Filter Quick Guide

When looking to purchase filters, the terminology can be quite overwhelming. In this section, we’ll try to break down the meaning of each section of naming and go into the specifics of the filter’s darkness. Let’s use an example: “3 stop Medium GND”. There are three sections to discuss: ‘3 stop’ refers to the amount of light that a filter blocks. ‘Medium’ refers to the way in which a filter transitions from clear to dark glass. GND refers to the type of filter. The following is a way to conceptualise just how much light a filter blocks and therefore how much longer an exposure it allows you to create without overexposing an image.

1. For any given photograph, a certain amount of light is needed to ‘expose’ the photograph correctly.
2. Filters, depending their darkness (formally ‘optical density’) block a certain amount of light.
3. The amount of light blocked is classified by the unit ‘stops’ of light (formally known as the ‘filter factor’).
4. For each stop of light blocked, the required duration of exposure doubles.
5. Therefore, the denser the filter the required duration of exposure increases exponentially, not linearly. Let’s take an example situation, firstly without a filter:

No filter: 1 second exposure required for a correctly exposed image

‘1 stop’ of light blocked: 2 second exposure required (2×1)

‘2 stops’ of light blocked: 4 second exposure required (2X2X1)

‘3 stops’ of light blocked: 8 second exposure required (2X2X2X1)

‘6 stops’ of light blocked: 64 second exposure required (2x2x2x2x2x2x1)

@ Brent Hall

In practice, unless you have a mathematical mind, there are phone applications that allow you to calculate an appropriate exposure time depending on the strength of filter you are using.