What is a Filter in Photography?
A filter is a product, usually made of glass, but sometimes made of resin or other material, which is designed to have some impact on the light hitting your camera’s sensor. For example, a UV filter is designed to filter out UV light – a throwback from film days, as modern camera sensors tend to have built-in UV filters. A polarizing filter filters out polarized light, good for cutting down on reflections and making blue skies pop.
What does a Neutral Density Filter do?
A Neutral Density Filter reduces the amount of light that passes through it and therefore, the amount of light that ends up on the camera’s sensor. It is essentially a darkened piece of glass (or resin) that is designed not to change anything other than the quantity of light that passes through it. Other factors such as the color of the light or polarization of the light should not be affected.
This allows for creative effects such as using a wider aperture (for depth of field effects) or a longer shutter speed (for time-based effects) than would not be possible otherwise. They are most useful in bright conditions where there is a lot of available light.
Other helpful uses of ND filters include:
Reducing the depth of field in bright sunlight
Adding motion blur to moving objects
When using a wider aperture
Types of ND Filters
There are several types of ND available – screw in ones come in various filter thread sizes and slot in ones fit into a holder that screws onto the lens. They are available in different strengths too, known as the filter exposure factor as follows:
ND filters and their exposure factors:
2x one stop
4x two stops
8x three stops
64x six stops
1024x ten stops (Lee Big Stopper region)
Using an ND filter is simple, you just either screw on or slot into a holder and leave the camera’s automatic exposure system to work out the filter factor. If it’s an 8x, for example, the camera will reduce the shutter speed from, say, 1/125sec to 1/15sec to compensate for the three stops extra light required. Or the aperture will be opened up from f/22 to f/8.
What Is a Graduated Neutral Density Filter?
Some neutral density filters aren’t so “neutral” after all. A graduated neutral density filter blocks more light on side than the other. Half of the filter might block out 4 f-stops on one side and only one f-stop on another. Where might be this be useful? Landscape photography is an immediate candidate. When you are taking pictures of landscapes, the sky tends to be much brighter than the ground underneath it. By using a graduated neutral density filter, you can get the right brightness levels in the sky and the ground at the same time.
Using an ND filter in this way is not its primary benefit though. Have you seen those shots of waterfalls that look ethereal with blurry cotton wool water? Well the chances are an ND filter will have been used. Here the filter is used to reduce the shutter speed so that blur occurs. If you are out in a bright location the shutter speed will be at least 1/125sec and ideally you need 1/15sec or slower. So pop on the necessary ND filter and you’ll gain the effect you’re after.
Architecture photographers have a useful technique that often needs an ND filter to work. When photographing famous landmarks you often have problems with tourists getting in the way. If the shutter speed is slow enough it will be open long enough to ensure the moving people are so blurred they cannot be seen on the image. A 1/2sec exposure may record a streak of someone walking while a 4 sec exposure will make them vanish. Simple, but very effective.
It’s not just the shutter speed that you may want to improve either. If you are shooting in bright conditions you may find the aperture the camera is selecting is small and the resulting picture will have far too much front-to-back sharpness known as depth-of-field. This is often the case with portraiture or flower photography where a distracting background ruins the photo. Using an ND filter will help you open up the lens and provide a shallow depth-of-field.
Another use for an ND filter would be when using flash. You can often reduce the exposure of the flash using auto settings, but for close ups that may not be possible. The ND filter will provide the key to this essential barrier.