speters wrote:
The only question I can see about this is, how many brackets one wants to do in a certain situation, stick to 3, do 20? But all that is a choice of personal preference (and should only be influenced by that)! I guess, a little depends on too, how wuch time you're willing to spend doing pp work/fun!
There is a rule of thumb, which can help determine the number of brackets necessary. For HDR, the lightest frame should have the darkest shadows present with detail, and the darkest should have NO blown highlights (apart from the sun or bright point sources of light). If the normal exposure meets those conditions there is no reason to do HDR, as normal post processing can give you any tonal spread that HDR tonemapping can.
If the dynamic range of the scene exceeds that of the camera, then you bracket. Now let's take a case where you need a frame that is 2 stops darker than the normal frame to keep the highlights from blowing out, and you need a frame 2 stops brighter than the normal one to retain detail in the deep shadows. In that case three brackets -2,0,+2 will be absolutely sufficient to do a decent HDR image. However in a more extreme case, in which you need the darkest frame at -4EV and the brightest at +4EV, then you will need five frames: -4, -2, 0, +2, +4. If you need -3EV to +3EV, I would go with -3, -1, +1, +3.
I have done experimentation to see if there is any advantage to doing brackets of less than 1EV steps, and my result has been that it is unnecessary. I have done 9EV brackets (from -4 to +4EV in 1EV steps) and done HDRs with every frame and with every other frame, and they were basically identical.
There is one potential advantage to doing 1EV or less brackets, and that is if you use Photomatix and need to do selective deghosting. In that case having more frames gives you that many more options for choosing base frames for the ghosted areas. Selective deghosting base frames have to be within 2EV of the optimal exposure for that section of the image to really succeed. If they are too dark there will be too much noise in those areas, and if too light you will have blown highlights, which look terrible. But if you do not intend to deghost, then steps of 2 EV are absolutely fine.
It's worth noting that including more bracketed images in the HDR process takes only a little more time in the initial process of making the .hdr image to process. Generally speaking, when I do an HDR image, I always shoot 9 brackets 1EV apart. I've had too many good images ruined because my darkest frame was too light and/or my lightest frame was too dark, which I discovered only in the processing. It's easy enough to delete frames you don't need, and you generally can't reshoot if you discover you don't have enough after the fact.