So I get asked a lot of questions about filters, how they work, what to get, why this does that. So heres my run down on filters...

Theres two main types of filter: Screw-ins and Slot-ins.
Most of you will be familiar with the screw-in type, as I hope you all have UV/Skylight filters on the front of your lenses! [Cheaper to replace one of them than a whole lens, remember]
The slot-in is a more exotic type.

You can get most types of filter in both screw in and slot in, but if you have a variety of lenses with different filter ring sizes, buying one filter of each size can become costly rapidly.
I favour the slot-in type. I have screw in UV filters on all my lenses, but ontop of that I always have a slot-in filter holders attachment ring.

Slot-in filters seem [very] costly to begin with, and sometimes rather complicated. But you need to weigh up the price of the initial start up cost and filters, versus buying the same filter for each of your lenses.

How it works -
Camera and Lens > Attachment ring, screwed to the lens > Filter Holder, which clips to the attachment ring > Filters, which slot into the holder.

The main manufacturers of slot-in filters are Cokin and Lee.

I started with Cokin filters, as they're the best place to start. They're relatively inexpensive, decent quality, and they do the job. But they do have their flaws.
Their "neutral" density filters aren't what you'd call "neutral". They do have a rather distinct magenta cast to them, but their quality is reflected in their price.
If you're justing getting into using filters, I'd recommend starting with the Cokin P size filters.

Next in the filter food chain are Lee's.
Lee filters are world renowned for their quality, from the filter holds to the filters themselves, no expense is spared, as this is also reflected in their price.
Where as you can get a Cokin ND grad for £15 at Warehouse Express, a Lee equivelent will set you back 4 times that at £60 each.
The quality is incredibly, only matched [and bettered] bit Singh Ray, an American version.

There are alternatives though.
My current filter complement is comprised of filters from a company called Hitech.
They come in two sizes, Cokin P, which is 85mm across, and 100mm, which is the same as Lee's.
They're a big step up from Cokins filters and sit just below Lee in terms of quality, but come at half the price. A Hitech ND grad will set you back £35 including VAT, nearly half the price of a Lee, but with almost the same quality.

If you're serious about slot-in filters, but can't justify the price of Lee's, I'd highly recommend Hitechs.

Another company to look out for is Kood. I have a Kood .6 [2-stop] solid ND filter. Its neutral[ish], solid, and does the job.
They also make grads in both Cokin P and Lee 100mm sizes.

If you really have some cash to blow, I'd recommend investing in Singh Ray filters.
They're a US based company producing some of the best filters on the planet. They make ND grads and solid ND's, as-well as some special, unique to Singh Ray polarisers. More on them later.
Singh Ray really became a hit with the crowds when they released the Daryl Benson reverse grad filter, which I'll explain in a bit.

Like I said, my filters are a mix Hitechs and Lee's, with one Kood ND and a Heliopan polariser thrown int oo.
If you're in the market for some serious filter goodness, I'd highly recommend Hitech filters and Lee holders.
The Lee holders and attachment rings are expensive, but fully customisable and incredibly well made. You even get a special screw driver with them!

Do some research and set yourself a budget.
If you know anyone that uses them, ask to borrow them for a weekend.
You wont be able to do without after...

Filter Types -

Theres a few main types of filter you should be looking into. You've probably heard the terms a hundred times on Flickr or in photography mags, but heres my rundown on the most popular.

The UV Filter -

As I said, you should all have one on each of your filters.
A decent UV filter = £40ish. A new lens = £400ish. Its a no brainer really.
A decent UV or Skylight filter wont affect the image quality at all, it'll serve as a shield for the main element in your lens.
UV and Skylight filters were used back in the film era as film was suseptable to UV polution. Sky's would become hazy and you'd experience a drop in colour.
Digital sensors aren't as prone to UV polution as film, hence why UV's are now used as protection filters.

