Wednesday, August 17, 2011

Should I Use Polarizing Filter?

If you are new to digital photography you might have heard some photographers talking about using polarizing filters (or CPL Filters) to improve images. That might have left you wondering what they are, what they are used for and why you might want to have one. I'm going to post some simple examples that show what a circular polarizing filter can do for you, whether you need one is up to you.

The photo below was taken without a polarizing filter. In this image the sky looks a little washed out, it isn't as blue as it should be. The clouds in the sky also lack definition, due to the same brightness from the sun that dulls the colour in the sky. In addition the water in the marsh in the lower half of the image is reflecting the sky, which could be hiding important details that you may want to capture. You can fix some of these issues by editing an image like this in an image editor like Aperture 3 or Lightroom 3, but that doesn't help you if you do not shoot RAW or if you dislike editing your images. What can you do to fix these problems while shooting?


The answer is to use a circular polarizing filter (CPL filter). A polarizing filter works similarly to sunglasses; it enhances colour, contrast and can cut down reflections on water, rock and even plant life. A CPL is a filter that screws onto the front of your lens (check the filter thread size of your lens, which is normally marked on the lens cap or the bottom of the lens itself). Once the filter is screwed onto the lens you turn the front of the filter until you achieve the desired effect. You can see the effect of using a CPL filter in the image below.


There are a few things you need to keep in mind when using a CPL filter, first of all it does not work when you are pointing it directly at the sun. You need to be facing 90ยบ from the light source (the sun) for the filter to be fully effective (have the sun behind you). Secondly, a CPL filter will not work on an overcast day, at least not very well anyway. Thirdly, using a CPL filter cuts down the light coming into the lens, so if you are shooting in lower light you may need to use a tripod. Fourthly, if you are using a wide angle lens (example: Nikon AF-S 16-35mm F4 VR, Canon 16-35mm F2.8L, Tokina 12-24mm F4, Tokina 11-16mm F2.8) you might want to consider buying a thin or wideband CPL, otherwise you might get unwanted vignetting from the filter. I use a Keno Pro1 Digital Wideband CPL, which is multi-coated to help prevent flare. Finally and most importantly, don't use matrix or evaluative metering when using a CPL filter, it will throw off the exposure. I recommend using centre weighted meetering or spot metering if needed.