Postby mikemeister_admin » Mon Mar 19, 2007 3:20 pm
I've learned more about HDR than I ever cared to by having to take pictures of the inside of churches for my lighting engineering job. Since there is almost nothing on the Web that is useful on HDR, I'll give you my experience that may be useful.
The first rule of HDR is to only do it when you HAVE to. If you don't have to, you'll almost always get better pictures. Why that is true is explained in how HDR works.
High Dynamic Range photography is the art and science of combining multiple exposures of one scene to capture all of the detail in the highlights and shadows of the scene when a normal, single exposure can't. This is done by capturing multiple exposures that properly expose all relevant parts of a scene. It takes a 32 bit color file to hold all this information. These are then blended or combined using Tone Mapping. Blending and Tone Mapping squeeze the HDR picture down to 8 or 16 bit files that are actually useful.
Very often beginners in HDR try to use HDR on scenes that don't need it. This typically gives very flat and ugly pictures. So, first know what the dynamic range is of your camera. Know what it will capture in a single exposure. Then you need to know what you need to capture in the scene. Many technical photographers believe they need to capture detail in absolutely every part of the scene. This may or may not be true. You do have to be honest with yourself about what is needed to tell the story. Sometimes letting parts of the scene go detailless as pure black or white may be better for telling the story.
Once you know what your camera's dynamic range is, you should notice that it is fairly wide. It has to be to capture good pictures in normal lighting scenes - like sunny days. The camera was designed to handle most of these situations without having to use HDR. You may be surprised to learn that there are fewer HDR scenes than you thought. A good example of a HDR scene is what I've been shooting. The inside of churches tend to have VERY dark places. At the same time they can have VERY bright light streaming in the windows. I also try to capture the light sources themselves (the bulbs) that are also VERY bright in relation to the dark places.
Almost any indoor scene where you need to capture what is outside the windows is a pretty good HDR scene. The same could be said for the reverse - an outside shot of a building where you have to show what is on the inside. Any scene where you have to show the light source and what it is illuminating at the same time is a HDR scene. A thick forest with a shaft of light stabbing down into it would also be a good HDR situation. A heavy storm that is breaking up and letting sunlight hit parts of the scene would also be good for HDR. I hope these examples help your visulization of what is a HDR scene.
Another thing you learn by knowing your camera's dynamic range is that one stop of exposure variation isn't enough. If your camera will cover 4 to 6 stops of exposure, one stop is much too fine of a jump. You will probably find that 2 or 3 stops will more than cover exposure jumps for HDR. One stop jumps will just be wasting your time and filling up memory.
If at all possible, do not use Photoshop's "Merge to HDR". It isn't very good or very easy to learn. Many people have been confused by it and no one that I know makes good HDR photos from it. If you can, buy Photomatix Pro. It is much better, easier to learn, and has more options.
I don't know all the science behind the two different merging methods, but I find that blending usually works better for my exterior shots while Tone Mapping works better for my interior shots. I'm not sure why, but 95% of the time this rule of thumb works for me.
I think part of the reason is that Tone Mapping can do some weird things to photos. One is that it can make big, fat halos around things. These often show up at the boarder of the sky and other objects. Frankly, these don't look very good. Tone Mapping can also create very surrealistic and "painterly" affects. Many people instantly see these photos as "not real".
The key to avoiding these affects is to do the very minimum of Tone Mapping. Watch your histogram carefully when you are adjusting all the settings. Remember that you are trying to squeeze the high dynamic range of the 32 bit color picture down into an 8 or 16 bit picture that is actually useable. A mistake that most beginners make is to squeeze it down too far. This makes very flat and ugly pictures. You want to squeeze it down only as far as you still have black and white in your picture or close to it. i.e. Use all your histogram.
This is key to getting photos that still look like photos. This is important for Tone Mapping, but it should be used when you do blending too. Hey, it's a rule of good photography for many pictures - usually you want as much contrast as you can in the picture. OK, the 32 bit contrast of the HDR picture is way more than you can see on the screen, your printer, or anywhere. That's why you do the second step of squeezing it down into that 8 bit picture. (I use 8 bit to fit the lowest common denominator and because I don't SEE any difference from 16 bit.)
A key reason why you only want to use HDR when you have to is that is often screws up the color. The reason is that the dark and light spots in you scene are often lit by different color sources of light. This is particularly true for the scenes that include both indoor and outdoor lighting. The reason is that Tone Mapping in particularly only uses the correctly exposed parts of each exposure. So your poorly lit, yellow incandescent indoor spot is a very different color than the very blue outdoor spot. In normal dynamic photography these aren't captured together. In HDR they come out at the same exposure level.
As wonderful as Curvemeister is for color correctly, it doesn't really work to overcome this. The reason is that Tone Mapping has created an unnaturally colored scene. (An assumption of CM is that fixing one or a few spots in the picture will give you correct colors for any normally colored scene.) I use the paint chips and pins in CM to easily and quickly color correct for all kinds of lighting in single exposures. It is amazing how accurately I can nail the correct colors in a picture. However, this has never worked for me in HDR. There is always some color in the picture that is thrown out of wack. Sometimes it is only a little bit and I can manually adjust; sometimes it's a pain.
I don't use the pins in HDR photos. I always have to manually adjust. I can usually get something that looks pretty good. Of course, I shoot more of those interior shots that you may. The fewer outdoor scenes I've shot typically don't have the color problem. It helps to have only one light source - the sun. Then again, I don't find that many outdoor scenes that need HDR.
One other tip is the change your exposures by changing the speed and not the aperature. Changing the f-stops will change the depth-of-field. This can cause shifting in the positions of things and they won't line up in the computer. Oh, and always use a good tripod.
To summarize:
* Only do HDR when you have to
* Know what your camera's dynamic range is
* Evaluate the story you are telling to be sure you have to have to use HDR
* Shoot exposures 2 or 3 stops apart
* Use Photomatix Pro - if you can
* Try both blending and Tone Mapping and many variations of the settings
* Use Tone Mapping to the minimum for as realistic photos as possible
* Squeeze the 32 bit HDR down only to the widest range that the 8 bit histogram will go
* Watch the color of your light sources
Other than that, you need to give yourself a lot of learning time. My suggestions should help, but you've got a lot of experimenting ahead of you. This is a new and very different area of photography that isn't documented very well. So, shoot, blend, Tone Map, and try every possible setting combination that you can. You'll begin to learn what works best for your type of photography.
HDR isn't the perfect solution to anything. It has its flaws and issues. However, it can be the ONLY solution to some photography. When it is, it is great. Learn to use it to the best you can.
Clyde