Polarising filters stop polarised light from entering a camera lens. By filtering out this light, a polarising filter eliminates reflections and haze from your photos. It also increases the contrast and depth of colour of objects, such as the sky or foliage.
The density of light that enters a camera is measured by f-stops. For example, a large aperture such as f-2 will let more light into the camera, compared to a smaller aperture, like f-10, which will block out more light.
When you use a polarising filter, it cuts out the equivalent of around 2 f-stops of light. This means you’d need to compensate for 2 f-stops of light exposure with this filter on your lens.
Look at the shutter speed on your lens before and after you put the polarising filter on. This should give you an indication of the difference in light exposure. Make sure you’re in aperture priority mode first, however.
Although 2 f-stops are the average amount of light a polarising filter blocks, this can vary according to the scene and your position relative to the source of light, ie, the sun.
When you rotate a polarising filter, you adjust how much polarised light you block out. The maximum polarisation effect is when the lens is pointed at 90 degrees to the sun or other light sources. To achieve this, make an L shape with your first finger and thumb, pointing the thumb at the sun. Where your pointer finger rests (you may need to rotate your wrist) will indicate where the polariser has the optimum impact. Of course, this doesn’t always work for every scenario, especially if you use a wide-angle lens, so take it (literally) as a general rule of thumb!