photographywired.com

Filters: What and Why?

Filters offer a host of potential to a photographer – both on a functional and creative level.  To provide an initial insight I thought I’d put together a list of filter types – explain their function, and give an example of what you can achieve with some of them.

Before we get into that though, just a quick word about filter systems.  When you’re looking at buying filters there are two distinct types (although some filter types are only available as one or the other):

• the circular type which screw into the end of your lens
  • these must be the appropriate size for your lens  – if you check the front of your lens, around the rim there will be a symbol like a circle with a vertical line through.  The figure next to this tells you the required filter size for your lens.
  • adapter rings can also be purchased which allow you to use different sized filters on different lenses.
• square filter system lenses
  • these work in conjunction with a filter holder, which attaches to the end of the lens.
  • common systems are provided by Cokin (the ‘P’ range is a popular place for amateurs to start) and Lee.

Clear Filters: A clear filter has no purpose other than to protect the end of your lens.  These are not as common as they used to be as most people now use UV or Skylight filters (see below) for lens protection.

UV/Skylight Filters: Although these are primarily used for lens protection they were designed to improve image quality by reducing the amount of UV light which penetrates the lens – reducing haziness.  Although cheaper quality filters can have a detrimental effect on image quality – including increased risk of flare and image aberration.

Strong UV Filters: Not so widely used.  These are yellow in colour (as opposed to clear), and severely reduce the amount of UV and also a small amount of violet light which reaches the lens.  They can also help to reduce the blue fringing common in digital images.

Polarisers: Polarising filters reduce the amount of haze and reflected sunlight in a shot.  They are primarily used to darken skies, and reduce reflections in shots (particularly reflections on water – allowing the lens to penetrate the surface and pick up underwater details).  They can really increase the sense of drama in a sky.  While some filter effects can be recreated using post-processing software – the effects achieved using polarising filters cannot be, which explains their continuing popularity.

Important: if your camera has a thru-the-lens (TTL) autofocus and light metering facility (which all digital cameras do) you must use Circular Polarising (CPL) filters and not Linear Polarising filters!  Linear Polarisers are not compatible with TTL metering and autofocus systems.

Neutral Density (ND) Filters: These are essentially grey filters, which reduce the amount of light entering the lens.  They are used to allow longer shutter speeds (or larger apertures) to be employed, without blowing the exposure.  They are numbered according to the increase of f/stops they allow – with 1 extra f/stop everytime the ND figure doubles (beginning with 1 f/stop from an ND2).  So an ND4 (or ND104) filter allows an increase of 2 f/stops, and an ND8 an increase of 3 f/stops etc.  Some great creative effects (e.g. light trails, tide blurring, etc) are achievable using the incredibly dense filters – such as ND110 (also referred to as ND1000) – which would otherwise only be achievable in near darkness.

ND Grad Filters: These are most commonly found in square format filter systems, although some are available as circular thread filters.  They are used to darken portions of a shot, but leave others unaffected.  The most common use is to allow a foreground to be correctly exposed without blowing the exposure on the sky.  The ND effect is slowly graduated from dark to clear (hence the name).  As with plain ND filters they are available in differing strengths. Hard Grad filters are also available, which feature little or no graduation between dark and clear.

While ND Grad effects can now be widely recreated in post-processing software (Lightroom 2 has an ND Grad function) some of us still prefer to get the image right in the camera to begin with (using the real thing).  But the preference is entirely personal.

Macro Filters:  These are exactly like little filter mounted magnifying glasses which you attach to the end of your lens in order to get up close.  They are a cheap way to get into macro photography – but as with most cheap things, you’ll be making some sacrifices in terms of image quality (over the use of a dedicated macro lens).  They are available in a range of dioptres (from +1 to around +10).

Coloured Filters: These are largely no longer used in the world of digital photography, as their effects can be easily recreated in post-processing software (and usually to better and more controllable standards).  In the past they were largely used for colour correction (e.g. a blue filter corrected the orange tones of tungsten lighting) – but this is now overcome using the White Balance feature in modern cameras.

They are still useful, however, in the realm of black and white film photography – for increasing the contrast of certain colour ranges.  For example, an orange or red filter will darken the sky, but not the clouds – thus increasing the contrast and drama in a sky (in a similar way to a Polarising filter – but to an even greater extreme).  A green filter will also darken the sky, but will lighten any green (e.g. trees, grass) – increasing the contrast between the blue and green elements in shot.

Coloured Grad filters are also available (the most common ones being Tobacco and Sunset).

Diffusion Filters: Again, a filter type largely consigned to the history bin.  This filter (incredibly popular in the 70s and early 80s!) softens an image, and was most commonly used to produce that steamy boudoir portrait look that was so popular back in the day (and was much better for your lens than the old trick of smearing it with vaseline).  Similar effects are incredibly easy to create using software (should you wish to revisit the 1970s anytime soon).

Infrared Filters: These will block all visible light, and only allow infrared light to pass.  They are used in conjunction with infrared film.  (Quite an interesting subject for another day!)

Star Filters: These were also very popular back in the day, however unlike diffusion filters they are enjoying something of a renaissance.  They turn any bright point of light in your shot into a bright shining star (by means of diffraction or prisms – depending on the filter).  They are numbered according to the number of points the resulting star will have (i.e. Star 4 filter creates 4-pointed stars, etc).  While the effect can be created during post-processing – it’s a much more laborious process than simply attaching a filter, hence it’s return to (cult) popularity.

There are also a large range of ‘special effects’ filters (largely produced by Cokin) which you may or may not wish to investigate.  Most of the effects they produce are incredibly easy to reproduce using post-processing software – so their use is really limited to that hardy species, the dedicated in-camera photographer.  Some of the most popular are:

Spot Filters: Soften the majority of the image, while leaving a clear area in the centre.

Prism Filters: Also known as ‘multi-image’ filters – because that’s what they do.  They produce a kind of kaleidoscopic effect.

Speed Filters: Create a simulated motion blur on objects in the shot.

Split-Field Filters: Allow you to focus very close to a subject and retain the same clarity for the background. This is achieved by placing a diopter (close-up lens) in one half of the lens frame.

---

Feel free to share your filter-related advice or questions below!

Tags: circular, cokin, filters, grad, hoya, lee, lens, macro, nd, neutral density, polariser, polarizer, protection, special effects, square, star, UV

2 Comments to Filters: What and Why?

1. 

   Just quick fix on ND filters…

   A ND4 filter reduces by 2 stops not 4.

   ND2 = 1 stop
   ND4 = 2 stops
   ND8 = 3 stops

   It’s known as the Attenuation Factor.
James Hill on February 8th, 2009
2. 

   Yep – dead right. What he said!

   Unfortunately my proof reader was indisposed when I posted this last night
Paul B on February 8th, 2009

Leave a comment