If you enjoy taking HDR images, then you know that the more exposure reference you have, the better HDR image result will be as transition is smoother and more realistic looking.
So how many exposure reference is needed? Between 3 and 9 is optimal depending on image scenario.
With Nikon D90 (or D80), bracketing has 3 options: 3F, +2F, and -2F. For better HDR work, always set +/-2EV for F.
So by default, D90 can achieve 0EV, -2EV, and +2EV in one setting (3F). This is my default option when doing casual HDR images. But for serious or really extreme scenario, I usually employ 9 exposures. Here's my step:
1) Use a tripod to minimize post-processing alignment.
2) Turn OFF Auto ISO and set a fix White Balance value (or shoot in RAW to fix WB later).
3) Use aperture-priority mode. For landscape work, f8 and above (smaller aperture opening) is recommended.
4) Make sure exposure compensation is set at 0EV. See page 90 of Nikon D90 manual.
5) Make your composition. Focus and then disable autofocus, if necessary.
6) Set bracketing to 3F 2.0. Or 3F 1.0, it's all up to you. See page 92 of Nikon manual.
7) Press shutter button 3 times to take your 1st 0EV, -2EV, and +2EV images. To minimize camera vibration, use ML-L3 remote control or use burst-mode to take 3 shots with one continued button press. See page 64 of Nikon D90 manual on release mode.
8) Change exposure compensation to -5EV (or -3EV for 3F 1.0 bracket mode). See page 90 of Nikon D90 manual.
9) Repeat step 7. This results to -5EV, -3EV, and -7EV images. Tripod is generally needed on this low-shutter speed region.
10) Change exposure compensation to +5EV (or +3EV for 3F 1.0 bracket mode). See page 90 of Nikon D90 manual.
11) Repeat step 7. This results to +5EV, +3EV, and +7EV images.
12) Merge images in your favorite HDR software (e.g. Photoshop, Photomatix, Picturenaut, etc.)
1) It's best to work fast from steps 7 to 11 to minimize movements in the image like clouds, shadows, changing ambient light levels, etc.
2) Nikon D200, D300 and other high-end models can do 2-9 frames with just one setting but maximum EV is 1.0 only. So for D300 to take 4-stop range (-2EV, 0EV, +2EV), it has to take 5 exposures.
For any comments or suggestions, please let me know. Thanks!