Online Help for Chasys Draw IES: Processes : Stacking : Merge to HDR

Processes : Stacking : Merge to HDR

What is Merge-to-HDR?

Merge-to-HDR is an image stacking operation that combines multiple images taken at different exposure levels to give a resulting image with a greater dynamic range than any of the individual source images.

Most cameras can't capture the entire range of tonal values available in a scene that combines both bright and dark areas (e.g. sky and shadow in the same photo); the photographer has to adjust the exposure setting of the camera to optimize details in either the bright part or the dark part at the expense of the other.

The Merge-to-HDR function allows the photographer to take multiple shots of the same scene at different exposure levels (called “bracketed exposures”), then combine these into a single photo that captures the entire tonal range of the scene as shown below:


The photos on the left are the originals, while the one on the right is the result of Merge-to-HDR. The resulting image captures both the cloud details (absent in all but the darkest image) and the details of the grass in the shadow (absent in the darkest image).


The “how” bit

You need at least 2 same-size images of the same scene to run this engine. It is of paramount importance that the camera doesn’t move between the shots; for which reason the use of a tripod is highly recommended.

The idea is to take several photos the same scene with varying exposures and merge them all to form one HDR photo. So, in theory, the photos can be taken with any camera that allows manual shutter speed adjustment. The exposure between shots must be changed with shutter speed because changing the aperture would change depth of field. Other than that, all other camera settings must remain the same.

It is recommended that you space the brackets by 2EV, above and below. 3 images should be enough for some situations, but you can use 5 or even 7 if you wish. Going beyond 7 brackets is over-kill, but Chasys Draw IES accepts up to 32 brackets.

Exposure Values (EV) or “F-stops” are a measure of exposure in photography, with each F-stop being a doubling or halving of the amount of light being received by the recording medium, be it film or a digital sensor.

Some digital cameras include an Auto Exposure Bracketing (AEB) option. When AEB is selected, the camera automatically takes three or more shots with a different exposure for each frame. This can be very useful for shooting HDR scenes because it eliminates the need to manually operate the camera between shots (reducing the risk of moving the camera) while at the same time reducing the time difference between shots (reducing the effect of motion within the scene being captured). Where available, the use of this feature is recommended.

AEB was not really intended for HDR shots initially, but rather for ensuring that at least one of the shots will be as close to perfectly exposed as possible. This is probably why some camera models only make it possible to auto bracket three shots at a maximum of one EV step difference or even less. Unfortunately, three shots spaced by one EV are often not sufficient for properly taking HDR scenes. For this reason, it is important to look at the settings of the Auto Exposure Bracketing features of a camera model to determine whether such camera will be convenient for HDR shooting.

This is the GUI for the Merge-to-HDR feature:

The sliders for exposure and gamma operate on the high-precision summation data to minimize rounding errors.

It is of paramount importance that the camera doesn’t move between the shots; for which reason the use of a tripod is highly recommended.



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