How to use a CPL filter (in)correctly
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That’s what it can seem like.
Under the right circumstances.
Turning the ring of a CPL filter, looking through the viewfinder to see what happens.
Watching the glare from foliage disappear, the reflections from water being removed, or the sky darken and the clouds and colors pop.
The situations are many, where the correct use of a CPL filter can seem to work like magic on the scene in front of you.
Let’s take a look at how to use, and sometimes overuse, the polarizer effect.
When to use a CPL filter?
A CPL, circular polarizing filter, or polarizer filter is a piece of glass you attach to the front of a camera lens, which filters out certain parts of the light from entering the lens.
The really geeky, nerdy, scientific explanation of how this type of filter works can be found here.
The CPL can be used for darkening skies, reducing reflections and glare from foliage, water, glass and other reflective surfaces.
By doing this it will also in many cases enhance the color saturation and contrast of a photograph.
Again, this in turn, can help “cut through” or reduce haze by increasing the contrast in an image.
Some of these effects are difficult/impossible to do in editing software such as Adobe Photoshop.
That’s why this is the most important filter you could add to your photographic arsenal.
What’s the rotating ring for?
This type of filter has an outer ring that rotates. With this ring, you can “dial” in the desired strength of the polarizer effect.
You’ll find that often a subtle effect from the polarizer goes a long way.
A little tip is always rotating the ring clockwise, like when you screw the filter ON the lens. Otherwise, you might accidentally unscrew the filter when you rotate the ring, and it might drop and break (yes of course it has happened to clumsy ol’ me 😉 ).
A couple of things to consider when using a CPL filter
The angle of the lens relative to the direction of the sunlight
When using the filter for darkening the sky, you have to take into account where the sun is. If the sun is directly behind you, or in front of you, using the filter to darken the sky will have little to no effect. It doesn’t matter how much you turn the ring.
At 90 degrees to the sun is where you will get the most significant effect of the polarizing filter.
Knowing this, you’ll have to be especially aware of the strength of the effect, when using a wide-angle lens.
The following image was taken with my Sigma 10-20mm lens.
At 10mm, this lens has a 102-degree angle of view. This means that in landscape shots like this it will cover a great portion of the sky.
As you can see in this image, the right part of the sky is much darker than the left. This is due to what I described earlier.
The portion of the image that is 90 degrees to the sun is the darkest, and then it tapers off to each side.
This is why you should take care when using a CPL filter with a wide-angle lens, or not use it at all, depending on the circumstances.
The filter steals some light
A standard polarizer steals about 2-EV (aka f-stops) worth of light, meaning you will have to compensate by using a longer shutter speed, raise the ISO or use a larger aperture compared to when not using the filter.
This may also be used to your advantage, for example if you need a little longer exposure time to blur water. Then you’ll have both the benefit of the longer exposure time AND the removal of the reflections.
Any additional piece of glass you stick in front of your lens will make it more prone to lens flare. This concerns all filters, not just the polarizer.
Just be aware of it. Use your lens hood and/or shield the lens from the side with your baseball cap or another suitable item.
A few things to consider when buying a CPL filter
You’ll have to know what size of filter that fits your lens. This is usually printed in mm close to the filter threads on your lens. Some common sizes are 49mm, 52mm, 55mm, 58mm, 72mm, 77mm, 86mm.
If you’re buying a CPL filter for a super wide-angle lens like the Sigma 10-20, it would be wise to get a so-called “slim” version. This filter is thinner than the regular filters to avoid vignetting (dark corners in the image).
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