Circular & Linear Polarizing & UV (Ultraviolet) lens filters

What do the different size lens filters mean?  It indicates the size of the actual lens and gets put on to filter out light.  Obviously, a 62mm UV filter will not fit on a 67mm lens.  So, make sure you have the right size for a specific lens.  This also applies to camera hoods – it is sized the same way.

THE 3 MUST HAVE LENS FILTERS

Polarizing filters eliminate reflected light from an image, acting like sunglasses for your lens.  They remove some reflective glares.  Polarizers are one of the few universally useful filters.  They’re effective for both film and digital and unlike warming and other color correcting filters, they really can’t be digitally simulated.  They can increase color saturation, reduce reflections and darken a blue sky.

  1. A Circular Polarizing filter is much darker than the UV (Ultraviolet) filter and should  be used outdoors in bright sunlight.  This filter lets in more light than the linear polarizer and is a little more expensive.  This is preferred for auto-focus lenses.  While most serious photographer will use this lens on a sunny day, you have to know your particular camera and what works best with each lens.
  2. A Linear Polarizing filter lets in light from one direction.  A linear polarizer is a device which selectively allows the passage of only certain orientations of plane polarized light.  At one orientation it might allow the passage of only vertically polarized light, while if rotated by 90 degrees it would allow the passage of only horizontally polarized light.  Half way in between, at say 45 degrees rotation, it would allow passage of only 45 degree plane polarized light.  Partially reflecting surfaces, such as a window or the surface of a pond, often reflect one, linearly polarized, component of the incoming randomly polarized light much more strongly than the others (keep this in mind when we talk about how SLR mirrors are partial reflectors!).  If a linear polarizer is rotated so as to block this reflected linear component, the reflection will be suppressed.  This is one classic use of a linear polarizer, to remove reflections.
  3. A UV filter comes with a multi-resistent coating which strips the UV light and haze from your images.  This helps reduce lens flare and sun rays or spots when you are shooting in sunlight.   Many photographers use this filter to protect their lens from dirt and smudges.

If you mix linear polarizing and circular polarizing lenses you will get an elliptical light effect.  Technically, polarized light is light with its electric vector oriented in a direction which is predictable rather than random.

Most light sources (except for some lasers) put out randomly polarized light, i.e. an equal mixture of every kind of polarization.  If objects reflected this light with no change, all a polarizer would do would be to act like a neutral density filter, no matter how it was oriented, but that’s not what happens.

If a linear polarizer blocks or allows the passage of linear (or plane) polarized light, you might justifiably think that a circular polarizer should do the same thing for circularly polarized light – and it does, but it is also sensitive to linear polarization.  This is just as well, since the phenomena we have just looked at, such as reflection of randomly polarized light from a partially reflecting surface, result in linear, not circular polarization.

A “circular polarizer” actually consists of two elements. The first is a linear polarizer, exactly the same thing as the linear polarizer we have just discussed. The second element is called a quarter wave plate and it is cemented to the back of the linear polarizer with a specific orientation such that the light emerging from the quarter wave plate is circularly polarized (hence the “circular” polarizer name).

Remember that the intensity of the reflection from partially reflecting surfaces can be dependent on the linear polarization angle of the incoming light. Windows and pond surfaces are two examples of partially reflecting surfaces, but another is the main reflex mirror in many modern SLR cameras. Older SLRs used fully reflecting mirrors, which reflect all polarizations with equal intensity. Newer SLRs, and particularly autofocus SLRs, often use partially reflecting mirrors. The reflected light goes to the viewfinder and metering systems, while the transmitted light goes on to the auto focus sensors. If a linear polarizer was used on such an SLR, the intensity of light sent to the metering system would depend not only on the intensity of the light, but also it’s polarization angle. However, when a picture is taken the mirror is out of the light path and the film is only sensitive to the intensity of the light, not its polarization.  Thus, metering errors can (and do) occur when a linear polarizer is used with an SLR which has a partially reflecting main mirror.

If a circular polarizer is used, the circularly polarized light that emerges from the polarizer is reflected by the partially reflecting mirror with an intensity which is independent of the angle at which the polarizer is set, eliminating metering errors.

A circular polarizer acts just like a linear polarizer as far as photographic results are concerned. It has to be adjusted for angle, and when properly aligned it darkens blue skies, saturates colors and eliminates reflections from water and windows. Indeed, and this is the crucial point, it is a linear polarizer, followed by a component which circularizes the resultant light only for the sake of the camera sensors but which has no visible or photographic effect on the image. There is no difference between how and when you use a linear and a circular polarizer. The fact that it is a circular polarizer matters only to the camera, not to you or to the final image on the film.

Polarizing filters also help in cutting out atmospheric haze, since the source of this haze is scattered light.  They saturate colors and work great with blue skies. Any camera can use a circular polarizer. The only downside to using a circular polarizer where only a linear polarizer is needed is that circular polarizers are a little more expensive than the linear variety.

You can also easily tell if a polarizer is linear or circular even if it is not marked. Hold the polarizer about 2 or 3 inches in front of your eye, and look at your reflection in a mirror. Looking from the camera’s viewpoint (i.e. with the filter threads pointing towards you) you should see the image of your eye in the mirror (as if looking through a neutral density filter). Now turn the filter around so that the filter threads point at the mirror. If the polarizer is linear you should see the same thing that you saw before, but if it is circular it will appear black and you will not see the reflected image of your eye. This is because light reflected from your eye passing through the circular polarizer exits in the direction of the mirror as circularly polarized light (let’s say it’s left handed circular polarization). When it reflects from the mirror it reverses its polarization to the opposite sense – in this case it is reflected as right handed circular polarization. The filter only passes left handed circular polarization when the light enters from the “camera side” of the filter, so the reflected light is blocked. The situation is analogous to the effect of a vertically polarized linear filter on horizontally polarized light – little or no transmission of the light can occur.

Polarizers are one of the few universally useful filters. They’re effective for both film and digital and unlike warming and other color correcting filters, they really can’t be digitally simulated. They can increase color saturation, reduce reflections and darken a blue sky.

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