It's been a while since I posted a somewhat educational post on my blog, but the topic of GND filters has been on my list for a while, so I decided to publish an introduction to GND filters.
I've reproduced it here for everyone's benefit, and hope that it will be useful.
Introduction to Graduated Neutral-Density Filters
Graduated neutral-density (GND) filters were used in the days of film to lower the contrast (sometimes expressed as 'dynamic range') in a scene.
When shooting a landscape or seascape image (especially when shooting towards a rising or setting sun), the difference in brightness between land (or sea) and sky can be significantly different, in the order of up to seven or eight stops.
The solution to this problem is GND filters, which are positioned in front of the lens, and optically even out the brightness levels between sky and foreground by darkening the much brighter sky via a semi-opaque darkened portion of the filter and transitioning to a non-filtered (ie, 100% transparent) portion of the filter.
Neutral refers to the grey, non-colour-altering property of the filter.
By positioning the filter such that the darker portion covers the sky and transitions to unfiltered where sky and land meet, the dynamic range in the scene can be reduced, which makes exposure easier.
GND filters come in various grades of darkness, measured in stops.
- GND2 or 0.3 = one stop.
- GND4 or 0.6 = two stops.
- GND8 or 0.9 = three stops.
One major brand even offers intermediate GND filters, such as 0.75 (2.5 stops).
Transitions are 'soft' or 'hard', with soft GND filters transitioning subtly from dark to light, and hard filters transitioning more abruptly.
Soft filters are best used for uneven landscapes (eg, mountains) and hard filters for horizons (eg, oceans or flat landscapes).
So, what does 'GND8' mean?
There is a simple formula for translating the filter nomenclature into the number of stops of filtration offered by the filter. The formula is as follows:
GNDx = 2 ^ y stops of darkness
- GND2 = 2 ^ 1 (ie, one stop)
- GND4 = 2 ^ 2 (ie, two stops)
- GND8 = 2 ^ 3 (ie, three stops)
Are GND Filters Necessary?
In the digital age, some people might propose, as an alternative to optical filters, exposure bracketing and blending of multiple exposures either manually, or using Photoshop's own gradient filters.
My own philosophy is that while you can do this type of work in post-production, attempting to decrease the dynamic range in a scene during the capture phase makes capture and post-processing easier in the long term.
Depending on the nature and intensity of the light and cloud cover, the use of a GND filter at the capture phase may eliminate the need to employ exposure bracketing and blending in post-processing at a later stage. In other cases, even stacking multiple GND filters may not produce a balanced single-frame exposure. I have certainly experienced cases where the sky is still blown out despite me having stacked my two-stop and three-stop GND filters.
My approach is to use both GND filters at the capture phase, and blending during the post-processing phase. I want to give myself the most flexibility and ensure I have a good range of exposures to cater for the dynamic range in the scene I have captured.
Recommended GND Filters
I personally use and recommend the Lee creative filter system. It is a modular system, consisting of a filter holder, an adapter ring (for mounting the filter holder onto the lens) and the filters themselves. Lee GND filters are 4 x 6" (100 x 150mm) in size, and made of resin.
A filter system such as this is quite handy, as the filters are large enough to cover ultra-wide lenses without introducing vignetting, and if you have lenses with different filter thread sizes, all you need to do is buy an adapter ring of the appropriate size. Adapter rings are quite inexpensive.
Unfortunately, the filters themselves are very expensive. I chose the more expensive Lee filters because I had heard good things about Lee filters, and I had also heard about, and seen, the magenta colour cast introduced by Cokin filters. Unfortunately the magenta colour cast issue is far from a simple case of brand, as I have personally experienced a magenta colour cast with my Lee filters when stacking GND and ND filters (both Lee-branded). It may be a combination of the colour of the light, the white balance, the filters and even the camera. The jury seems to be hung, but in my experience with Lee filters, colour casts have not been problematic for me with the exception of one dawn shoot.
Many people use the much less costly Cokin filters, which are more readily available, and have also shown themselves to work quite well. Someone using the Cokin system may never experience magenta colour casts. In my case, I decided not to risk it, so I opted for the Lee brand, which generally had much better feedback.
My recommendation is always to give yourself the most options when it comes to capturing images. You cannot easily add what wasn't captured, and in my experience, using filters can eliminate the need for exposure blending. However, I still bracket so that I have the most options, and I will mostly continue to blend exposures that were captured with filters.
Where filters can also help is in cases where there is movement in a scene. Wind has a nasty habit of blowing branches and leaves, and if these are in the sky portion of a scene, blending multiple exposures will be somewhat tricky, whereas the use of a filter can eliminate that problem.