Teaming ND Gradient Filters with HDR Software

With the rise in popularity of HDR software, many landscape photographers have opted to toss their Neutral Density (ND) gradient filters and rely solely on multi-exposure techniques. This is not limited just to HDR but also to "simulating" gradient filters in Photoshop (overlaying two exposures separated by a gradient layer-mask). I was slowly falling into this trend until some experimenting has convinced me otherwise. Now, as with an American Express Card, I'll never leave home without my grad filters.

Actually, I had sometimes used gradient filters and HDR together, especially when dealing with very high contrast scenes such as during dawn or dusk. Except, my procedures were inconsistent and I was always looking for an excuse to toss the filters just to avoid their hassle. However, I feel the best results are when using both and I have now established one simplified procedure to use in all my HDR shots. Now to be clear, when I say all HDR shots,  I’m referring mostly to landscape shots taken typically during dawn, sunrise, sunset and dusk. 

Some Past Habits
I found dawn and dusk most problematic when shooting in the sun's direction since the contrast between sky and foreground could be enormous. Often I'd shoot several exposures (usually seven) to span the entire dynamic range. The problem with too many exposures is I use a small aperture for maximum depth-of-field and this leads to long exposure times. When shooting more than three bracketed shots, even seemingly stationary clouds will move a little when the exposure series is stretched over one or more minutes. Yes you can use the deghosting feature in either HDR Efex or Photomatix, but this is what further contributes to the problem. It’s been my experience that deghosting in either HDR Efex or Photomatix readily leads to aggravating image problems. As a rule, I avoid deghosting like the plague.

For less contrast situations than dusk or dawn, such as at sunrise or sunset and shooting opposite the sun's position, I may have shot several exposures as just described, or instead use a 3-stop ND gradient filter with some minor bracketing (usually one or less stops). Then I'd experiment between the middle 3-stop grad filter shot or combine all three frames in HDR to see what worked best. The problem here is a 3-stop gradient filter can be problematic with creating dark areas or unbalanced illumination of foreground lake or river reflections.

I have now changed my procedure to limit the number of exposures to three frames for all situations (using my camera's auto bracketing feature) unless there is a compelling reason to do otherwise. 

Shooting During Sunrise and Sunset.

First let me describe a typical landscape scene of a bright distant sky and mountain range (either lit by a rising/setting sun or backlit by a below-horizon sun) with some sort of shadowed foreground subject (such as a river, lake, flowers, angry moose, whatever). I'm going to ignore situations when shooting directly into the sun, which is a whole different set of problems.

During sunrise or sunset, the sky will be very bright with the sunlit mountain peaks a little less so, and the shadowed foreground significantly darker. During my film days I could capture most sunrise or sunset situations such as this with a 3-stop ND gradient filter; and that’s still the case. So I still take a lone shot with just a 3-stop gradient filter when feasible. The reason is I can preview the shot (which you can’t do when shooting HDR) and have a safe backup in case the HDR version turns sour.

The pitfalls with relying only on a graduated filter are when the horizon is uneven and/or there are foreground lake or river illuminated reflections. Uneven horizons can lead to dreaded "dark zones" that scream grad filter (especially when using a large filter strength), or lake reflections may turn out brighter than the reflected image, which isn’t natural. In the past I tried to avoid these problems by shooting a long series of exposures, usually spaced 1-stop apart without any grad filter. Under "ideal" conditions, that was a good approach; but for most real-world situations, I feel only three frames spaced no more than 2-stops apart works best. 

To keep the dynamic range within the ± 2-stop bracketing range, I now use a 1-stop gradient filter. A light-strength filter like this avoids the dark zone problem and maintains proper light balance with foreground reflections (actually it improves balance). As always, I need to examine the histogram for each bracketed exposure to ensure both the highlights and shadows are pulled back from each histogram end-point by about one-third. If I can't capture the entire range for some unusual reason, then I'll switch to a 2-stop gradient filter. Conversely, if either the highlights or shadows are pulled back too far, then I narrow the exposure bracket and possibly re-center the mid-exposure. However, I think 2-stop bracketing with a 1-stop gradient filter should work in most cases.

Shooting During Dawn and Dusk

During either dawn or dusk, I’m usually shooting into the direction of the below-horizon sun to capture the magic light. This poses a higher contrast situation than a sunrise or sunset lit scene. In this case, I start with a 2-stop gradient filter first. There are various ways to figure what strength gradient filter value is best (e.g., using either your in-camera spot meter or an external meter), but I find taking a quick 3-shot series (starting with the 2-stop gradient filter) and checking the histogram is easiest. I may have to iterate a few times with bracket spacing and mid-exposure offset (as well as possibly switching to a 1- or 3- stop filter) to capture and center the full dynamic range, but it doesn’t take long.

Summary

So in a nutshell, my new procedure is to capture all my HDR images with only three frames in combination with a 1-stop gradient filter for sunrise/sunset scenes opposite the sun's position, and a 2-stop (or in certain cases a 1-stop or 3-stop gradient filter) for dawn/dusk scenes towards the sun's position. If I'm including the sun just above the horizon, then I'll have to shoot more than three frames. Finally, for sunrise/sunset scenes, I'll shoot a backup 3-stop gradient filter shot if there are no issues with an uneven horizon and/or foreground reflections. In most cases, I'll favor a soft-edge filter rather than a hard-edge. This is now my standard approach unless something dictates otherwise. Of course I know there'll always be an oddball situation, but my motive is to avoid over analyzing every scene and keep things simple so I don't increase the risk of screwing up.

I have to admit this originally ran against my original notion that several 1-stop bracketed shots were better than three, wider-spaced shots. But this doesn't seem to be the case and a lot of that has to do with inter-frame motion (along with the deghosting issues). Plus, on the practical side, extra exposures make the HDR processing time excruciatingly long (especially with HDR Efex Pro); and additionally creates that many more files to archive. 

Examples Photos

Below are a couple examples from my recent trip to the California Eastern Sierras this October. The first is after sunset in Yosemite’s Tuolumne Meadows taken with three, 2-stop bracketed frames. Here I used a 1-stop ND gradient filter since the overhead clouds kept the dynamic range down and I was shooting off-angled to the sun's position. Note that the sky and its reflection were adequately balanced. I had compared this image to a longer seven-exposure series that I also took, which suffered from cloud movement and deghosting issues. It was because of this and similar experiences that convinced me to limit, when all possible, my HDR shots to just three frames by using an appropriate strength gradient filter. 

The second shot is a straightforward sunrise shot of Convict Lake. I used a 1-stop gradient filter and three 2-stop bracketed frames. Here, as with the Tuolumne shot, the reflection’s luminance came out properly balanced.

Tuolumne Meadows

Convict Lake