The ND filter -

ND stands for Neutral Density. They're [supposed to be] completely neutral gray.
ND's are mostly used for gaining slower shutter speeds, for blurring movement, or for shooting at wide open apertures in bright sunlight.
ND's normally come in densities of .3 [1-stop], .6 [2-stops] and .9 [3-stops]. Though you can get them in others, like 1.2 [4-stops].

A big hit recently has been the B+W 10 Stop filter. Its a screw in filter, which has a massive density of 3.0, 10 stops of light reduction, allowing you to achieve multiple second exposures even in broad daylight.
Lee recently released their own slot-in version, named "The Big Stopper". Its a standard square 100x100mm slot in filter, with a special foam gasket on the back to provide a light-tight seal.

If you want to achieve longer exposures for creative effect, ND's are the way to go.

ND Grads -

The essential for the self respecting landscape photographer.
Like the ND filter, they're neutral, providing varying degrees in light stopping power, but unlike the standard ND filter, only part of the filter is coated, the rest remains clear.

The ND Grads main purpose in existance is to provide balance between a foreground and the sky. Our eyes can balance the bright sky with the darker foreground, but our cameras sensors aren't as lucky.
The ND grad helps solve this matter.

Slotting [or screwing] the filter in, you'll see the sky slowly start to darken [assuming you have it the right way up].
Positioning the graduation on the horizon will help the camera's sensor record a better balanced scene, with less of a radical difference between light levels.

The graduation of the filter comes in two main types. Soft or Hard.
Soft grads provide a more leniant fade between the darker top and clear bottom, making them ideal for uneven horizons, such as landscapes in the hills.
Hard grads provide a sharper, more definted graduation between dark and clear, suiting them for even horizons, such as seascapes.
Hitech used to make a "Razor" grad, which has a very sharp, abrupt graduation, but they've stopped producing them now.

I have both Soft and Hard grad filters, in .3 [1-stop], .6 [2-stops] and .9 [3-stops], aswell as a hard 1.2 [4-stop].

ND grads mostly come in densities of .3 [1-stop], .6 [2-stops] and .9 [3-stops], like standard ND's, but they are obtainable in various other densities.
I own a 1.2 [4-stop] ND grad, for when shooting directly towards a bright sunset. Back in the day this was a custom item, but you can now order them directly though Hitech's website.

Another variation on the ND grad is the "Reverse ND Grad". Originally pioneered by Singh Ray, and now copied by Hitech, its the perfect ND grad filter for when the sun is low to the horizon.
It has two graduation. A sharp graduation going from the clear to dark, but then dark then slowly graduates back to a lesser density, ideal for when the horizon is a lot brighter than the higher parts of the sky.

I recently invested in the Hitech .9 [3-stop] Reverse ND Grad and its easily my favourite filter for sunsets over the sea now.
Combining it with a .6 [2-stop] soft edged ND grad usually gives the perfect combination, using the reverse the hold back the horizon region, and the standard grad slightly placed below the horizon to hold back any blow out on the sea.

Polarisers -

Polarisers are a pretty essential piece of kit for the landscape photographer, as its effects can't be replicated in post processing.
They work in two ways; they reduce glare on reflective surfaces like water and windows and boost contrast and saturation.

Theres two main types of polariser. A linear polariser and a circular polariser.
First off, its nothing to do with their shape. Both come in rectangular and circular screw-in formats.
Linear polarisers are cheaper, as they're easier to manufacturer but will mess with your cameras autofocus and metering abilities, which is why the circular polariser came along.

You're best off getting a circular polariser, they rotate freely and are easier to use.
Main problem with screw-ins are their size, they're quite deep, meaning they stick out from the front of the lens, even ones which are classed as "slim". This becomes a serious problem when using the filters with wide angle lenses.

I use a 105mm Heliopan slim polariser, which connects to a special ring on the front of one of my Lee filter holders.
Its a pricey investment, but it means I can use with my standard filter set up, without having to remove the attachment ring, mount the filter, mount the attachment ring on the filter, then the holder, blah blah.

