It's fairly well known that the more close-together exposures you can supply a tonemapping algorithm, the better the final output will be. You'll get more natural rolloff off highlights and shadows, better colour gradation with less posterisation, lower noise, and generally more information to play with when cleaning up the tonemap into a final image. Here's a quick trick to get around the limitations of auto bracketing still found on many entry- and mid-level cameras, particularly in the Canon lineup, without having to install custom firmware.
If you've never used the custom modes on your camera dial before, this quick tip should help explain how they work and what they're for.
1. Set Up Your Camera
Set up your camera exactly as you would for HDR photography; aperture priority or manual, f/8 to f/22, minimum ISO, two second timer, etc. These settings will be the basis for the three custom modes we're going to create.
2. The Bracketing Math
Now, go into the menu and turn on auto-bracketing. What we're going for here is to get as many exposures around one stop apart as possible. How great a range you can cover depends on your camera's auto-bracketing ability and how many stops of exposure compensation it has.
On my EOS 40D, I only have three bracketing points up to 2EV apart, and +/-2EV of compensation. Not a lot, as will be the case on many older model cameras before HDR became more popular. This means I have a maximum possible range of -4EV to +4EV, which is a nice 9-stop range, ideal for what we're doing. Unfortunately, I can't access all of it using this technique because I can't get to +/-3EV on the compensation dial, and any other method leaves me without certain stops.
So instead, I fudge it slightly and do 1.3 stop intervals at -2, 0 and +2EV, which gives me a total sequence of -3.3, -2, -1.3, -0.6, 0, 0.6, 1.3, 2, 3.3 EV. If I made it asymmetrical, I could probably spread a little wider, but since I don't know in advance whether I'll be shooting night HDR, where I need to dig into shadows with high EVs, or daytime HDR, where I may well be shooting into low sun and have to recover all of those sky details, I prefer to have it symmetrical.
I could do a dual-bracket and single shot 7-step sequence of -3/-2/-1, 0, +1/+2/+3, but I prefer having the nine-shot sequence with the slightly wider range.
On the T3i, the camera allows three-point bracketing at up to +/-4EV, so the full nine-stop range is possible. Other models and manufacturers will vary.
3. Dial in Your Customisations
Once you've figured out your sequence for maximum range, minimum step size, and what your camera will allow, set up the auto-bracketing to the correct amount (1.3 stops for me) and then set your exposure compensation to the lowest number you're using, for me -2EV.
Go back into the menu, head to Camera User Setting or equivalent, and set the current camera settings to Custom Mode 1.
Back out of the menu, move your compensation to zero, then set those settings to Custom Mode 2. Repeat for the higher exposure bracket for Custom Mode 3. You now have all three sets of brackets at the tips of your fingers, without knocking the camera to change your compensation.
4. How to Use It
Out in the field, meter off a midtone object in your scene, and the camera should take care of the rest. Just turn the mode dial to C1, hit the shutter, then C2, three more clicks, then C3. Within seconds, with no camera movement as long as your tripod is solid, you should have seven to nine identical images at reasonably close exposure intervals. Don't worry if they're not in exposure level order, tonemapping software takes care of that for you.
Turning the mode dial is less of a rotational force applied to the camera body than clicking the exposure compensation dial, and thus is much less likely to move the camera between frames, requiring much less image alignment from the tonemapping software.
5. Buttery Smooth Rolloff
Once back on the computer, if your bracketed sequence includes any duplicate exposures, delete any duplicates so that you only have one at each level, to eliminate unnecessary processing. Now load them up into Photoshop, Photomatix, Luminance, or other tonemapping software. You should notice a fairly significant difference, particularly in high-contrast areas, between the nine-image tonemap and your old three-image tonemaps.
I haven't shown it here, but the result of the three-exposure image had much more contrast in the right tree, and was less smooth and painterly. Compare the two at 100%, and you can see noise and banding decreased, and you should have less tidying up to do when you take the tonemap on for final processing.
That's all there is to it! It's an old trick, but still useful in many cases. Even if you're limited to five brackets, you could still use this in two of the custom modes for a 10-stop sequence, rendering those contrasty edges nice and smooth.
Questions? Comments? Hit up the comments below!
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