Smart_Photography_-_December_2015_

LEARNING

Polarising Filters

A

polarising filter is one of the

three very important filters for

use in digital photography – the

other two being the Neutral Density

and Graduated Neutral Density filter.

Used correctly, a polarising filter

reduces atmospheric haze, removes

glare, saturate colours, deepens blue

sky and to a certain extent increases

shadow details.

First things first. There are two types

of polarising filters: Linear and

Circular. Don’t confuse the ‘Circular’

variety as being round in shape and

the other, some other shape. Both are

round in shape! The difference lies

in the way they are constructed. Film

SLRs (with some exceptions) used the

Linear Polariser. With the advent of

digital SLRs, it became necessary to

have a Circular polarising filter.

Note: The physics involved in

understanding ‘polarisation of light’

and how a Circular Polariser (CPL)

actually works, is, to say the least,

beyond the scope of this article. The

explanation given below is over-

simplified and hence from a point

of view of those deeply involved in

physics, may not be entirely correct.

Those who want to delve deep in its

theory, can look up a highly technical

write-up by Bob Atkins (www.

bobatkins.com)

See the sketch below. The incoming ray

of light is un-polarised light (vibrating

SP reader Arijit Das had requested us to explain, in Ask

ncle onnieǡ ho™ to correctly use a ’olarising �ilter.

Since the explanation is rather lengthy, we decided to

have a separate write-up on it.

in all directions). When it gets reflected

off any surface, it gets polarised

(vibrates in any one direction only).

Now if you were to place a polarising

filter in the path of the polarised light,

depending on the angle of polarisation,

it could partially, or to a great extent,

block the light, thereby also reducing

the reflection from the surface. This is

basically how a polariser works.

Modern D-SLRs use a reflex mirror

that reflect some amount of light to the

metering system and the viewfinder,

and also transmit some light to the AF

sensors. If you were to use a Linear

Polariser on such a camera, there

could be metering errors as well as

Rohinton Mehta

Incoming ray of light, radiating

in all directions

e�lected ray of lightǡ radiating

in only one direction

e�lecting surface

autofocussing errors. Hence, with

D-SLRs, we need to use a Circular

Polariser.

Note: You can safely use a Linear

Polariser with mirrorless cameras;

there is no mirror where the problem-

causing secondary polarisation could

take place. Note also that you can use a

CPL with film cameras.

Any filter placed in front of the lens

causes a loss in the amount of light

entering the lens. Since all modern

cameras use Through-the-Lens (TTL)

metering, this problem is automatically

taken care of. Typically, a polariser

reduces about 1.5 -2 stops of light.

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Smart Photography December 2015