HDR Technique
The sensor of our cameras capture less tones that your eye might catch which is 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 we choose to correctly expose the main subject ,but almost certainly we loose the details of the sky by getting blown effect. Likewise if we try to properly expose the sky we will get the main subject underexposed and lacking in detail that will be dominated by the 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)
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 (see the Tutorial about Shooting without a tripod).
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 we want. Typically in the case of 3 pictures the camera will fire the first shot with the exposure that we measured, the second picture underexposed by n-stop and the third one overexposed by n stop. The 3 shots can appear approximately like these:
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To help taking 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 Qtpfsgui.
The composition of the HDR image takes essentially two phases:
- merging photos with emphasis on 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 our personal taste.
The HDR process inevitably involves the introduction of noise sometimes quite invasive and 'therefore you’re strongly advised to use a filter to reduce noise at the end of post production.
The final result of processing the 3 pictured above is this:
Enjoy your HDR experience
ScriS
