HDR is a photographic technique that allows you to expand the dynamic tones range in a photo (HDR stands for High Dynamic Range). The sensor of our cameras capture less tones that a human eye can catch and it’s impossible to reproduce all lighted details and shaded areas as our eye is able to see with a single shot .For example, if we are photographing a landscape usually we choose to correctly expose the main subject, but almost certainly we loose the details of the sky. Otherwise if we try to properly expose the sky we will get the main subject underexposed and all details will be masked by shadows. The HDR technique helps us to extend the tonal range of shots and get more rich details. To do this we need to merge more photographs of the same subject taken with different exposures. To create an HDR image we need at least 3 photos done in this way:

• a picture with proper exposure on the main subject
• an underexposed photo (usually -2 stops)
• an overexposed photo (usually +2 stop)

To increase the final effect it’s also possible to take more than 3 photos increasing or decreasing the exposure bracket depending on the subject you’re shooting. Pictures taken will be merged in post production so they must be taken with the help of a tripod or (in case you do not have a tripod) shooting freehand with some tricks to avoid unwanted movements and ghosts on the image. All modern cameras have the ability to set via  menu the exposure range between shoots (bracketing). Activating this function the machine will take a sequence of  3 or more photo automatically setting exposure by adding and subtracting the number of stops desired. Typically in case of 3 pictures the camera will fire the first shot with the exposure measured, the second picture underexposed by n-stop and the third one overexposed by n stop. The 3 shots can appear approximately like these:

To properly take the sequence set the camera for shooting in bursts mode instead of single shot so with a single press of the shutter  you can produce the entire sequence of pictures needed (especially if you’re shooting without tripod). At this point we have to merge our shots in a single image. To do this we can use the built-in Photoshop tool for HDR images or we can use a dedicated software to manage the HDR processing. The most used is Photomatix but you can find a lot of other programs including the excellent open source freeware Luminance HDR. The composition of the HDR image requires essentially two phases:

• merging photos enhancing areas of greater detail for each level
• Final photo postproduction working on levels and color saturation

Touching more or less incisively color saturation and curves you can obtain photos very similar to reality or very similar to a cartoon according to your personal taste. The HDR process inevitably involves the introduction of noise sometimes quite invasive and therefore you’re strongly suggested to use a filter to reduce noise at the end of post production. The final result of processing the 3 pictured above is this:

Let’s go out to take your first HDR!

ScriS