Using Graduated ND Filters
Selecting and Using NiSi Graduated ND Filters
IntroductionOne of the biggest challenges in photography is managing the light in your scene. It is for this reason that many landscape photographers love to be out shooting during the golden hours or blue hour when the light is beautiful and the contrast is manageable. Contrast is tough to manage on bright days and in certain scenes, but there are a few ways to work around this. In this article we are going to look at the usefulness of using neutral density gradient filters (aka ND grads). These filters have been around for a long time, most landscape photographers will have a set of them in their camera bag.
Filters or Photoshop?Tips for selecting and using graduated neutral density filters
Controlling and balancing the exposure throughout an entire image can be difficult when you have brighter or darker elements in different parts of a scene. More often than not with landscape photography the sky will be much brighter than the foreground – up to 6 or 7 stops difference depending on what time of the day you are shooting. When shooting things like a city scene during the night you will tend to find the reverse happens – the foreground will be much brighter than the sky with light trails and city lights tending to become overexposed with longer exposures. It is possible with post processing techniques in software packages like Adobe Photoshop to capture multiple images (bracketed exposures) for different parts of the scene and to blend them together, or to use software adjustments like the “gradient filter” in Adobe Lightroom or Adobe Camera RAW to change the exposure and colours in certain parts of an image like the sky.
The other option would be to use graduated neutral density filters to control the exposure in the brightest part of the scene, therefore capturing images with a balanced sky and foreground, and contrast throughout the entire image. The benefits of using filters include being able to expose the entire scene correctly in a single shot, or to get different parts of the scene closer together in exposure when choosing to blend multiple images together in post processing, reducing time required when editing images.
What is a graduated neutral density filter?
I use the NiSi complete range of Neutral Density Graduated Filters, for more information please see here
Graduated neutral density filters are typically rectangular pieces of glass (although they are also available as circular filters in the NiSi product range as well) with a gradient transition from dark to light in varying strengths. The gradient can run from darkest at the top to lighter in the middle for filters such as a soft or medium grad, or from darker in the middle to lighter at the top for reverse graduated filters. The strength or density of the gradient is represented in stops of light in the darkest part of the filter, with each stop of light being a 0.3 decimal value. So for a 3 stop soft grad, you will see the value of 0.9 printed on the filter. For a 4 stop medium grad it will be represented as a 1.2 and so on.
Graduated neutral density filters are called “neutral” because they should add no colour cast or any noticeable change in the white balance in the area of impact of an image. The gradient is designed to darken either the sky or foreground depending on how it is aligned, but colour reproduction should not be impacted when using high quality optical glass to manufacture ND filters.
The focal length and physical build size of any individual lens will determine what system or size of graduated neutral density filters and filter holder will be required. NiSi offer systems in 75mm for smaller compact cameras, 100mm for standard focal lengths as wide as 16mm (and sometimes wider depending on the specific lens) and lens screw thread sizes to 82mm like the Canon 16-35mm f/2.8L, or 150mm or 180mm systems for lenses wider than 16mm and those that tend to have a “bulbous” front glass element such as the very popular 14-24mm Nikon f/2.8.
Selecting the correct density of graduated ND filter to useChoosing the correct graduated neutral density filter will depend on a couple of major things – the amount of difference in light (measured in stops) between different parts of the scene, and environmental factors like whether there is a flat, uninterrupted horizon line or elements like mountains, trees or buildings above the horizon. Where the sun or other bright parts of the scene are positioned will impact on the selection of ND filter as well.
To make a decision about what density of graduated ND to use (eg. 3 stop or 4 stop) a meter reading of both the sky and foreground will need to be done. To do so you will need to –
1. Set your camera up on a tripod and ensure your camera is in Manual mode. To take a meter reading of the foreground in your scene get your shutter speed, ISO and aperture correct, and ensure the foreground it is being exposed at 0 EV. This is best done in Evaluative or Centre Weighted metering modes.
2. By either moving your metering area using the camera controls, or tilting your camera upwards towards the sky and not moving your metering point (or in whichever direction the brighter parts of the scene are), take note of the meter reading and the variation of stops of light within this part of the scene. Do not change the camera settings that were set to meter correctly for the foreground. The difference in exposure level will be measured in variations of +/- 1EV (or even +/- 1/3EV) on most cameras.
3. The difference in EV levels (or stops of light) will be the requirement when choosing what strength of grad filter to use. The aim is to get the foreground and sky within 1 stop of light difference of each other. So if the sky is 3 stops brighter than the foreground, either a 2 stop or 3 stop graduated ND will be required to help balance the exposure of the brighter part of the scene.
ND Optical Density F-stop Reduction Transmittance %ND4 0.6 2 stop 25%
ND8 0.9 3 stop 12.5%
ND16 1.2 4 stop 6.25%
ND32 1.5 5 stop 3.12%