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Documentation / How to calculate the Exposure Value (EV)

EasyHDR automatically calculates the Exposure Values needed in the HDR radiance map generation process. It is done upon the exposure parametres that are read out from each photo's EXIF headers (EXIF is read from JPEG and RAW file formats). If the exposure parametres (exposure time, f-number and the ISO sensitivity) are not available, easyHDR sorts the photos from the darkest to the brightest and assumes an EV for each photo. The middle exposed photo is assumed as 0 EV.

The knowledge of the proper Exposure Values is cruicial in the generation of a realistic High Dynamic Range radiance map with a sequence of photos. Important is the spacing between the values, not the values themselves. In example if the true EV for the sequence of 3 photos are: -2, 0 and +2, also the values (respectively) +5, +7 and +9 are correct.

How to interpret the Exposure Value?

Difference of 1 EV between two photos means that the one with higher value is twice as much exposed as the second one. Difference of 2 EV means 4 times stronger exposure, 3 EV - 8 times and so on.

Auto Exposure Bracketting (AEB)

Most digital cameras have an AEB option. When it's enabled a specified amount of photos is automatically taken by the camera in one sequence. Each photo is taken at slightly different exposure settings to meet the user-predefined EV spacing. The middle exposed photo (0 EV) is taken for the camera measured or manually selected settings. The photos with negative Exposure Values are underexposed as compared to 0 EV photo, while the positive EV means overexposure. The AEB option is mainly used to take a series of photos in hard lighting conditions when it is difficult to assume best exposure settings. One of those photos simply must be properly exposed. In HDR photography it is used to acquire the full sequence of differently exposed photos as fast as it is possible, with minimal differences in the photographed scene (i.e. moving clouds).

In order to take the best photo sequence for HDR processing only the exposure time should be tweaked. Modification of the f-number causes change in the depth of focus as well as in vignetting. Modification of ISO gives little flexibility and also the higher the sensitivity, the image quality is lower due to noise. Cameras like Canon 350D sweep through exposure time as well as f-number when the AEB is enabled for P (full automatic) mode. The manual mode should be used in order to have the f-number fixed.

How is the Exposure Value calculated?

Why is there a difference between the EV calculated by the camera and easyHDR?

The camera stores the EV information for the photos taken in the AEB mode. The photographer predefines the EV spacing at which the photos should be taken in a series, however the camera is not able to meet the requirements accurately. The exposure time, f-number and ISO can be set only to some fixed values. In example the exposure time cannot be set to an arbitrary value. A photo can be taken at exposure time of 1/4 sec, but if we want to take another photo exactly 4 times less exposed we have to use 1/15 instead of 1/16 sec. See the example #1 in the above calculator and compare rows 2 and 3.