4. Bracketing and dynamic range of digital camera's.

If you use a digital camera you will probably mostly use it on automatic exposure mode. In this mode, the camera tries to estimate an overall average exposure for the picture. If you are in a situation with great contrast and lighting differences, like a dark church with light windows, or bright sun with deep shadows, this will mostly result in overexposed light areas, loosing details, and underexposed dark areas, again loosing details. This due to the (limited) dynamic range of digital camera's (see middle picture in table below).

To compensate for this you can also make an under-exposed and over-exposed picture of the same scene. The underexposed picture "will take care" of the details in the light sections, which were overblown in the original (see left picture in table below). The overexposed picture will now properly register the details in the darker sections. You can merge these pictures into one final picture taking the relevant information from all pictures and creating the best possible optimum. Making a "normal" exposure picture, one or more underexposed pictures, and one or more overexposed pictures is known as (exposure) bracketing. Most camera's today have an automatic exposure bracketing function. Almost all camera's have a manual exposure compensation of -2 or -3 EV and +2 or +3 EV.

-3EV exposure (underexposed) 0EV exposure (normal exposure) +3EV exposure (overexposed)
The final aligned and enfused picture.

There are two techniques of creating these pictures and ImageFuser (or actually Enfuse) is using the exposure blending (exposure fusion) technique, as Enfuse is based on this algorithm. It is best to use a tripod (and a steady subject) to shoot your bracketed series. In case you don't have a tripod (with you), and sometimes even in cases with a tripod, your images might not be correctly aligned. This will result in a smudged, unsharp final picture. To compensate for this we need align_image_stack. Align_image_stack will analyse your pictures and align them correctly on top of each other based on equal points in these pictures. Enfuse now can merge these "pretreated" pictures. ImageFuser uses both command line tools Enfuse and Align_Image_Stack and gives you a user friendly access to all options and settings of both tools.

See below an example of an enfused image from a handheld bracketed series, one not corrected with align_image_stack and the second one corrected with align_mage_stack.

© 2008-2010, Harry van der Wolf