• Amber Jones

How Many F-Stops of Light Does an ND Filter Reduce?

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Understanding ND filters and their f-stop reductions is key to capturing great photographs. Learn which ND filter you might need for different purposes.

Words and Photography by Urth HQ

An ND filter, or neutral density lens filter, reduces the amount of light entering your camera. ND lens filters vary according to how much light they let in, and this is determined by a filter’s f-stop reduction.

Essentially, an f-stop relates to the aperture size, ie, how open or closed the aperture of your lens is, for any given photo. If you want to shoot with a wide aperture to capture a shallow depth of field, you’ll need to set your f-stop to a low number like F2 or F4. But doing so means you’re letting a lot of light into your lens, and if you’re shooting on a sunny day there’s a good chance your photo will be overexposed. This is where ND filters come in to reduce the amount of light that enters your lens. 

Keep reading to understand how many f-stops of light each ND filter blocks out. 

You get a wide choice of options when choosing ND lens filters. This includes ND4, ND8, ND64 filters, and even as far as ND1000. Each time you double the number in the ND name, this equates to a 1 f-stop reduction in light hitting your camera’s sensor. For example, an ND2 blocks out 1 f-stop of light, an ND4 blocks out 2 f-stops, an ND8 blocks out 3 f-stops and so on. 

how to shoot in harsh light

Understanding stops on an ND filter

When deciding which ND filter you need, knowing its f-stop reduction, optical density or lens opening percentage is helpful. If you choose an ND2 filter, this equates to a 50% light reduction,  or a 1 f-stop reduction. An ND4 filter lets in just 25% of the amount of light equal to 2 f-stops of light reduction. Each time you double the number in the ND filter name, this equates to a 1 f-stop reduction in light hitting your camera’s sensor. 

It’s important to know how many f-stops of light an ND filter is blocking out, so you can adjust your camera settings accordingly. Doing so opens up a world of creative possibilities, like shooting beautifully shallow depth of field with wide apertures, or shooting soft, magical motion blur with slow shutter speeds.  

Imagine your camera is set up in manual mode with ISO 100, f/2.8 and a shutter speed of 1/250s. If an ND4 filter is applied, this will now show a 2 stop underexposure for the same shot. To balance the shot now that the ND filter has been applied , you would need to widen the lens aperture to allow more light to pass, increase the ISO to make the sensor more sensitive and/or slow the shutter speed down to allow for more light.

Shot with no filter.
Shot with an ND2.
Shot with an ND4.

What does a variable ND filter do?

Variable ND lens filters differ from fixed ND lens filters in that you have greater control over the amount of light they block out. As we’ve seen, a standard ND filter reduces light by a specific f-stop. However, a variable ND lens filter covers a range of f-stop reductions in one single filter, and you can rotate the filter to adjust the number of f-stops it blocks.

The f-stop ranges on a variable ND lens filter vary slightly, depending on which brand of filter you buy. For example, Urth variable ND lens filters come in the following ranges: 

1. Variable ND2-32 (blocks between 1 and 5 f-stops of light)

2. Variable ND2-400 (blocks between 1 and 8.65 f-stops of light) 

3. Variable ND8-128 (blocks between 3 and 7 f-stops of light) 

4. Variable ND64-1000 (blocks between 6 and 10 f-stops of light) 

Variable ND filters are especially useful when shooting outdoors, because they allow you to adapt to changing light when the sun moves behind the clouds, or when you want to transition from shooting underneath a shaded tree canopy, to shooting in a brightly lit scene like the beach. 

If you’d like to experiment with ND filters but you’re not sure which one to pick, variable NDs are a great option because they give you lots of light reduction levels to try. We’d recommend starting with the Variable ND2-400, as it gives you the greatest range of light reduction levels. 

Shot with the ND2-400 filter from the Urth Explorer Kit.

By turning the filter and adjusting the stops of light exposure you can slow down shutter speeds to introduce motion or blur to your images, or widen your aperture to produce a shallow depth of field between objects in the forefront and background of your scene.

How many f-stops you reduce light by on your variable ND lens filter largely depends on how much daylight you are exposed to at any given time. A gloomy, cloudy day may require a different f-stop light reduction compared to a bright sunny day at noon or under snowy conditions. For example, shooting by the sea at midday using long exposures can give the water a really silky, smooth effect, particularly compared to later on in the day. 

If you want a sharply focused photo, you need to allow as much light as possible to your camera, so if the light is dull, opt for a low f-stop to boost the aperture. To capture a shallow depth of field in an image, consider choosing around f/1.2 or f1/4, but for deep depth of field, especially in landscape photography, high f-stops in the range of f/16 or f/22 may be preferable.

Choosing which f-stop to use is often a matter of personal taste based on the end result of your image. Shoot the same scene using different f-stop ranges by turning the variable ND lens filter to see how they differ.

Shop our range of fixed and variable ND filters that plant 5 trees with each filter purchased.

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