By Jeremy Lavender

 

HDR (High Dynamic Range) is a revolution in digital photography. It's a process in which multiple exposures of a same photograph are captured, aligned and merged to generate a single image that allows you to see a much wider range of colors and tonalities. HDR techniques may be used to enhance and create photographs from realistic to artistic.

 

When people are looking at a photograph taken and processed using HDR techniques, most will see at first a painting or even a retouched photograph using Photoshop or other editing image software. Convincing viewers that a HDR picture is a pure photography in all the sense of the term can be very difficult. The basic results obtained in HDR photography are nothing more than what you'll get if you could use your eyes instead of a digital camera to record a photography.

 

Let me explain a little further...

 

First of all, we need to understand what the HDR technique brings us. When using your camera to take a traditional photograph, your camera will record a certain amount of colour information, a ceratin amount of brightness information and also a certain amount of contrast information. Following the basic physic laws, the sensor of your digital camera can only capture a limited amount of all this information and again, far away from what the human eye could do. 

 

To take one single shot, the range of mixed information (colour, contrast and brightness) that a digital camera captures is called "Dynamic Range of the Sensor". Most of the recent digital cameras (DSLRs or DSLTs) have a Dynamic Range comprised between 7 and 11 stops of exposure. To compare, the human eye can easily recognize up to 15 stops of exposure. Anyway, when it comes to HDR photography, we are very concerned about capturing the widest possible gamut of contrast range. Actual digital sensors have a contrast of up to 2048:1 and your eye has a detection range from 1024:1 up to 16384:1. This is why at human sight we can detect and see perfectly the green tree leaves even under a high contrast of a very sunny day with a deep blue sky. Try to reproduce what you see at human sight with your digital camera and the result should be really deceptive as the beautiful and sunny blue sky will go to a plain white colour and the popping green colour of the leaves will turn black. 

 

Let's get a little more technical to really understand what I'm trying to explain above......

 

We have detected three different sources on the photograph of the green leaves under a very sunny blue sky. What are they?

 

So how to do to catch as much information as possible on each of the three areas described above?

 

HDR photography is the technique of taking several photographs, without moving the camera, of the three areas we just spoke of so as to get them as best exposed as possible. Recent DSLRs/DSLTs cameras allow users to select "Exposure Bracketing". Exposure bracketing is an automated function where the camera can record a set number of photos (3, 5, 7 or more) of one scene and at different exposure levels. Most digital cameras allow only three exposures within an exposure difference of 2 stops, meaning, if the base exposure is set at zero, then the 2nd exposure will be under-exposed by 2 stops and the third exposure will be over-exposed by 2 stops.

 

There are three important things to always keep in mind when attempting to do HDR photography...

Once multiple exposures have been shot, they need to be processed in software developed to create HDR images out of multiple exposures. Don't think it is as simple as taking different parts of an image and pasting it onto a single image. The process involves reading color, brightness and contrast information in the photos and merging that information into a single file with all the information blending in smoothly, as if it were all part of a singular image to begin with. This process is called 'Tone Mapping', where the overall contrast ratio is reduced to that of a normal photograph, but the local contrast of each pixel is maintained with respect to its neighboring pixel. Tone mapping offers a plethora of settings that control almost every aspect of the merged image, from amount of contrast to amount of saturation to the amount of 'blend' of the three (or more) images. This is where, as an artist, you would decide whether to keep the HDR photograph blend look realistic or turn it into a more personal artistic final result.

 

The best HDR photographs are created through a very arduous process, from the initial choice of the shot in the field to all computerized variations and strength levels of tone-mapping depending on the subject of the photograph itself and the final touch that’s exclusive to the photographer's artistic talent.

 

HDR photography existed in the era of film, where photographers would splice together negatives of different exposures to create one perfectly blended positive image, and now, the same technique is being carried out through the digital workflow. The process of HDR requires just as much creativity as it does technical skill, so the only way to master it, as one would in photography, is through a lot of practice.
 

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