Olympus 45mm 1.8 - Polarizing filter at widest apeture?

Started Jul 17, 2012 | Discussions thread
rrr_hhh Veteran Member • Posts: 6,023
Difference between a polarizer and a neutral density (ND) filter

akelu wrote:

I'm looking to get a polarizing filter for my olympus 45mm 1.8 lens.

I've heard that when using a polarizing lens on a wide-angle lens i will have issues with uneven polarizing effect in the sky.

I was wondering if i will have this issue with the 45mm 1.8 at its widest?

I was thinking of getting B+W 37mm 0.6 ND MRC 102M Filter

1) A polarizing filter is not an ND filter (aka grey filter, neutral density filter);

2) A grey filter affects the whole picture similarly, it will lower the exposure; in your case, you will gain two stops; with very fast lens (1.4 to 1.8) this may not be enough if you want to make use of the thin DOF, but enough if you just want to keep the lens at his sweet spot;

3) A polarizing filter filters unwanted reflections : avoiding you to get too shiny leaves, allowing you to see through the waters, or to look at the content of a show window without being reflected in the glass. It helps darken blue skies too. Since it filters the light it does also have a darkening effect.

4) The polarizing filter filters the light coming from a specific direction. It is built of two rings and you determines the direction of the light filtered by rotating the thinner external ring untill the sky is darker, or the unwanted reflexions disappear.

5) Any filter ring may add vignetting to a picture. The larger the aperture, the thicker the ring and the wider the lens, the higher the risk of vignetting will be. Vignetting adds darker corners to a picture and can be easily corrected in postprocessing. A neutral density filter will present that risk, but it will be moderate on a short tele lens, especially if you look for a filter having thin edges, like the pro serie of B&W.

6) With polarizing filters you have two things to consider : the risk of vignetting mentioned in point 5) will be higher, because polarizing filters have two rings. Then with wide lens the direction of the light to filter will change over the frame, leading to a lack of uniformity in the skies. This has nothing to do with vignetting. For instance the left side could become darker than the right one, or you may get a darker sky at some distance of the center and with the edge becoming somewhat lighter again. Where the darker/lighter part strikes in depends of the position of the sun. The two effects can be cumulated in one of the corners. Unlike vignetting, that lack of uniformity is very hard to correct in post processing.

To sum up : you may get some vignetting at very large aperture if you add an ND filter on the 45mm, but nothing very dramatic and it will disappear as soon as you close the aperture a little. However, if your goal is to exploit the thin DOF of fast lenses in bright sun, you may rather look for a filter allowing you to shoot at -3 stops than -2 stops.

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