Are Graduated Neutral Density Filters Obsolete in the Age of HDR?
Photography has a very limited range of exposure latitude. Overcoming that limitation has long been a challenge for outdoor photographers. But has the preferred method of extending exposure range shifted with advances in digital photography? Are Graduated Neutral Density filters now obsolete?
Every photographer who has shot outdoors on a sunny day has encountered this problem. Without some means of encompassing or compressing the wide range of brightness they are likely to encounter, either they sky will be rendered as burned out white or the foreground will fall into darkness. The difference in brightness between the two ends of the frame is just too much for either film or standard digital image capture to record in such conditions. The problem is most acute at times when the best conditions for photography occur at sunrise and sunset. During those magical "golden hours" the foreground landscape is deep in shadows while the sky above is well lit in amazing colors.
One way to get around this problem, and the only practical one available back in the age when image manipulation and editing wasn't easily available, was to compress the brightness range in the scene by blocking some of the light in the brightest area of the frame from reaching the camera's recording media. This is the principal on which the graduated neutral density filter (or GND for short) operates. Such filters are dark gray at one end ("gray" being the common man's way of describing what in technical parlance is referred to as "neutral density"), and clear on the opposite end. Between these two ends, gray density tapers gradually (the "graduated" part of the GND acronym). The theory is that if the transition between gray and clear is smooth enough the effects of the filter can be hidden within the frame by positioning it along the horizon line or other natural transition area. The good kind of GND filters are rectangular and fit into a frame made by Cokin or compatible manufacturer in a way that allows the photographer to slide the filter up and down as needed as well as to rotate the frame as a whole independently of the lens orientation to help with positioning the transition.
Over my years of shooting outdoors on film, I used to carry an increasing number of graduated neutral density filters. Filter companies make these things in varying strengths (densities) and various types of gradations from relatively abrupt transitions to smoother and more subtle but elongated transition zones. No one filter worked best in every circumstance. In some situations I even stacked up more than one GND filter to get enough density, or positioned multiple filters over multiple areas of the frame to knock down the brightness from both sky above and mountain lake reflection below and similar tricky situations. At the peak of my GND filter usage I used to carry up to a dozen such filters.
Even with such practiced dexterity in positioning graduated neutral density filters it was simply not always possible to achieve satisfactory results. As a common example, the horizon may bisect the frame conveniently into bright and dark ends, the trees, mountains and other desirable subject elements tended to be less cooperative. Any object that crossed the area where the transition zone of the GND filter fell would give away the use of the filter to a viewer familiar with the tricks of the trade. A tree trunk does not naturally start out well let at the bottom of the frame where the clear portion of the filter fell, and abruptly get darker as it crosses the horizon and goes into the sky. Although the gray portion of the filter positioned there helped bring the sky itself into the range of what could be recorded, it also made that tree trunk darker that it should be since it was effectively already in shadow, being backlit by that bright sky behind and around it. Such peculiar, unnatural shadow effects, particularly when crossing the horizon, are the unavoidable tell that gives away the secret of how the image was made. And once someone knows what to look for they can't help seeing this defect.
The use of graduated neutral density filters isn't limited to film photography of course. They do exactly the same thing when shooting digitally. With digital editing (either images shot digitally or film images that were scanned) the optimization process doesn't have to end when the shutter release gets pressed though. The judicious use of Levels and Curves in Photoshop and similar tools in other image editing applications allow the user to attempt to smooth out such tell-tale shadow transitions to make them less obvious. There are limits of course. If the camera captured nothing but shadow, there would be no detail there to be restored. You could increase the brightness, but it still wouldn't look right without bark or other detail on that part of the tree.
Digital editing techniques can also allow the photographer to completely bypass the use of GND filters in some less harsh conditions. If, without the filter, the sky would be overly bright but not completely burned out, it can be shot as is and then masked and adjusted appropriately in Photoshop later to produce a more pleasing rendition closer to what the human eye would see in nature. This wouldn't work if the brightness difference between sky and foreground were too great, but when it could be used it no longer mattered if the tree trunks crossed the horizon and such. One merely used appropriate selection and masking tools before bringing down the brightness of the sky in the digital darkroom.
Digital also lets the photographer combine two separate shots taken at different exposures, one correct for the sky, and one for the foreground shot at a slower shutter speed. Numerous techniques exist to help create pleasing results when doing this, although none that will work well in every case. And of course if anything in the scene moved at all between the two exposures, the difficulty in digitally blending them increases greatly.
If you've ever tried it, using any filter at all on extreme wide angle and fisheye lenses is nearly impossible so there are some limits to the use of graduated neutral density techniques that can't be resolved. On a fisheye in particular, you will be able to seen whatever filter holder you try to use. Even your delicate fingers can't be kept out of the way. In such cases, digital techniques are the only possible approach.
High Dynamic Range (HDR) imaging is, in a sense, an extension of digital blending methods. Two or more frames are exposed separately and then combined digitally later. Technically, true HDR photography involves 32-bit floating point file formats that differ radically from the usual 8-bit or 16-bit per pixel jpeg, PSD or even RAW formats, but the details of these differences are beyond the scope of this article. Suffice it to say though that when done well, the exposure brightness range that can be combined into a single image file is nearly limitless with HDR. Suffice it to say as well though that doing a good job of HDR is generally not an easy task. So much so that whole online communities of photographers exist who have taken the garish, haloed look of bad HDR and turned it into a new pop art sensation to be striven for. For some, the more bizarre, but still reasonably recognizable, the better. To me, the proper use of HDR should in a sense be similar to the proper use of graduated neutral density filters: both should yield images that defy common methods of capture yet still appear completely real and generally natural. To each his own though.
But regardless of the difficulties inherent in any given digital blending technique, all generally involve multiple shot capture. Yes, it is possible to take a single RAW image capture and convert it at different exposures to produce one bright and one dark resultant image that can then be digitally blended, but as you can understand, this still limits you to what a single digital frame can record as RAW. And whenever you are relying on combining more than one frame later, you can't really tell what you will end up with when shooting. The final image won't materialize until worked on digitally later. If you've tried shooting for HDR, you should already be familiar with the fact that no one image in a series looks very good as shot. If you show someone your unmerged images they'll likely thing you aren't a very good photographer. Hopefully though if you later show them what you did with those unflattering original images they'll be suitably impressed.
This is one area where the use of graduated neutral density filters in the field still holds a significant edge. You can see the results while still on location. With a digital camera, you can zoom in as necessary on the camera back LCD image review to evaluate the result and then retake as needed while still on site and hopefully still in close to the same light. In a sense, HDR returns photographers to the days of film where they aren't able to see the results of their efforts until hours, days or even weeks later, once the film had been developed and returned to them. I doubt many outdoor photographers if any take a laptop with Photoshop and other HDR software with them as they trek up the side of some alpine mountain vista, camera in one hand and computer in the other.
So the bottom line for me is that I still employ both approaches as they seem applicable. I no longer take quite so many GND filters with me on a trip because beyond a certain point I know I'm better off with the delayed gratification of HDR. But when the lighting conditions aren't too unforgiving, and the trees crossing the horizon aren't too problematic, I will still reach for my old trusty graduated neutral density filters and Cokin holder so I can tell what I have as soon as I shoot it.
No, graduated neutral density filters aren't obsolete, but they're no longer the only way to go beyond the range of brightness a camera can normally record with the press of the shutter release.