ND and GND filters

Learning goal

You will be able to recognize two ways of deliberately reducing light in daylight, and use both with stop calculations. You will learn that an ND filter reduces the light falling on the whole frame evenly, and to convert the density value on it into stops; you will calculate how much the shutter speed will lengthen when an ND filter is fitted; you will see how the same filter makes it possible to shoot a portrait with a wide aperture in the sun; you will tell apart how a GND filter brings a bright sky together with rocks in shadow, and why it causes trouble with mountains or heads cutting the horizon; you will make a habit of setting focus and framing beforehand when shooting with a filter fitted; and you will take into account how people, on every skin, can blur in a long exposure.

The question

At noon you are on a rocky shore. You want to photograph the waves crashing against the rocks and pulling back, in a frame where the water softens like mist around the rocks. But the sun is so bright that even with the narrowest aperture and the lowest ISO, the camera won't allow a time longer than 1/60, and the waves come out as frozen, sharp foam. On top of that, the sky is pure white and the rocks pitch black.

Is a long exposure possible in daylight? It is, but you need to reduce the light; either over the whole scene, or only in part of it. In this lesson we will talk about the ND filter, which darkens the whole frame, the GND filter, which darkens only part of it, converting the numbers on both into stops, and what to watch for when shooting with these filters.

Context

When we photographed the boatman on the river, the turnable filter lengthened the time only a little. This loss is enough to soften water slightly; but for an exposure of several seconds under the sun, the light needs to be reduced much more, by six, ten, sometimes more stops. ND filters are made for this job.

Balancing the big difference between a bright sky and a dark foreground is another job. In the exposure module we saw shooting several frames and merging them; a GND filter is the way to build this balance in a single frame, at the moment of shooting. Both filters are simple tools that fit in front of the glass, but using them correctly relies on stop calculations.

Simple explanation

What does an ND filter do and how is it read?

An ND filter, short for neutral density filter, is a dark glass that reduces the light falling on the whole frame evenly, without changing color. On it, an optical density value is often written: like 0.3, 0.6, 0.9. To convert this value into stops, it is enough to divide it by 0.3: ND 0.3 cuts one stop of light, ND 0.6 two stops, ND 0.9 three stops, ND 1.8 six stops, and ND 3.0 about ten stops. On some filters, what is written instead is how many times the light is divided: ND8 means three stops, ND64 six stops, ND1000 about ten stops.

Once you know the stops, calculating the new shutter speed is easy: each stop doubles the time. In a scene that asks for 1/125 without a filter, with a six-stop ND the time goes up to about half a second, and with a ten-stop ND to about eight seconds. An exposure this long requires holding the camera on a sturdy tripod as in Lesson 13.1. Cheap ND filters can give the frame a slight color shift, often a warm or purplish tone; you can correct this with white balance.

What else is an ND filter for?

An ND filter isn't only for softening water. It is also useful when you want to shoot a portrait with a wide aperture under the sun. If on a sunny afternoon f/16 with 1/125 at ISO 100 gives a suitable exposure, opening to f/2 in the same scene means six stops more light, and a time like 1/8000 is needed. If your camera can go up to at most 1/4000, one stop of extra light remains; a one- or two-stop ND makes a wide aperture and a soft background possible in the sun too.

A long exposure blurs everything that moves. In a busy square, in an exposure of several seconds, walking people turn into semi-transparent traces or disappear completely; still buildings, on the other hand, stay sharp. This is sometimes a wanted effect. But if someone in the scene is standing and posing, it is hard for them to stay completely still for several seconds, and their face, light or dark, softens. In a frame where people need to stay sharp, you need to shorten the time or choose a lighter ND.

What does a GND filter do?

A GND filter, short for graduated neutral density filter, is a glass whose top half is dark and bottom half clear, with a transition between the two. It is placed so that the dark part falls on the bright sky and the clear part on the dark foreground; that way the sky darkens by a few stops while the foreground stays as it is. A GND 0.6 darkens the sky by two stops.

The transition hardness matters too. A GND with a hard transition suits a straight, sharp line like a sea horizon; a GND with a soft transition, on the other hand, doesn't make the transition stand out in scenes where hills or trees cut the horizon unevenly. A GND's biggest problem is things that cut the horizon: a mountain, a building or a person's head reaching into the sky also enters the filter's dark part and darkens. If a person's head rises above the horizon, their forehead and hair darken together with the sky; this can bury the top half of the face in shadow on dark skin, and split the face into two tones on light skin. That is why you need to place the transition above the heads, or choose another way instead of a GND.

How do you work with a filter fitted, and what are the alternatives?

When a dark ND is fitted, the viewfinder or screen can get very dark and autofocus may not find sharpness. That is why you first build the frame without the filter, find focus, switch focus to manual, and only then fit the filter. On cameras with an optical viewfinder, light entering through the viewfinder during a long exposure can leak into the frame; covering the viewfinder prevents this leak. The meter sometimes can't measure correctly in this much darkness; checking the time you found with stop calculations with a test frame is safe.

