The circular polarizer (or CPL) is a must-have for any landscape photographer. This handy filter can be used in many different situations, but like any piece of photo gear it helps greatly to know exactly what it does and what its benefits and downsides are. This is the first of two parts.
WHAT A CPL IS & HOW TO USE IT
- A circular polarizer is a filter that screws on to the threaded front end of your lens. It has two pieces of glass sandwiched together. So it also has two rings for you to grip. If you grip the ring closest to the threads you will be able to screw the filter on and back off your lens.
- Once it’s on (not too tight!), grasp the ring furthest from the threads to rotate the front piece of glass relative to the other (now fixed in place). This is the way you adjust the filter’s strength. It goes from minimum to maximum effect with 90 degrees of rotation, then back to minimum if you continue rotating all the way to 180 degrees.
HOW A CPL WORKS
- The CPL filter works by polarizing light in a couple different ways. When light is reflected it becomes polarized to one degree or another. Light rays can be thought of as vibrating waves. When emitted by some source (like the sun), the light waves vibrate in all directions. When light hits a reflective surface and bounces off it, the waves vibrate mostly in one direction, parallel to the reflecting surface. The light has become linearly (or plane-) polarized.
- A circular polarizer works by first polarizing the light linearly, then turning it into circularly polarized light. In the case of the plane-polarized reflected light above, the front glass element of the CPL acts as if it has slits, either allowing the polarized rays through or (partly or fully) blocking them.
- The rear glass element, the 2nd one the light passes through, takes that linearly polarized light and polarizes it further, but this time circularly. If you think of the linearly polarized light as a line on a graph, with both horizontal and vertical (X and Y) components, the CPL is blocking one component (vertical, for example) more than the other. It turns it into a vibrating wave that sort of spirals. The light that finally reaches your lens is now circularly polarized!
- As described above, you adjust a CPL by rotating the front glass element. This increases or decreases the degree of circular polarization. And if you have reflected light, off a lake or river for example, rotating the filter also changes how much of that plane-polarized light you’re blocking. Again, think of that front glass element as having ‘slits’, which when crossed at an angle to reflected light will prevent some of that naturally polarized light from getting through.
- By the way, that crossing of the slits to plane-polarized light is called cross-polarization, and it’s how polarized sunglasses work. Their “slits” are fixed in a vertical position, enabling them to block the plane-polarized light reflected off of water, roadways and other horizontal surfaces. Look at the reflection off a vertical store window with your sunglasses on and you’ll see they allow that light right on through.
BASIC FIELD USE
- Again the effect increases as you rotate the moveable (outer) ring. In the case of light reflected off water or glass, rotating the filter to its max. position (90 degrees from minimum) will cut the reflection dramatically.
- And for similar reasons, as you point the camera close to right angles (90 degrees) with the sun or other light source, the polarization effect increase dramatically as you rotate the filter to its max. position.
Those are just the two basic ways to use a circular polarizer in the field. There are quite a number of other, more subtle ways to use a CPL in photography. And next time we’ll look at using the filter to improve your images, all the while emphasizing its strengths and dodging the inevitable drawbacks. Have a great weekend and happy shooting!