ND filter query: How many f-stop reductions is this filter?

If you use ND filters, get to grips with f-stop reductions.



An ND filter, or neutral density lens filter, reduces 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.


Understanding f-stops

The f-stop reduction of an ND lens filter, therefore, differs based on how much light you want to block out. Essentially, an f-stop relates to the aperture size, ie, how open or closed the aperture is, for any given photo.

You get a wide choice of options when choosing ND lens filters. This includes 2x, 4x, 8x, 16x, 32x filters, and even as far as 8192x and beyond. Each time you increase the multiple, this equates to a 1 f-stop reduction in light hitting your camera’s sensor.

When deciding which ND filter you need, knowing its f-stop reduction, optical density or lens opening percentage is helpful. If you choose a 2x filter, this equates to 50% of the lens opening percentage or a 1 f-stop reduction. A 4x filter has a 25% opening percentage or 2 f-stops, whilst an 8x filter represents a 12.5% lens opening percentage and a 3 f-stop reduction. 

The higher up the filter number you go, the larger the f-stop reduction, but the smaller the lens opening percentage becomes. Thus, a 256x ND filter will have an f-stop reduction of 8, but just a 0.39% lens opening percentage area. The smaller the f-stop number, the larger the aperture.

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 x filter is applied, this will now show a 2 stop underexposure for the same shot. Essentially, if you increase the exposure by one stop, you would be doubling the exposure.

To achieve the same exposure before the ND filter was applied, you would need to adjust the lens aperture, opening it up 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.


Choosing f-stops

You can choose which f-stop reduction ND filter to use, depending on what effects you want to create. Plus, how much daylight is present. In bright midday sunshine, this filter lets you open up the lens aperture fully for a range of shutter speeds. You can also take longer exposures without actually overexposing any subjects in your photo.

When sunlight is very bright, it might be tempting to go for as dark a filter as possible. But, the higher up you go in filter numbers, such as 16x, 32x or more, the harder it could be for your camera to focus properly.

As well as taking f-stop reductions into consideration when choosing an ND lens filter, always purchase filters with high quality at the forefront of your mind. A premium grade lens filter will maximise the quality of your shots, prevent ghosting and guarantee durability.