ND Filters - Long Exposures / by James Brown

Picture: Peter Mead©

Picture: Peter Mead©

If you want to improve your photography then having the right equipment is essential. One of the most basic things you can buy to achieve better images is an ND filter (Neutral Density Filter). An ND filter reduces the amount of light reaching the sensor without affecting the colours, hence the term ‘Neutral’ this then allows you to use long shutter speeds during day time. 

If you take a 3 second exposure outside during the day the image would most certainly be overexposed. If you use an ND filter you would be able to achieve long exposure times and at the same time capture any moment, such as water, clouds or trees. The most popular images using ND filters are ones of waterfalls that look like mist, steam or paintings. 

If you're in the right place and the wether conditions are just right you can create stunning images. most ND filters variety in price but at around £20 - £30 you can purchase a relatively good filter which will make your images look like a professional photographers. 

When you use an ND filter you will need to calculate the new exposure which will be explained below. You can download apps that help you work it all but but its quite simple to work out. 

The filter number is what is used to calculate the exposure times, for instance an ND64 will take out 6 stops and an ND1000 will take out 10 stops. 

A stop in either direction is either double (an increase in a stop) or half of the amount of light (a decrease in a stop) entering the lens. This can be achieved by either changing the aperture, shutter speed or ISO. 

Because we want long exposures its common sense to lower the shutter speed. If the exposure goes over 30’’ you will need to use BULB mode found at the end of the shutter speeds or displayed as B on the camera dial and a remote to time it right. 

Step 1

Place your camera on to a tripod

Step 2

Get the composition correct and focus using M or MF (manual focus)

Step 3

Set the cameras ISO to the lowest possible for example (100 ISO)

Step 4

Take a light reeding known as a base exposure  

Example

Shutter speed 1/120 

Aperture F8.0 

ISO 100

Step 5

Attach the ND filter

Step 6

Use the chart below or an app to determine the new correct exposure. 

Step 7

Take the picture (You may want to use a remote or delayed timer to stop camera shake)

Example

Calculation using an ND1000 filter. Notice that the shutter speed is the only setting changed. 

Base exposure

Shutter speed 1/120 

Aperture F8.0 

ISO 100

Calculated Exposure

Shutter speed 8 Seconds 

Aperture F8.0 

ISO 100

Calculation chart for a 6, 10 and 15 ND filter.