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They are two different ways to accomplish a similar task. HDR uses a lot of math to reverse engineer the photos to a simple exposure value and manipulates the data from that point. While "Exposure Blending" cuts parts out of the photos and sticks them together as if you were doing it via regular photos. We can't always choose which one to use, so we included both.
This is a really special file format as it allows the program to keep your exposures and all your settings together in a way that you can re-open and edit your HDR. Say you'd like to change your HDR to an Exposure Blended image, simply re-open it and change the settings. You can also use this format to share your HDR images with friends or family who might want to adjust them.
The "Exposure" slider in the "Inspector" is used to adjust the overall exposure of your HDR. If you increase the exposure, it'll brighten a dark HDR. It works the other way also, decrease the exposure and your HDR will darken.
HDRtist Pro, uses the Apple RAW convertor and so you can find a list of compatible cameras on Apple's website. It's different for Mac OS X 10.6 and Mac OS X 10.5.
Mac OS X 10.6 (Snow Leopard)
Mac OS X 10.5 (Leopard)
As for Mac OS X 10.4 Tiger. I cannot find one nice complete list, instead it seems like every update adds a little more.
HDRtist Pro features a RAW converter which'll try to convert RAW image data into HDR data. However, if you already have multiple exposures (even if they're in RAW format) it'll try to make the HDR from the multiple exposures as opposed to trying to use the data from a single image.
Select "Preferences" from the "HDRtist Pro" menu. In the Preferences window use the "Photo Organizer" buttons to choose which one you'd prefer.
In the Open dialog (or Add Photos dialog) of HDRtist Pro, scroll down the left column so you can see "Photos", click on "Photos" and then you'll see "Aperture" in the right hand list.
This file is used by HDRtist Pro to store your HDR settings when you save your HDR as a JPEG or TIFF. If you delete this file you'll no longer be able to edit these images in HDRtist Pro. This doesn't affect any "HDRtist Pro documents"" (when you choose HDRtist Pro document while saving).