Basic Filters

One of the first things a beginning photographer looks at after deciding to become more serious about taking pictures is filters. Filters change the light before it hits your sensor which allows for a great many of special and useful effects. However, if you're just getting started it can be a bit dounting to understand what each does and why you need it.

In the digital age, photographic filters come in two flavours: effects you can achieve in postprocessing and effects you can't. Generally the consesus is that if you can do it in post, do it there because you'll have more freedom to play around and change things and get it exactly like you want.

Examples of filters that give effects that can be done in post:

• colour filters (blue/red/mauve/whatever)
• blurring/antialias filters (as long as you don't have moiré)
• White balance filters (really just colour filters)
• Special effects (starburst, glitter, triplicate, ...)

Examples of effects that cannot (easily) be achieved in post:

• Neutral Density filters
• Polarizers
• Gradual neutral density filters
• Infrared filters
• Close-up filters

Each of these latter ones is discussed on this page.

Types of Filters

Filters come in two distinct types: circular screw-on and square slide-in. The former screw directly onto the lens while the latter require an adaptor. Both have advantages and disadvantages.

Square filter advantages

• The same filter can be used on lenses with different diameters by using different adaptors (circular ones can use step-up rings, but this might induce vignetting).
• Filters can be slid up or down, which is great for filters that aren't the same all over (like ND Grad filters, see later)
• Quick-use filters by holding them against the lens without an adaptor
• 90° rotation notches
• Quick and easy changing of filters

Circular filter advantages

• Smaller size due to not needing an adaptor, especially handy for storage and transport
• Can be kept on the lens all the time (good for protectors)
• Can be used with a lens hood (but rotating filters become inaccessible)

Which one is best? Well, it depends and it's actually often a matter of personal preference. For most filters I prefer circular ones, but for ND Grad filters I'd go for square ones.

Ultra-thin filters

Most circular filters can be found in an ultra-thin form. These minimize the thickness of the filter to reduce vignetting on wide-angle lenses. They are almost never needed on any other lenses. Even on ultra-wide lenses like the ZD 9-18mm a normal filter will usually suffice and ultra-thin ones are only needed if you want to combine two filters on top of eachother (say put a polarizer on top of a protector).

Protection/UV filter

I'm taking these two filters together because these days they are virtually the same thing. In the film era, UV light would create a purple cast in the image which would skew the colours, especially of the sky. This is why UV filters are often called skylight filters. Digital cameras generally have UV filters built in on top of the sensor already, so a separate UV filter is mostly superfluous. This means that UV filters really have only a neglible effect on the image. Because of this, UV filters are popular to protect the front element of expensive lenses. After all, a UV filter is cheaper than a lens (be careful though: a big shock can splinter the filter which could scratch the front element). In the past few years the major filter manufacturers have been releasing plain (but coated) glass filters for the same purposes. These tend to be cheaper and they block marginally less light while being stronger (thicker).

Whether or not to use a protection or UV filter at all is a bit of a contentious issue with adherants on both sides. I'm generally in favour of using them on more expensive lenses (since I have some scratches on the front of one of my lenses), but do take this advice: don't go cheap. If you use a protection filter, make sure it's a high quality multi-coated filter of a well regarded brand (like Hoya HMC/Super HMC, or the expensive line of B+W). If you do not wish to make that expense, skip it altogether. A cheap UV filter can significantly reduce your image quality!

Neutral Density (ND)

Neutral density filters block part of the available light. That's it. They come in various strengths, with a number generally indicating the number of stops of light they block, so an ND-1 filter blocks one stop (half the light), an ND-2 filter blocks two stops (three quarters) and an ND-8 filter turns a sunny summer day in spain into Moria. The "neutral" part of the name means they block all wavelenghts equally: they don't change any colours. For this reason they are sometimes also called gray filters or gray density filters (or even neutral gray filters).

You might be wondering why anybody would want to reduce the light. After all, much of the time we find ourselves with too little light. This is why many are obsessed with noiseless high-iso images: to get more light. Neutral density filters come in useful when we want to use slower shutter speeds than the light allows. Examples of this happening are legion: dreamy waterfalls/wave effects in the sunlight, bright snow, using a fill-in flash outside when shooting against the sun, ultra-long exposures for those fancy taillight-streaks on highways, etc etc. A neutral density filter solves all of those.

