HDR creation on flashes without exposure information

Question

There is a challenge to shoot HDR of a subject in a studio, but the problem is that the subject is illuminated with flashes that flash momentarily in complete darkness. Because of this, it is not possible to change the exposure on the camera. Therefore, it was decided to change the flash power between shots for HDR. But we faced the problem that the software for creating HDR from a series of images requires information about the exposure of the camera. In our case, we only changed the flash power, and the camera settings remain unchanged. Therefore, it is not possible to calculate HDR.
As a result, the question arises, is it possible to calculate HDR in some software without information about the exposure of the images? Or does it all make no sense, and is this information required? And if necessary, then there are suggestions on how you can shoot HDR in such conditions?

Steven Kersting · Accepted Answer

Software like Photomatix will let you specify the exposure offsets for the images. If trying to use Adobe, then you will need to modify the exif with an external editor.
Alternatively, shutter speed does not affect the flash exposure and will allow you to bracket using flash power and cause an exif difference.
But your premise is wrong from what I can see... the subject being illuminated solely by flash does not prevent bracketing the exposure with camera settings. ISO affects the flash exposure, so you could bracket using ISO instead; and w/o significant penalty with many cameras as long as you don't go too far with it. Or you could bracket with the aperture setting which also affects the flash exposure... but that will affect the DoF which might be undesirable.
What strikes me as odd is the need to do HDR in the first place. HDR is really only useful when the dynamic range of the scene/lighting exceeds the camera's capability; but you are creating the lighting and dynamic range... it's within your control.

xenoid · Answer

A solution is to alter the EXIF data of the photos to make the HDR software believe that the exposure is different.
Normally, a factor of two on the power (halved or doubled) should entail difference of one EV (in the EXIF, better translated as a change is exposure duration, since this doesn't change the DoF in case it matters).
The problem is figuring out how accurate are the flash settings.
Otherwise, on the camera you can sill change the aperture, and if you don't want to change the aperture you can change the ISO.

HDR creation on flashes without exposure information

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