Imagine you are traveling in the northern regions, near the Arctic Circle. You know the time but it’s not getting dark. Your GPS device has failed and it is very cloudy and you can’t see the sun. You have lost your bearings and your magnetic compass is showing you the wrong direction due to your proximity to the polar region. It’s enough that you can’t trust the magnetic compass. Bad luck, I would say. Now what? How do you find the cardinal points? Not you, but your digital camera :-), your CPL filter and your tripod with degree scale. Assuming that you know how a polarizing filter works. I assume you do, otherwise you wouldn’t have one with you.
So the aperture priority. Make sure that your ISO value is fixed and open your aperture. Activate the live view. Point your camera at a fixed point in the sky (first measurement). Now slowly rotate your CPL filter and observe the shutter speed on the display. Note the range (fastest to slowest shutter speed). Turn the camera counter-clockwise by 10 degrees on your scale and measure the shutter speed range again. Do this until you are all the way around. The slowest shutter speed shows you the area 90 degrees to the sun. The fastest shutter speed shows you the direction of the sun (or directly opposite). The brighter of these two “fastest shutter speeds” is the actual direction of the sun. The shutter speed is your main indicator: a longer time means maximum polarization, a shorter time means minimum polarization. If you know the time and the position of the sun you can determine the cardinal directions.
On a polar day, when the sun is visible for a long period of time (even if obscured by clouds), it still moves in a circle around you. If you know where it should be at a certain time of day (e.g. at midnight in the north at midday in the south – this depends on the exact latitude), you can deduce the other cardinal points from this. This article with this content was first published by me on 03.07.2025. All subsequent articles on this topic have been based on this article. PS: How did I come up with this idea? It was probably the Vikings who used crystals for the double refraction of light for nautical navigation.