The balance a GND can build can be built in other ways today too. Shooting the same scene at different exposures and merging them, or shooting RAW and darkening the sky with the graduated adjustment in the editing program, give results close to the balance built with a filter. But these ways can't do what a long exposure does: water or clouds softening as they flow for several seconds forms only in a single frame taken during those several seconds. That is the one thing a daytime long exposure gives that nothing else can.

Visual evidence

You go back to the rocky shore. You set the tripod firmly on the rocks, build the frame, put focus on the edge of a rock and switch focus to manual. The metering without a filter is f/16, ISO 100 and 1/125. You fit a ten-stop ND and lengthen the time to eight seconds. In the frame, the waves have turned into a milky white, soft mist around the rocks; the roughness of the rocks and the moss are sharp.

The sky is still much brighter than the foreground. You lower a soft-transition, two-stop GND to the level of the horizon; the clouds come back, and the rocks stay as they are. Then your friend, an olive-skinned young man, sits on the rocks; you take off the GND, fit a light one-stop ND and take his portrait at f/2 and 1/4000. His face is sharp and detailed, and the sea and rocks behind are a soft background. The same sun, the same shore; how much and where you reduced the light turned each frame into something else.

Common mistakes

Summary

An ND filter reduces the light falling on the whole frame evenly; the density value on it is converted into stops by dividing by 0.3, and each stop doubles the time: a ten-stop ND lengthens 1/125 to about eight seconds. The same filter also makes a wide-aperture portrait possible in the sun; a long exposure, on the other hand, blurs moving people, on every skin. A GND filter balances a bright sky with a dark foreground, but also darkens mountains and heads that cut the horizon; when shooting with a filter fitted, you need to set focus beforehand, place the transition carefully, and if needed choose ways such as merging or graduated editing.

Mini assignment

In this assignment you will try reducing light with ND and GND filters, and complete the module project. You don't need to buy a filter; you can get one for a few days from a club or an acquaintance.

1. Convert the density value of the ND filters you have into stops, and calculate what the shutter speed will be with each in a scene that asks for 1/125 without a filter.
2. In daylight, photograph flowing water or a moving crowd on a tripod, without a filter and with an ND.
3. Balance a scene with a bright sky and a dark foreground, with a GND, or by shooting RAW and using the graduated adjustment in editing.
4. Module project: photograph the same scene with a polarizing or ND filter and without a filter; compare the frames and write a paragraph on how the sky, the reflection or the movement changed.

Check yourself

• Did the time you calculated with the ND give the right exposure in the test frame?
• How did the water or the moving people change in the long exposure?
• Did the GND or the graduated adjustment balance the scene without darkening something that cuts the horizon, for example a head, regardless of skin tone?

Glossary

Mini quiz

1. How many stops of light does an ND 0.9 filter cut?

• A) Nine stops
• B) A loss of exactly three stops
• C) Half a stop
• D) Ten stops

2. In a scene that asks for 1/125 without a filter, about how long does the time become with a ten-stop ND?

• A) About eight seconds
• B) 1/250
• C) About half a second
• D) One minute

3. When using a GND filter, if a person's head rises above the horizon, what happens?

• A) Nothing happens
• B) The person's head gets sharper
• C) The person's forehead and hair darken together with the sky
• D) The white balance goes wrong

4. In the sun, when f/16 and 1/125 at ISO 100 is a suitable exposure, what do you do to shoot a portrait at f/2 if the camera's shortest time is 1/4000?

• A) Raise the ISO
• B) Stopping down the aperture is a must
• C) Lengthen the shutter speed
• D) Fit a one- or two-stop ND filter

Answers

1. B — Optical density is converted into stops by dividing by 0.3: 0.9 ÷ 0.3 = 3 stops.
2. A — Ten stops lengthens the time by 1,024 times; 1/125 × 1,024 comes to about eight seconds.
3. C — A GND's dark part darkens everything above the horizon; the top half of a head enters the shadow too.
4. D — Opening six stops from f/16 to f/2 requires 1/8000; the one stop that doesn't fit into 1/4000 is cut by the ND.

Summary card

• ND: darkens the whole frame evenly; density ÷ 0.3 = stops.
• Each stop doubles the time: 10 stops lengthen 1/125 to ~8 seconds.
• An ND also makes a wide-aperture portrait possible in the sun.
• GND: darkens the sky; it also darkens mountains and heads that cut the horizon.
• Focus first, then fit the filter; in a long exposure people blur on every skin.

Bridge to the next lesson

In the eight-second exposure on the shore, the moment you touched the camera the frame on the tripod could have shaken slightly; that is why you shot with the self-timer. But sometimes even a two-second wait makes you miss the moment, and sometimes you are far from the camera, waiting for a bird to land or to take your place in a group. How do I shoot without touching the camera? The next lesson talks about the remote shutter release, the self-timer, and vibration-free shooting with phone apps.