Chapter 2 - Light and its Measurement - continued

Having received a helpful comment from Isabel about my previous blog concerning the use of a hand-held light meter with a circular polarising filter, I have investigated this further.

My initial thought was that it would be very difficult correcting for such a filter as this would be different depending upon the rotation of the filter i.e. how much the light was polarised. This turns out to be incorrrect although it is a common misconception, judging by various comments on internet forums. Apparently, the 'filter factor' (required exposure correction) does not vary with the rotation of the filter as this only affects the amount of polarized light allowed through the lens, not the total amount of light per se. In this respect it acts like a neutral density filter which limits the amount of light passing throough the lens.

The actual 'filter factor' will vary from manufacturer to manufacturer. For my Jessops circular polariser, the filter factor is 3.0. A useful chart that I found on the internet (www.dpfwiw.com/filters.htm) indicated that a filter factor of 3.0 was equivalent to -1 2/3 stops which means that, once I have taken a light reading with the hand-held meter, I must open the aperture 1 2/3 stops more than the reading shown (or reduce the shutter speed by the same amount). I would have liked to show the chart here but I was unable to cut and paste it from the web site).

Having tried this out by applying the suggested correction, it seems to work fine.