7. Long exposure photography part 2: with an ND filter

In my guide 6. Long exposure photography part 1: without an ND filter I shared the exposure technique for before sunrise, while the sky is still dark, or for after sunset, when it is almost dark. At these times you can expose without needing an ND filter.

As sunrise approaches and the sky gets brighter, or before sunset, the sky is very bright. At these times, to take a long-exposure photo you need an ND filter. It cuts the light across the whole frame, so you can use a slow shutter speed and get the long-exposure effect.

The creative effects of long exposure

Unlike the usual way of taking photos with a fast shutter speed, where every movement in the scene at that moment is “frozen,” long exposure lets those movements play out over a long time to create interesting effects. Here are the effects of long exposure:

• Removes ripples on the surface of ponds and lakes, creating a mirror reflection that is crystal clear and still.
• Makes the sea surface smooth as silk, and waves crashing on rocks soft and floating like clouds.
• Makes pouring water, streams or waterfalls stretch into silky strands.
• Creates eye-catching long streaks for prominent moving objects such as clouds, mist, car lights, debris moving on the water surface, etc.
• Erases unwanted objects that are moving, such as people and vehicles during the day.

How to calculate the optimal exposure

To meter and work out the correct and optimal exposure, do not attach the ND filter yet and follow these steps:

Step 1: Attach the Grad ND filter

A Grad ND filter is a filter that is half dark and half clear, used to hold back some of the light in the sky area so you can make the foreground brighter, with a better balance between sky and ground. A Grad ND will not interfere with the camera’s automatic metering, and the camera will also take this filter’s light balancing into account in the next step.

Step 2: Switch to A/Av mode (aperture priority) to meter

Switch to A/Av mode so the camera can meter automatically. Note that in the camera’s metering settings, you should use a default metering mode such as whole-frame Matrix metering or Center Weighted, not Spot, because we need to meter the whole scene. Note down the shutter speed measured at the optimal aperture and ISO.

Example: the camera meters 8 s at f/11 ISO 100.

Step 3: Take test shots to find the optimal exposure

After noting down the shutter speed the camera just read (e.g., 8 s f/11 ISO 100), switch to M mode, set those settings again and start taking test shots. After shooting, check the histogram to see whether it touches the right edge (or is at the optimal level); if not, increase or decrease the shutter speed and shoot again until the histogram is optimal.

The reason I use the histogram instead of the in-camera automatic metering result is that the light meter is not as accurate, especially in low-light conditions like these. But I still need to meter, because it gives me a starting setting and saves time searching for the optimal one. The metered settings are not far from the optimal ones, so after 1 or 2 test shots and adjustments you will reach the optimal exposure.

See also: 4. How to optimize exposure with the histogram

For example, after adjusting, your optimal exposure has a shutter speed of 15 s (at f/11 ISO 100).

Step 4: Attach the ND filter

The ND filter is attached closest to the front of the lens, behind the Grad ND filter, to limit gaps and light leaks. During the long exposure you need both filters stacked at the same time because the two filters serve two different purposes. The Grad ND balances the light in the sky, and the ND lowers the shutter speed to allow a long exposure.

For example, you need a 1-minute long exposure but the current shutter speed, as in the example above, is 15 s, which is too fast; to lower the shutter speed you have to attach a 4-, 5- or 6-stop ND filter, and for this example I use a 5-stop filter. Because a 5-stop ND filter reduces the light by 5 stops, you have to make up for those 5 stops by lowering the shutter speed to give a longer time so more light can be collected.

As I shared in my guide 3. How to increase or decrease exposure with shutter speed, aperture and ISO, each time the shutter speed is doubled (×2), that is 1 stop, as follows:

Making up the 5 stops of lost light with shutter speed
Starting from 15 s
15 x 2 = 30 s = 1 stop
30 x 2 = 60 s (1 minute) = 2 stops
1 minute x 2 = 2 minutes = 3 stops
2 minutes x 2 = 4 minutes = 4 stops
4 minutes x 2 = 8 minutes = 5 stops

So after attaching the 5-stop ND, you have to lower the shutter speed to 8 minutes at f/11 ISO 100 to make up the exposure.

But 8 minutes is too long because we only need 1 minute to smooth out the water surface. But if we use 1 minute, it only makes up 2 stops, still 3 stops short. So you can use the following ways to compensate besides shutter speed.

Adding more light with aperture
For example, if you are using a 16mm focal length on full-frame, f/11 is 2 stops smaller than the minimum optimal aperture of f/5.6, so you can switch to f/5.6 as follows:

f/11 > f/8 = 1 stop
f/8 > f/5.6 = 2 stops.

After switching to f/5.6 you have made up a total of 4 stops, still 1 stop short because the ND filter has cut as much as 5 stops. So you can compensate further using ISO.

Add more exposure with ISO
The current ISO is 100.

100 x 2 = 200 = 1 stop.

So after increasing the exposure by lowering the shutter speed 2 stops (increasing the time), increasing the exposure 2 stops with aperture and increasing the exposure 1 stop with ISO, you have made up the full 5 stops for the newly attached ND filter.

Step 5: Compensate for the changing light

Sunrise: The earlier it is, while the sky is still dark, the faster the sky brightens, because during the 1 minute of your long exposure the light in the sky will change brighter by 1 stop to 1/3 stop. So you need to adjust beforehand by subtracting exposure for that brightening so the exposure stays correct and optimal.

This is a difficult step that requires experience in compensating according to the light conditions in the sky, but understanding this theory helps you compensate closer to optimal and limits overexposure when the long exposure time is over.

E.g.: Subtract 1 stop with ISO
The current ISO from the step above is ISO 200.
200/2 = 100 = 1 stop

So the shooting settings are: 1 minute f/5.6 ISO 100 with a 5-stop ND

Sunset: Similar to sunrise but the other way around: after the sun sets, the closer it gets to dark, the more it darkens, because during the 1 minute of your long exposure the light in the sky will change darker by 1 stop to 1/3 stop. So you need to adjust beforehand by adding more exposure for that darkening so the exposure stays correct and optimal.

E.g.: Add 1/2 stop with shutter speed, which means lengthening the time by 1/2 stop.

The current shutter speed is 1 minute or 60 s

Find the 1/2-stop increase in time
60 x (1/2) = 30 s
the new shutter speed is 1 minute + 30 s = 1:30 minutes

So the shooting settings are: 1:30 minutes (90 s) f/5.6 ISO 200.

Step 6: How to shoot longer than 30 s with B (Bulb) mode

You need to switch to B (Bulb) mode to be able to use a remote to shoot longer than the 30 s available in the camera.

A mechanical remote has a HOLD button that keeps the camera shooting until you return it to its original position. With this remote you have to watch the clock yourself (older Nikon, Sony) or watch the seconds counting on the camera (newer Canon, Nikon and Sony) and then end the shot yourself.

• Nikon and Sony: Turn the shutter speed dial past 30 s and you will see B or BULB.
• Canon: Turn the mode dial to B or BULB.

A remote timer lets you program exactly the time you need to shoot, and it automatically ends the shot once the time is up.

Step 7: Check, adjust and shoot again

After each shot you have to check the histogram and readjust the shutter speed, compensating for the brightening and darkening so the exposure is as optimal as possible. Once you have mastered this technique, you can also bring in aperture and ISO to calculate the compensation more conveniently.

Other long exposure photos