Blending vs. Double Processing

After the discussion about using GND filters and learning about a thing called "blending," I checked it out and it looks interesting. Although I look forward to trying it out, it seems like double-processing -- if it works as well -- would be superior because there'd be no need to do an overlap of two separately made images (where some element of it might have moved between exposures). Is double-processing an alternative to blending, or something completely different? Or do you still need to blend a double-processed image?

Yes it is an alternative and the advantage is, as you noted, the perfect registration of the 2 images.
The down side is that a single capture may have artifacts like noisy shadows or (less likely) lack of highlight detail. Processing can make the shadow areas brighter or the highlights darker but will not solve the noise/detail issues.

Guy

I believe that with double processing, you're also limited to a smaller dynamic range of brightness values than with two exposures, since one RAW file is still limited by what your camera can capture in one shot to avoid over/under exposure.

So in something with, say, bright sun and very dark shadows, you could "fit" more stops into the image with two exposures. Of course, this is getting into HDR images, but that usually refers to multiple exposures. And much like HDR, results can look unnatural at times.

(Having not worked with RAW, I'm not certain how much you can get from just double processing....maybe someone else can enlighten us...)

Double processing is and excellent alternative to blending multiple exposures, but it is still going to require blending of the two differently processed images. The goal of double processing is to extract all the available data from a single raw file, but as Guy pointed out, there is only so much information available before the image begins to degrade. On the other hand, creating separate exposure with the precision needed for blending can be extremely frustrating (it's not unusual for it to be impossible). I've used both techniques frequently, and I'm always happy when I'm able generate an image from a single exposure, no matter how many times I need to process it. Still, in the field, I attempt to record compositionally identical photos at different exposures of any scene with excessive dynamic range on the chance that one raw file just won't have what it takes to hold details through the entire scene.

What I usually do is combining the use of physical GND filters with doble-processing (generating two images from a single exposure) to get the best of both worlds.

BR,