People are excited about HDR and, for good or ill, it appears to have some long legs. Until it becomes yesterday’s effect or tomorrow’s standard, you need to understand some basic HDR processes. These will help ensure that you are creating a great image-not just an effects demo.
Just because you are going to capture a bracketed series and use a ‘wow’ technique like tone-mapping, doesn’t mean you will automatically create a great work of art. Several things are requisite to the image capture in order to complement that ‘wow’ factor.
Good technique in the field is paramount. This means
- Use a tripod
- Use a remote release
- Use mirror lockup
- Use a sharp aperture for your lens and still get the DOF you require
- Select a Shutter Speed for the conditions-primarily wind
- Turn off any stabilization (unless you opt to hand-hold)
- Focus and then turn off autofocus
It also means that you evaluate your histogram. Many people make the mistake of coming to a scene and judging exposure from the normal metered exposure. Looking at the image and its associated histogram, all you can tell is that you are blowing highlights and plugging shadows. The histogram suggests an HDR solution but it doesn’t tell you how many exposures or what bracket separation you need.
I always start by using a test under exposure. I set my scene (more about that later) and then I use the exposure compensation to set the exposure downwards in 1 stop increments until I have captured the highlights. Notice the left side of the histogram has a little room to spare before blowing them out. I want a 1/3 stop safety margin-minimum here. Let’s say that I was –1 2/3 stops to retain highlights. (I know the example shows –2 stops but that is explained in the next paragraph.)
Then I crank my exposure compensation upwards until the shadow section of the histogram is almost a full stop from the left edge. This ensures that I will have good shadow detail and have negligible noise. Noise as we all know, lives in the darkest stop of the image. For the sake of argument let’s say this was +1 stops. For the sake of argument you ask? The reason I am not more precise is that I discard these two exposures in the field and no longer have them as examples to show. That is why the under-exposure example was off by 1/3 stop earlier.
Here is where we have to do the math… (sigh). I have an exposure spread on this particular camera (Canon 5DMKII) of 2 2/3 stop (found by adding the over and under from the above example. Splitting the difference means I need to bracket by 1 1/3 stop to cover the range. But, I also know that my exposure was offset by 1/3 stop to the under-exposed side so my exposure bias during bracketing needs to be -1/3 to compensate. Looking at the first histogram, it seems obvious now. We have a solid wall at highlights but are just touching the shadows.
I know I can get the entire exposure in three shots because my difference is under 4 stops (for this particular camera, which does ± 2 stops). I give myself an extra 1/3 stop for subtle changes in light and set my exposure compensation to –1/3 and my bracket to ±1 2/3.
Just because you are shooting HDR doesn’t mean that composition goes out the window. Examine the shot. I have placed the horizon at the upper rule of thirds line, put the buildings (our center of interest) offset and to the left with the brightest part of the sky behind them, and we have used the leading line of the river to pull us into the shot. A weak composition would have given us pretty colors but not held together.
- Carefully place your horizon
- Lead your eye into the brightest spot on the photo
- Have something interesting to view when you get there
- Keep out distractions on the sides, front, etc.
Here is a shot of the Photomatix dialog showing the settings used for the tone-mapping of this image. You can see that we didn’t go all-the-way like so many people do when they first become enamored with HDR. Backing off on the controls-especially saturation keeps the image stunning-yet real. I also know that I am going back into my image editing software where I can execute finer control so I don’t want to go overboard in this limited environment. Get the tone-mapping done and get out to a more powerful tool is my mantra.
The tone-mapped results show me an image ripe with possibilities. I have all my shadow detail, all my highlight detail and I have a good composition from which to work. I like my tone-mapped image from the HDR software to look just like a raw image unprocessed. I think this one fits the bill quite nicely.
A few adjustments later, a minor crop and voila!
Remember, your workflow can and should vary. This is the technique that works for me. Quickly in the field, I can find the over and under on exposure that will keep me will within the bounds of highlight and shadow detail and the balance point at which to set my exposure compensation. Following good technique I can then create an image both pleasing to sensibility and stunning to the eye.
Rikk Flohr © 2010