Monday, August 27, 2007

HDR Image Processing -
I have recently had some discussions about HDR image processing and its pro's and con's. Most folks that are doing HDR (High Dynamic Range) processing on their images are using a product called Photomatixs. This is a great program when used correctly but it can offer up some strange and unnatural results with its tone mapping features. Photoshop (I believe starting in ver. CS2) included the ability to create HDR images but most folks don't really have any idea of how to use it to achieve the best results. I know that the first time I tried it in CS2 without any guidance I was very disappointed with the resulting image. First of all, I wasn't aware that the 32-bit image that was being displayed on my monitor as a result of combining my bracketed images could not properly be displayed by my monitor. I also was not aware of what was possible by playing with the adjustment curve when converting it to a 16-bit image. After stumbling across some great web site tutorials, I found that I too could get some great imagery by using this technique. The best results come from scenes containing a wide latitude of tonal ranges where you really want to pull out and keep all of the shadow and highlight details that you couldn't possibly capture in a single exposure (picture a dark object in the snow). If you take a median exposure you will, in essence, get a nice flat image. By taking separate exposures to capture all of the highlight values and then another for the shadow details, and finally one more for the mid-tones, you can then blend all of them using the HDR function in Photoshop or Photomatixs. I would suggest shooting in the RAW image format to preserve as much image detail and tonal range as possible (the Merge to HDR function in the Automate menu will still process JPEG and TIFF images if you don't have RAW images to work from). Also, if you really want to get some great flexibility, try using 5 or 7 bracketed images. It will take some time to process them all but your dynamic range will be phenomenal. Finally, you can actually produce an HDR from a single image but you most definitely need to be shooting RAW for this. You can process the RAW image in an editor of your choosing, such as Adobe Camera Raw, and save three differing exposures of the same image by adjusting the exposure slider (just make sure you don't change the color temperature). You can now proceed as if you shot bracketed exposures. Just for the record, you will not achieve nearly the expanded range and depth of exposure from using one image and your resulting image will also contain more noise than it would from using multiple exposures. This technique works best if you want to create an HDR from subjects that tend to move. The problem with bracketing exposures of, say, a landscape with big fluffy, fast moving clouds, is that the multiple exposures will not align well due to the moving clouds. This is definitely a case where a single exposure will give you the best results. (The last WAVE image I posted on here last Thursday is an HDR rendering from a single RAW exposure. The HDR image was generated by Photomatix and then processed in CS3)

Ok, so now that I have gone on and on without telling you how to do any of this, I will point you to some nice tutorials written by folks that have already done the hard work. If you are using the Photomatix software, or are thinking about it, try out this tutorial on the Vanilla Days blog site. It is very well written and gives some suggestions to setting those mysterious radial buttons and sliders.

I also found two excellent sites for processing HDR in Photoshop. The first is from Luminous Landscape which is also a great site for anyone that shoots landscapes or outdoorsy (technical term) images. Another outstanding tutorial was written by Ryan McGinnis on his blog, Backing Winds. His is one of the best step-by-step tutorials out there and will certainly get anyone with CS2 or CS3 up to speed in making great HDR images. So now that you are armed with all of this information, go grab your tripod and start bracketing those exposures!

2 comments:

ThiagoNZ said...

Hi Jeff,

Thanks for this post. There is also a very famous tutorial on HDR with Photomatix by "Stuck In Customs". Here is the link:
http://stuckincustoms.com/2006/06/06/548/

Jeff Revell said...

I have read the "Stuck in Customs" tutorial and you are right, it is very good I actually have the site bookmarked in my Firefox toolbar). The only thing about his shots are that they reflect that extreme halo look with unnatural like saturations that I referred to. That is pretty much his signature style and it works for him. I guess my point was that there are ways of using the same tools and achieving accurate looking results as well.