What are lens filters?

Lens filters are accessories that are attached onto the front of the lens.  These attachments may modify the image recorded unto the sensor.  The filters are typically circular but there are also square filters commonly used by landscape photographers.

The diameter of the filter will depend on the diameter of the lens' filter thread.  The filter thread size may be found marked on the lens usually with a Φ symbol.

What are the types of lens filters?

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Here are common filters usually used by photographers.

Clear and UV Filters

These type of filters are probably the most common filter.  Every lens should have this filter.  Clear filters are typically just plain glass.  A UV filter is used to reduce haze from ultraviolet light.  

These filters are basically used for protecting the front element of the lens from dust and scratches.  It's more economical to replace the filter than to replace the lens.

CPL or Circular Polarizers

A polarizer filters out light with a particular direction of polarisation. Basically it reduces the amount of reflection from a reflective surface.  CPLs can darken the sky, makes the sky bluer.  This filter is commonly found in a landscape photographer's bag.

Captured using an ND8 filter

ND or Neutral Density Filters

These are solid dark filters.  These are used to allow for long exposures.  It is used by landscape photographers to create motion blur especially in water.  There are different grades of ND filters.  Common is an ND8 filter, or 0.9 or a 3 stop ND filter.

ND filters come in circular and square filters.

GND filter

GND or Graduated Neutral Density

A GND filter is neutral density filter with different attenuation at different points, typically dark on one side and gradually clears on the opposite side.  It is used to balance out the exposure of a scene, like when the sky is too bright compared to the ground.

These are commonly square, but there are circular GNDs too.

IR Photography by Michael Ocaña

IR or Infrared Filters

IR filters capture infrared light.  These creates a dreamy, false color images.  Digital images captured by IR filters are post processed through photo editing software like Adobe's Photoshop to produce that dreamy effect.

What are square filters?

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A common filter is a threaded round filter and screws on directly onto the lens.  A square filter needs to have a holder set in order to be mounted.  

A holder set consists of two accessories: a thread adapter ring and a filter holder.

The thread adapter should have the same diameter with the lens' filter thread; much like a circular filter.

The filter holder is attached by sliding it on the thread adapter.  It will lock on place and is freely rotatable.  A Cokin holder may hold 3 filters at once.

A square filter is then attached to the holder by, again, sliding it in the holder mount.  Since the holder can accommodate 3 filters,  it is up to the photographer's creativity to mix different filters to get the desired effect.

There are different sizes of square filters and it's corresponding holder.  Common sizes are 85mm (Cokin P-series) and 100mm (Cokin Z pro).

A square holder set may differ with different brands but the function is the same, to hold square filters.

Graduated Color Square Filter

These filters are like GNDs but instead of one side neutral density, it is one side color.  Common colors are graduated blue,  red, orange, green, and sunset orange.  These are used for creativity effects.

There are other types of filters but these are the common ones.  So there you have it, any more queries about lens filters?  Here are some links for you.

Photographic Filters
Cokin
Lens Filters Explained