Polarizer

A polarization filter is a tricky one to explain. Remember that physics class where they said light was a wave?  Well, every lightbeam is actually composed of many of these waves, all going the same way. However each of them oscilates in a different direction. When seen head-on, each of them will have a different rotation. A polarizer filters out all rays except those with a certain rotation. This will darken the image (by about 1-1.5 stops) and in that respect they are similar to ND's (except polarizers aren't as neutral). After the light has gone through the polarizer all its waves oscilate in the same direction: the light has been polarized.

Polarizers are designed to rotate relative to the rest of the camera, which lets you select the rotation of the light that is allowed through. If you were to put two polarizers on top of eachother so that they align perfectly, the effect would be the same as a single polarizer: about 1-1.5 stops of light loss. However rotate one of them and the image will get gradually darker, until at a relative angle of 90° all light is blocked. At this point the first one is still letting through quite a bit of light, but none of this light lies in the region that the second lets through.

While this is nifty and makes for a neat variable-strength darkening filter (though one with a slight colour cast), the real use of polarizers is to filter out reflections. When light reflects off any object a natural polarization occurs. A polarizer can be used to either reduce this reflection (filter it out) or strengthen it (filter everything else out) depending on how it is rotated. For similar reasons a polarizer can be used to enhance the blue colour of a sky (blue light get refracted and reflected off tiny particles in the atmosphere). This effect will be strongest when shooting away from the sun and this is the most popular use for these filters. As a bonus any clouds can be made to stand out more or less by rotating the filter.

Other uses for polarizers:

• Strengthen rainbows
• Enhance or remove reflections from lakes
• Reduce reflections when shooting through glass (remember you're losing 1.5 stops of light though, so it's not ideal for indoors).
• Reduce reflections of a flash in glass indoors

Fun note: LCD screens use a polarisation technique. You can turn an LCD screen black by rotating a plarizer in front of them.

Polarizers and wide angle lenses

Wide angle lenses are often said to have a "natural polarizing effect", with which people mean that skies shot with a wide angle lens are generally quite blue on their own. This is not exactly a polarization effect and is actually mainly due to there simply being a lot of sky in the shot which makes them more important to the exposure meter.

That said, it's generally not adviced to use a polarizer on an ultra-wide lens because the polarization of reflections depends on where the light is coming from. If you have light coming from a lot of different angles (as with an ultra-wide lens) it will be polarized in many different directions. Putting a polarizer between that will result in a vertical band of your sky being noticeably darker than the rest because light coming in from that angle was being blocked more.

Gradual Neutral Density (ND Grad)

ND grad filters are part ND filter and part clear glass (generally speaking). Usually they are 50/50. They come in two flavours: soft grads and hard grads. Hard grads have a sudden change from the ND half to the clear half while soft grads take their time to change. As a result, hard grads are a lot more noticeable (when used incorrectly).

Either kind of filter is used for cases where part of your image is much brighter than another part. The most common case is for sunsets, where the light from the sun and the sky will flood out the land in the foreground. To remedy we use an ND grad filter with the dark half covering the sky. This way we can make an exposure that shows a beautiful sunset without reducing the foreground to underexposed blackness.

Hard grad filters are good when there's a clear horizon line, for example near the sea. Soft grads are useful when the horizon is very uneven, as it would be in the mountains. However, the closer the sun is to the horizon, the less useful a soft grad becomes.

Gradual (or graduated if you prefer) filters benefit greatly from a square or rectangular filter system because these let you place the dividing line at a height of your choosing while with a circular filter you're stuck with the line in the middle (limiting composition options).

A similar effect to using grad filters can be achieved by taking multiple exposures at different exposure levels and combining them carefully in HDR.

Infrared

Where an UV filter blocks ultraviolet light, an IR filter blocks everything except infrared light. Most digital cameras only let a bit of IR light through, but this is still enough to take IR photos if you don't mind using very long exposures. However, IR photography is outside the scope of this series.

Close-up filters

These are only filters in the sense that you attach them at the end of a lens. They differ from normal filters in that they do not have an equal thickness which really makes them lenses. They can be used for macro photography, which will be discussed at a later time.