Thanks for replying. I probably didn't explain my question very clearly. I am familiar with those blending tutorials (and others). Below is a sample of what I am talking about, with a 100 percent crop taken from an unsharpened D2X file converted in ACR. The first two frames show the same file converted with identical settings other than a 1-1/2 stop difference in exposure. The final frame shows the darker frame superimposed over the lighter one in Difference mode, illustrating that the images are not congruent along the horizon, even though they are the same image. The sky is one pixel larger in width in the dark version than the light one.
When I blend the two layers, it is usually possible to more or less obscure the "halo" effect either by using the gaussian blur effect or a soft edged brush or a gradient. Of course, the ability to obscure the halo comes from muddying the waters and using some of each image (or not blending at all near the areas of transition). The more one wants to take full advantage of the exposure differences between the two images (i.e., to present a properly exposed foreground right up to the horizon and a sky that is not overexposed right down to the horizon) the more difficult it is to obscure the halo. It can make for some fairly laborious editing of the mask in some situations.
In any event, assume I successfully obscure the halo by using some sort of blurred or soft-edged layer mask for blending. There is still some discontinuity in tonality from the halo present in the blended version but it isn't readily apparent because it it lurking just below our visual threshold. Unfortunately, that lurking halo can re-appear at the final stages of processing when it is time to sharpen the image. Sharpening of course accentuates any differences in tonality along edges in the image. The result often is an unrealisitic, oversharpened appearance along the horizon when I apply sharpening settings that are appropriate for the overall image.
I do often use the technique of making multiple exposures in the field. Of course the greater the exposure difference between the two images, the more apparent any halo will be. I deliberately chose the example of multiple conversion from one RAW file because I knew people would assume I had a problem with camera movement between multiple exposures. (I've experimented with HDR quite a bit, but I don't think the alignment feature works very well and I have yet to get results from an HDR that are as good overall as one of the more conventional exposure blending techniques. That's a topic for another discussion however.)
Anyway, I remain puzzled about why two conversions of the same file produce incongruent results and I am open to suggestions for easy ways to deal with it.