"Polar Express"

* * *  B L O G  W A R N I N G  ***

Right up front I want to let you know this is going to be a techie post. My daughter likes that so she can know to just skip today.

I am going to jump into the photo-techno-geek mode here and talk about polarized lens filters. The concept is the same for a circular polarizer or a linear polarizer. Ok, what is the differences between circular and linear. A linear filter is more of the dated traditional filter where rectangular glass (filter) is slid down one of the grooves in an apparatus mounted to the front of the camera lens. As the filter slides into frame, the filter effect will occur, be it polarization, neutral density (ND), or a graduated neutral density (ND-Grad). A circular filter is one that screws on directly onto the front of the lens. The drawback of a circular ND-Grad filter is the graduation line can not be adjusted for the horizon vertically in the frame. One circular filter that every, every, every lens should have on it is a simple multi coated ultra-violate (UV) filter. This filter actually does very little photographically but it does protect the front of your expensive glass. The circular polarizer mounts just the same but rather than sliding the filter down a groove, it is simply rotated ninety degrees to become effective. Take a look a the two images below.

Both of these images were from a Canon 40D with a EF 16-35 2.8L USM lens mounted. Shot at 35 MM at 1/40th second with an aperture of f/6.3. The circular polarizer was mounted on bot shots, the top image the polarizer was off and the bottom the polarizer was on.

That is impressive but how does it all work. Smoke and mirrors my friend... Smoke and mirrors. Well, Ok... Maybe not smoke, but certainly the refractiveness of glass and that is close to mirrors. Take a look at the diagram below.

I am no specular or optic expert but I did stay in a Holiday Inn Express last night. I am not kidding about that I am in my room in the Holiday Inn Express in Andover KS. Ok, back to the topic at hand. As light passes through polarized glass it refracts (bounces) away from the lens at the entry point as well as the exit point leaving a cleaner (filtered) value of light. That light then passes through a second element of polarized glass with the same result.

When a manufacture creates polarized glass the make-up of the glass is designed to align the light waves. Each element of the filtered glass addresses the orientation of the oscillation of the entry light wave. When these two sets of glass are oriented, the polarization effect is maximized or turned ON. Conversely when the filter elements are ninety degrees out of phase, the effect is minimized or turned OFF.

The three images below demonstrate the design and effectiveness of the circular polarized lens filter. The first image shows the lines of polarized glass out of phase effectively turns the filter OFF.

The second image shows the filter being rotated and the effect starts to become apparent.

Finally the filter bezel is rotated to the phased position and the effect of polarization is maximized.

A couple of items you need to consider when using a circular polarizer are the stop value and the other is the useful light direction. Just mounting a circular polarizer onto the front of your lens will provide a nice two stop reduction in light. Essentially having a 2X ND filter... Sweet! The other issue to consider is when will a polarizer be most effective. That is when the light (sun) is entering the lens at ninety degrees to the filter. Don't forget that you can still use the filter to block the glair on water just like those cool sunglasses.

Hey, If that was not entertaining, I hope it was educational. Cya tomorrow... Doug