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I bought a circular polarizing filter, to take some shots of people on a balcony with a city skyline behind them on a bright-but-cloudy day.

However, I've never used a CPL before and don't know what I'm doing — I'm s so far just rotating the filter randomly and trying it out. Despite that, it nevertheless does seem to really help darken the sky, make the texture of the cloud surfaces more visible, and create a better balance between the sky and the people standing on the balcony near my camera (with 2 off-camera flashguns to add some fill flash as well).

There is a ∆ (triangle) marking on the rotating ring, shown in the attached picture below. This marking really looks like it is trying to tell me something. (I mean something perhaps like: "For maximum polarization, rotate this marking to position it at the 12 o'clock position, if the sun is at position X and the lens direction is Y.")

The filter came with no instructions or manual. What does this marking mean, and how is it used in practice?

The filter is an 82mm Marumi EXUS Circular P.L filter, fitted onto a EF 24-70mm f/2.8L II USM lens. (The weird "P.L" punctuation is part of Marumi's product name.)

Marumi EXUS Circular P.L filter fitted onto a EF 24-70mm f/2.8L II USM lens

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Many polarizing filters have a marking like this — either a triangle or a dot or something. You'll get maximum effect if you align the camera so you're pointing in a direction roughly perpendicular to the sun in the sky and the mark is pointed at the sun.

This doesn't need to be exact, but it's an easy starting point. Of course, with through-the-lens viewfinders (optical or electronic), you can see the effect as you turn the filter, so you can just ignore the mark if you like. (Perpendicular orientation will still get you greatest effect, though, of course, mark or no mark.)

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According to this you are meant to align the triangular marker with the direction of the sun.

enter image description here

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  • \$\begingroup\$ I guess that must date from pre-SLR days. Nowadays, it makes much more sense to just look through the lens and rotate the filter to whatever angle looks best. \$\endgroup\$
    – David Richerby
    Mar 3, 2018 at 12:16
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    \$\begingroup\$ @DavidRicherby Sometimes a lens hood, matte box, or other object may be in the way of convenient access to the ring after the entire rig has been set up. The indexing mark allows the filter to be aligned for an intended effect prior to the fact. \$\endgroup\$
    – Michael C
    Mar 4, 2018 at 3:56

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