What is a Graduated Neutral Density (GND) filter? How would I use one?
GND filters are a square or rectangular filter.
They are 50% dark grey, and 50% clear.
The dark grey half will reduce the brightness over the area it is positioned, whilst maintaining an exposure for the clear part.
GND filters are useful for sunset photography as they darken out a bright sky.
When using a GND filter, you would expose for the foreground. This is the part that is covered by the clear part of the filter.
The GND filter therefore creates an even exposure. The foreground is bright and well lit, and the sky has been darkened by the dark part of the filter.
You need to make sure you are using the correct filter, and that it is aligned correctly. This might take a little time to perfect!
What are the different types of graduated filter?
There are two different types of graduation within these filters. One is a "soft" grad and the other is a "hard" grad. The soft and the hard refers to the transition from dark to clear - a soft graduated filter has a feathered, gradual transition that fades from dark to clear. A "hard" grad filter has a sold clean line that makes a quick transition between dark and clear.
How do you know whether to use a soft or hard grad filter?
- A hard grad filter should be used in a landscape that has a very clear defined horizon, for example seascapes or flat land.
- A soft grad filter is useful where the horizon is not a straight line, providing a gradual transition between shades.
What about different densities?
Graduated filters are available in different "darknesses" - the photographer is able to choose which darkness to apply, dependent upon the conditions. For example, for a really vibrant, bright, sunset sky a darker filter might be chosen. If the sky is not that bright, and the foreground quite light (i.e. the difference between the brightness of the sky and foreground is quite minimal), a different filter could be used.
The darkness of the filter is referred to as density.
The density of the filter is referred to in terms of "stops". I.e. the density (darkness) of the filter reduces the exposure in the brightest part by a certain number of stops.
These "stops" are the equivalent to the number of exposure stops in your camera. That is the difference in the exposure between the foreground and the sky. For example, the exposure for the sky is two stops faster (brighter) than the exposure of the foreground.
Graduated neutral density filters are available in a variety of densities, the most common being a 1 stop, 2 stop and 3 stop.
I use Lee Filters, and they refer to the filters as a 0.3 (1 stop), 0.6 (2 stop) and a 0.9 (3 stop). I have no idea why they call them that instead of one, two or three stop - maybe someone can enlighten me!!?
How the filters can be applied - some examples!
Typically for me and a bright sunset sky, I use the 0.9 GND the most. This is mostly used after the sun has gone down.
You can stack more than one filter if required, sometimes for a really vivid sunset I might use a 0.9 and a 0.3. Very occasionally I will use a 0.9 and a 0.6 filter together (but I can only think of a few times I have done this!).
My graduated filters:
0.3 stop soft GND
0.6 hard GND
0.9 hard GND
To create a softer transition I will sometimes use a hard and a soft together. A 0.3 soft grad is useful for me for this as I can position this filter further down in the frame to soften the transition between the darker hard graduated filter and the clear area at the bottom of the image.
Any problems with GNDs? Any tips?!
- I always position my darkest filter on the front of my lens - just so that I know which one to move when I need to. The softer/lighter one I position at the rear. There is no reason for this, other than me always knowing which filter I have positioned where!
- I always carry a damp (freshwater!) cloth with me, and a dry one as well. I am always around water and sea spray. Filters get cloudy and smeary very easily. I have found the best method for me to keep them clean is to always carry a wet cloth with me (I keep it in a freezer/food bag) and clean them as I go. Then dry the moisture off with a dry cloth. I never, ever get them out at home to clean them but find I do it more often on the go. This is also the best way to remove sea water as it happens - if you just carry a normal dry cloth to wipe away sea water splash, the filters get cloudy and smeary very quickly.
- Be careful where they are positioned - it is easy to mis-position a filter creating dark lines where there shouldn't be lines or gaps of light on the horizon. Make sure you check your images on the screen when they are taken (before moving the tripod), so that you can correct any errors.
- Don't be afraid to tilt the filters or use them on a 90 degree angle if needed. If you are photographing on a hill, you'll need to tilt the filter to the "shape" of that hill to even the exposure correctly!
- Beware of vignetting with certain lenses (i.e. darkness in the corners). Similarly, don't fall in to the trap of trying to darken the sky too much - I've done it, and I think everyone probably has! I've stacked too many filters for the sky and created unnecessary underexposure.
- To calculate the difference between the exposures between two areas, meter the area. I.e. test the exposure. Flick to aperture priority mode, and set your aperture and ISO. Point the camera and focal point at the foreground subject and remember the shutter speed. Then point at the sky. Calculate the difference between the shutter speeds and how many stops that difference is. This is obviously time consuming and is good whilst getting a feel for the filters and what the correct density is to use (and when). When out in the field and "waiting for the light" you will then be able to identify what filter should be used when to save this time. It all comes with practice!
Not everyone uses Graduated Neutral Density (GND) filters, so I will be also writing a post-processing tutorial on how you can blend exposures at a later date. (This is a "workaround" if you do not use GND filters!)
What are your experiences of using grad filters?
Any tips or ideas to add?