained within an image.

An image with excessively high or low contrast will lack dynamic range as it will contain fewer variants from white to black. This is some instances is quite normal and depends entirely upon the nature of the image an the intentions of the photographer. There are no rules as such.

When capturing an image in high contrast lighting conditions your camera will only be able to capture approximately 5 EV stops of light, the rest of the data captured will not contain any detail it will simple be under or over exposed. Photography is littered with examples of images where detail at either one end of the specturum or other is lost.

Normally exposed images will have a fairly well distributed range of tones, representative of normal lighting, such as the image below of the motor cyclist by Nigel Jepson. Conversely, images taken in darker environments will naturally have a lower number of bright pixels and a higher number of dark pixels such as the live music shot by Keith Trigwell.

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An accurate way of viewing your captured luminance is by looking at your histogram, this will give you a clear indication as to how many tones are visible in the image and how bright or dark they are with this plotted on an exposure graph.

The brightness of pixels is represented horizontally with the llower brightness on the left and the higher brightness on the right. the number of pixels with that level of brightnes os shown on the vertical axis

Below are three histograms for images at various levels of exposure

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What is HDR Photography

However, what do you do if the level of tones of the image you want exceeds the range that the combination of camera and lens can achieve?

For instance the normally exposed image of the middle histogram above is shown below

middle exposed image of histogramThe sky is over exposed (you cant see this on the histogram as the range of pixels approaching the top level (for the sky) is very narrow). But as you can see from the image, you cannot see clouds,which you could with the naked eye at the time. You also cannot see any detail in the shadow under the bush to the right.

This leads on to a growing photographic technique called HDR Photography or High Dynamic range photography.

HDR photography is a technique where you capture three or more images and combine their mid tones into one image. This will give you the optimal exposure in Shadow, Mid tone and Highlight areas; hence the name High Dynamic range, as the entire image is correctly exposed as a mid tone in a way which cannot be captured by a camera and lens in one setting of aperture and shutter (see our article on the laws of reciprocity) due tot he higher range of brightness to darkness.

How do I do HDR Photography?

HDR requires you to take multiple exposures of the same scene from the same angle, composition and importantly the same aperture setting (eg same depth of field). The under or over exposing is managed with alterations tot he shutter speed. Another reason to use a tripod as you will not manage to hand hold the shoot and keep exactly the same angle for all images. Teamed the tripod with a cable release and you will have the exact images on each exposure. with little chance of shake during an exposure.

Set your camera on burst mode, after this is done find your cameras bracketing mode and set the quantity and breadth at which you would like your exposures recorded. This is normally in the menu but on some cameras there is a dedicated button, once this is set compose your image and take the appropriate amount of shots.

Keep in mind, all images will need to be presented to the HDR software later on at the same colour temperature.

HDR Software

To merge the 3 or more files and optimise the use of their mid tones you need appropriate software.

Adobe Photoshop contains an HDR Merge tool or alternatively you can purchase dedicated HDR Merging software such as Photomatix. These require little more input then showing the software where to locate your files and choosing your preference in regards to its luminance point.

 

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HDR in action

The HDR software of your choice will combine these images to produce one image which captures all levels of detail.

You will need to convert your image to a 16bit image after running the software, and additionally you will need to convert it to an 8 bit image to save it as a JPeg.

The image shown below is an HDR merge of three images tagen from this shoot

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