One common question I see from photographers about lens filters is whether to get a hard or soft graduated neutral density filter. Which one is better? You’ve probably seen a lot of photographers recommend soft GNDs (if you can only choose one), since they’re more versatile when photographing uneven horizons. But that doesn’t capture the whole story. Instead, the optimal GND filter depends as much on your lens – wide vs telephoto – as the scene in front of you.
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Sample Photos
The easiest way to demonstrate my point is to show you a few sample photos. All of these are gray field images with a two stop graduated neutral density filter. In every photo, the filter’s transition point is in the center of the image.
In terms of post-processing, I corrected vignetting and made minor adjustments to equalize brightness. Otherwise, these are straight out of camera RAW files:
Do you notice anything interesting? When you’re zoomed in to 200mm, the soft GND has only the most minimal of gradients. What was once a specialty graduated filter is now acting as a watered-down standard ND filter! It provides little benefit aside from cutting out some light, which is usually more of a problem than a feature.
Note that I took all four photos above at the same aperture, f/11. But the appearance of the gradient also changes with aperture. Take a look at the two shots below, taken with exactly the same filter (2 stop hard grad) and focal length (70mm), only changing aperture:
As you can see, the optimal graduated ND filter depends on far more than just the scene in front of you. Your focal length and aperture both have important effects as well.
Focal Length Comparison
In addition to the extremes of 14mm and 200mm, I thought it would be useful to show how soft and hard GNDs look at some of the most common focal lengths in between. This way, you have a better idea of when each graduated filter is more useful than the other. Note that all of these shots are taken at f/11:
20mm
35mm
50mm
70mm
100mm
135mm
Note, of course, that a real-world scene will show these effects far more subtly. Even at the wider focal lengths, a hard GND is unlikely to have a noticeable line in the images, except for cases where the transition is over an empty area of sky.
When Should You Use a Soft or Hard GND?
If you’re shooting with an ultra-wide lens (anything wider than 20mm full-frame equivalent), a soft graduated neutral density filter is usually the way to go. Hard GNDs just transition too suddenly at such wide focal lengths, and they only work well for very flat horizons.
From 35mm to about 50mm, hard and soft GNDs both have a place. Something between the two would really be ideal; hard grads are a bit too harsh, and soft grads don’t have a massive effect. I would tend toward a soft GND when the horizon is broken by an obvious mountain, tree, or something similar. In most other cases, a hard GND will be preferable.
With telephoto lenses beyond 70mm, I really wouldn’t use a soft grad filter in most cases. They just don’t have enough of an effect at those focal lengths; it’s like not using a filter at all. Although hard GNDs are still fairly strong in the 70mm to 100mm range, real-world scenes will be much more accommodating than this torture-test of a gray field. For example, the photo below (taken at 100mm and f/5.6) is a good example of when a soft GND will have practically no effect, but a hard GND is quite valuable in equalizing the sky and foreground:
Conclusion
Hopefully, this article gave you a better understanding of how to use filters more effectively, specifically graduated neutral density filters. The typical logic of “flat horizon = hard GND, broken horizon = soft GND” is not totally wrong, but it’s based on the assumption that you are using a wide angle lens. Once you get to about 50mm, and especially at 100mm and beyond, the equation changes. Suddenly, even for mountain landscapes and broken horizons, a hard graduated filter is the way to go.
So, if you’re tempted just to buy one type of GND and not the other, make sure you consider more than just the types of subjects you plan to shoot. Your expected focal length range is an equally important factor, if not more so.