Shooting for HDR, And Whether You Even Need To
High Dynamic Range imaging is all the rage in some circles, extending the exposure of an image beyond what a camera is capable of usefully rendering in a single capture without editing. But doing a good job can be difficult and time consuming. Here are some thoughts.
To begin with, we need to agree on what we mean by HDR imaging. The typical range of brightness in the real world can be so extreme that it has long been troublesome for photographers to record. In the early days, photographers used dodging and burning techniques in the darkroom to return as much detail as they could to a useable range during printing. Anyone who's ever marveled at an Ansel Adams image can see the results. But there were limits to what was possible. The idea of combing multiple exposures into a single image dates back as far as the 1850's when French photographer Gustave Le Gray combined an image of the sea with a second shot exposed for the sky to create a seascape not possible otherwise.
Various ways of translating these ideas into the world of digital photography date from close to the beginning of the digital darkroom. But the digital HDR craze owes more to the world of movies, not still photography. The wizards at Industrial Light and Magic used floating point numbers to model objects in computer generated graphics. The CGI sequence could be modeled, and then the dynamic range compressed to where it could be printed to film stock as part of a special effects sequence in a motion picture. At the time, computers powerful enough to perform the needed calculations where quite expensive, and budgets for major motion pictures tended to be a tad more than what the typical still photographer, even professional ones, could afford. As computers became both more powerful and cheaper though, it was only a matter of time before HDR software and computers capable of running it started to appear that mere mortals could afford.
The problem was though that such early consumer HDR software wasn't really all that good. Sure, it could combine multiple exposures into a composite HDR image, but the tone mapping algorithms needed to compress the result back into something you could see on your screen or print out were crude at best. Sure, with enough tweaking you could get acceptable results, but most attempts came out with garish colors and over-exaggerated contrast. Suffice it to say, such images didn't look very realistic. But their distinctive appearance did give rise a whole subculture of people creating "HDR images" as a form of pop art. I use the term in quotes because such "images" simply weren't photography in any traditional sense. People creating them were actually trying to produce the sort of exaggerated appearance that traditional photographers were trying to avoid. This kind of "HDR images" isn't what I'm talking about here. Instead, what I want to discuss this week is how to shoot for HDR if your aim is to create images that still look like photographs. Hopefully this is the kind of HDR that you want to shoot too.
You're going to need a good tripod. Don't even try to shoot hand held if you want to combine multiple frames. Not only will the framing vary to some degree from frame to frame when hand holding, the relative alignment of objects could also shift if they aren't all shot from exactly the same spot. Shift your position by even a small amount and you will introduce parallax changes in how foreground objects line up with relationship to objects further away from the camera. If you want to merge your exposures together, I'm sure you'll agree that having everything line up properly would be helpful. Use a cable release or IR remote control too in order to avoid accidentally bumping your camera on one or more frames. You want everything locked down rock solid and stationary.
You also want to work as quickly as you can for the same reason. If anything moves between frames, even the most stable camera will record it in different positions. If the wind is blowing at all, your task will naturally be more difficult, but there's not much you can do about that. Some things are beyond your control.
Set your camera on fully manual focus and manual exposure. Do not use auto-white balance either. You want to keep all your settings exactly the same other than the exposure that you control yourself. To create different exposures, vary the shutter speed too, not the aperture. Shifting the aperture will alter the depth of field, creating changes in the sequence of images not related to exposure. Changes to shutter speed can be problematic when shooting moving water and similar subjects, but there are ways to deal with this after the fact that can't be done with depth of field changes.
Once you have your composition worked out and your camera locked down, it's time to figure out your exposures. Spot meter on the darkest part of the frame you want to have detail in. Then do the same for the brightest point in the frame. You want to start and end your sequence of exposures just outside the range you metered as a precaution in case you missed any points darker or lighter that you might later decide you care about. Especially on the highlight end, make certain you cover everything. Missed areas that are too dark will simply become shadows in the final frame. Areas you miss that are too bright will render as burned out spots that basically can't be salvaged. Most HDR tone mapping methods will create ugly borders around such points making them even more visible than otherwise.
After you've determined the range you will need to cover, it's time to decide how many discreet exposures you want to shoot between your dark and light endpoints. I generally try to shoot one frame every 1.3 stops which works out to four clicks on the shutter dial assuming the camera is set to 1/3 stop increments. If the lighting is tricky, you might want to shoot frames closer together, but the more frames you shoot the longer it will take to get from beginning to end, increasing the chances that something will move while you are shooting. Shoot frames too far apart though and you won't be able to blend them as well.
You'll want to shoot in RAW mode rather than jpeg too. Most software that can combine images for HDR supports RAW, and RAW formats are more forgiving of exposure problems There's no sense in making your task harder by working with a series of 8-bit jpeg files. It's a good idea to set your camera on a slow ISO speed too. No sense starting out with any noise that can be avoided.
So, now that I've covered all this, you're probably thinking "that's a lot of work," and you're right, it is. The obvious question then is whether you really need to shoot HDR. And the truth is that sometimes you don't. Especially with newer cameras, shots that used to be impossible without HDR can now be captured in a single shot. Even a few years ago, digital imaging was plagued by digital noise, especially in the shadows. In a return to the days of burning and dodging though, it is often now possible to pull usable detail out of shadow areas and thereby completely avoid the need for HDR compositing. I sometimes talk with people who shoot HDR because they think they're supposed to, not realizing that their nice, new camera inherently has enough dynamic range all on its own to get at least some difficult images in a single shot. Whether you need HDR or not obviously depends on your camera and your subject matter. One more reason it's a good idea to know what your camera is capable of.