HDR ISO Bracketing
I’m sure others have tried this but I’ve never seen anyone really talk about it or recommend it. Basically instead of varying the shutter speed to take 3 to 5 photos for creating a HDR I varied the ISO instead. The results were way better than I expected. Now I only wish I had taken a similar set with the shutter speed changed to compare the two methods. If nothing else it says it can be done with good results for a night HDR.
St. Paul Skyline Using HDR ISO Bracketing
I’ll explain a little about the setup and process. I’m using a Sony A99 with a Zeiss 24-70mm 2.8 lens at 60mm focal length. Each exposure was f/9 and a 30 second shutter speed. I took 5 shots from -2 EV to +2EV. I simply started at the dark exposure with ISO 50 and doubled it to 100, 200, 400 and finally 800 for the brightest shot. Doubling the ISO changes the exposure by ~1 EV step. This was then combined in Photomatix and finally tweaked in Lightroom which is my normal process.
Benefits of the HDR ISO Bracketing method.:
1) The shutter is the same on each exposure so moving objects create similar patterns. That can help with ghosting issues. In this scene the 1st Bank sign rotates and flashes on different sides. The shorter exposures which are most important for that sign can end up not capturing the sign as it is lit.
The second part is the water and traffic which often create similar patterns over 30 seconds where they can be quite different in a 2 second shot.
2) The Zeiss lens is pretty sharp at f/4 but hits it’s stride from f/5.6 to f/11. The f/9 aperture is right in the sweet spot. In using HDR ISO bracketing I choose manual mode and set the shutter speed to 30 seconds and the ISO to 50. That means I just need to find the aperture which is f/9 in this case. You could use a shorter shutter speed for all exposures but 30 seconds works well with painting all that light on the water.
Issues with this Method:
1) Noise could be a problem depending on the low light ability of the camera. The A99 does a great job even at higher ISO and it’s full frame. A camera with poor ISO performance like my old A55 might have issues with this. That’s one reason I like 5 exposures or more at night because noise is reduced with more exposures
2) While this method is great for repeating patterns of motion it’s not good for more random motion. There were airplane trails all over the place that I had to deal with after the fact. This is an issue in any time you combine multiple exposures but in this case each light trail is 30 seconds so there’s more to deal with.
3) It takes a bit longer. At 30 seconds + long exposure noise reduction (dark frame) that’s 6 minutes of time for one HDR. A normal bracket would have taken about 2 minutes with the same 5 exposures. At this time of night there’s virtually no change in exposure so time isn’t as much of a factor.
Would I recommend this method. Yes, in certain situations. If your camera has good ISO performance give it a shot. It probably makes most sense to use this at night after the blue hour has ended and on a clear night. Hopefully I get a chance soon to test this further.
HDRealPhoto.com 
Very interesting. You have some valid points and good reflections on the patterns of motion.
I’ve never tried this before, but I will definitely experiment with it.
Thanks Cardinal. Would be interested to know how it works out for you and others.