In a Flickr discussion, a poster asked whether he should get a hard stop graduated neutral density (GND) filter or a soft one. But wait, perhaps you would first like to know what a GND is so that I can explain to you why you probably DON’T need one. Here is a concise article, I’ll wait for you to come back.
In a nutshell, let’s dissect the words: Density means darkening – the denser the darker. Graduated means the amount of darkening changes from top to bottom. Neutral means that while darkening, the color is unchanged. A neutral density filter is one that darkens the scene to allow a longer exposure – e.g. of a waterfall or ocean to get the “silky water” effect. A good neutral density filter will not distort the color in the scene. The Cokin GND I’ve used, for example adds a purple cast (hue) to the captured image. Usually photographers employ a GND to darken the sky in a daytime scene so that the sky brightness is not so significantly different from the landscape. By darkening the sky a single exposure can capture sky details (e.g. bright clouds) as well as details in the landscape.
All that’s left now to describe is the difference between a HARD and a SOFT GND. If you read the linked article you already know, but if you didn’t, here is an easy way to disentangle the words: hard is ABRUPT while soft means GRADUAL. That is, a hard filter has an sharp transition between the dense area and the clear area while a soft filter has a smoother transition.
Can you use a GND at night? Yes! But normally you will end up using it in the opposite way of the normal use: e.g. to darken city lights at the ground but leave the night sky undarkened.
Here is an example where you might think of using a GND
The lights on the bridge deck and towers are very bright compared to the night sky. A GND in theory could be used to mask the lower portion of the bridge to control the brightness. And while it will work, it will not work well because somewhere there will be a transition between the darkened part at the bottom and the sky above. If there were no tower, the GND could be used to knock down the lights on the fences and deck with a relatively minor sacrifice.
This scenario repeats itself in daylight situations too. Imagine a lovely tree in your daylit scene instead of a tower and it quickly becomes clear that nature affords very few situations where there is a linear and sharp distinction between the dark area and the light area… the ocean being one clear possibility or perhaps a landscape in a flat-as-flat-can-be field in Kansas.
Here is a better candidate for use of a GND – but still far from ideal:While it is certainly desirable to knock down the excessive city lights there is no orientation for a GND that won’t also knock down the brightness of the mountain or a portion of the sky. A triangular GND – if it existed could control the overbright part but since every scene is different the chances of getting a well matching GND are slim.
90% of the time using bracketed exposures and High Dynamic Range processing techniques makes more sense than using a GND. Also by not using a GND there is no additional opportunity for glare, flare, reflections or color cast – and nothing additional to carry lose or break or spend money on!
But before I completely pull the plug on GNDs, here are a few extra tidbits you can employ to get the most out of a GND that you may have:
- You can turn a hard GND into a soft one, by moving it up and down while shooting. For this to work, you need a longish exposure and you must be careful not to shake your camera. You also won’t be using the filter holder.
- A lower tech approach that works is black carding. Instead of using a neutral density filter you use a flat black card. You hold it for a time (keeping it moving a bit) over the light part of the scene and then remove it. This is exactly analogous to the “dodging” and “burning” process used long ago by Ansel Adams when he wanted to tone down (or up) an area of a photographic print.
The effects of the GND can be duplicated on the computer – and better yet, without the tell tale darkening of things like trees and mountain tops that cross the transition area of the GND filter. Tools like Photomattix and Adobe’s Photoshop Merge-to-HDR can automate the merging of exposures, but to my eye the result is usually somewhere between bizarre and odd looking. A more effective process is to hand merge your HDR as described by Harold Davis in his book The Photoshop Darkroom 2 or his latest book dedicated to HDR processing - which begins shipping July, 2012.
GNDs work, but seldom does one find a bright area separated from a darker area by a straight line – whether gradual or otherwise. Trees, hills, mountains and man made things like to poke up into the sky and look “weird” when darkened via a GND.
The advantages to bracketed HDR photos are many, the drawbacks are few – and it’s one less filter, filter holder, and pouch to take along and one less source of additional reflections, flare, vignetting, or color problems.
In fact, I’m not really sure where my GND is anymore, or the holder for it which was a beast to manage.
- Jim Goldstein does great work and has an article on GNDs worth a read.
- Ron Niebrugge’s discussion, but also note the tell-tale dark areas where the foreground transitions to the water and glacier.
- Here is an example of a “hand HDR” technique which won me an international award. I’m quite sure a GND would not have “cut it” for this shot.
I did NOT use a GND on the Mary Avenue Bridge picture! I used a technique that will be covered in an upcoming webinar on Photo Manipulation for the Night Photographer. The very next webinar on photo manipulation is June 7, 2012.