Another problem with polarisers on super-wide lenses, is uneven polarisation.
Polarisation is at its maximum when 90° from the sun. Wide angle lenses take in so much of the view, that a lot of the scene will be a lot less that 90°.
When this happens, you'll get a big area of darker blue sky where the polariser is at its most effective, which will then fade into un-polarised sky.
It leaves you with a distinct and annoying wedge of dark blue in the sky. You need to take this into consideration when using a polariser on a wide angle.
My normal work around is to only semi-polarise the sky, and use an ND grad with it to help with the contrast.

Misc. Filters -

Theres hundreds of other types of filters out there, it'd take me a life time to explain them all. Heres a brief round up.

Colour filters -
Colour filters are used in black and white photography. Different colours have different effects to the final outcome. For instance, red will boost over all contrast, blue skies will turn dark and white clouds will become brighter.

Colour correction filters -
Name says it all really. Back in the film days, film was balanced to certain light sources, normally daylight, around 5,600°K, or Tungsten, around 3,200°K.
Colour correction filters where used so you didn't have to change films when changing locations and light sources.
They were also used for sunrise and sunsets, when the colour temperature of the light changes from normal daylight. Shooting a sunset on normal, uncorrected daylight film lacked the colour of the original scene. CC filters were applied, normally 81 series warming filters, to boost the temperature, giving it a more true to life feel.

The IR filter -
Infrared photograpy is a specialist subject. Specialised IR film is available, which is only sensetive to infrared light, but in the digital age, its more common to use a IR filter.
IR filters are almost opaque. They block most visible light, and only allow infrared light through. Some cameras are more sensetive to IR light, like the Nikon D70, which make them perfect candidates.
IR photography gives other-worldly results. Tree's and greenery turk spookily white, and blue skies turn almost to black.
I use a cheap eBay sourced IR filter. It was about £15, but does the job perfectly. For that price its hard not to buy one to try it out.
For more on infrared photography, check here.

Tacky 80's effect filters -
Back in the 80's, there were two types of filter that were all the rage.
Soft-focus filters and starburst filters.
Soft focus filters, as the name implies, give a soft, blurred feel. They were normally used for portraits, blurring the outer edge of the scene, giving a dreamy soft of feel. Reeeaaaaally tacky.
Starburst filters were just as tacky. Using small start shaped holes in the filter, any bright source of light becomes a big starburst. Depending on how many points the star shape has, determines the amount of points the starbust has. Great for night shots of the city, but still really tacky...

Round Up -

I hope this has cleared up some facts about filters and not made it more confusing.

If you're testing the water of landscape photography, and don't want to invest a fortune, look into Cokin P sized filters. They're relatively cheap and will give you good results.
If you're more serious about your landscape photography or are looking to upgrade your filters, I'd highly recommend looking into Hitech filters and Lee holders.
If you have some cash to burn, look into Lee's, they really are amazing.
If you have a lot of cash to burn, its Singh Ray all the way.

Recommendations -

Basic Set Up -

If you're just starting with filters, I'd suggest .6 [2-stop] and .9 [3-stop] soft edged grads, a .6 [2-stop] normal ND filter, a holder and the appropriate rings.
This should cover you in most circumstances.

Enthusiast Set Up -

If you're an enthusiast looking to upgrade to a high quality, versatile set up, I'd recommend the following -
Lee Foundation Kit - Filter Holder - £52.99 - Warehouse Express
Lee Wide Angle Attachment rings - £40 - depends on size - Warehouse Express
Hitech Filter Kit - Soft edged - £70 - Team Work Photo

Look to pay around £180 for a decent set up. Its pretty pricey to begin with, but it opens up a whole avenue of upgradability for the future.

Examples -

I hope you found the article helpful. Any questions, leave a comment or shoot me an email.

Cheers,
Matt.