In the days of film, filters were used for a variety of purposes, but since digital photography has developed, there is a growing trend towards getting similar effects in Photoshop. However, there are definite advantages in using filters to get it “as right as possible” in camera. I do use Photoshop, but while I might spend hours waiting for a shot until the light is just right or for a subject to appear, I’m reluctant to spend hours in front of a computer manipulating and correcting a photograph in Photoshop, I just don’t have either the time or that sort of patience for it. That is perhaps the obvious advantage of filters, to save time later. For me though, there are many other advantages and I will attempt to promote those reasons for using filters in the digital age.

There are many different filters available, some such as the warm up filters and various colour filters no longer have an important role to play in digital photography. Warm up (and cooling) filters have largely been replaced by white balance. Different film had different responses to light, so there were two main choices if light wasn’t ideal for that sort of film, you either had to use filters to balance the light or change film to a different type. For example, there were specific films for tungsten lighting. Likewise, coloured filters were used a lot in black and white photography to emphasise different tones, for example, a blue filter would absorb the blue light to darken a blue sky to enhance the mood. Photoshop has sliders that will easily mimic the same look and they are much more flexible and very quick. Other filters though, I believe are still as important as they ever were and while the effects of some can be mimicked in Photoshop, I don’t think the results are as good and some effects can’t be reproduced at all.

I will go through some of the more useful filters for digital photography and try to persuade others to make more use of filters. I will also look at some less useful filters that are often mis-sold with lenses. One thing to remember with filters though, the more glass you have between the sensor and the subject, the more glass light has to go through, reducing image quality each time, so always go for the least number of filters possible to get the effect you desire.

Ultraviolet (UV) Filters

In my opinion, UV filters are the biggest waste of money, yet they are the ones most often sold with new lenses. Their original purpose was to reduce haze (hence their other name haze filter) or to reduce the blue in skies that were unnaturally dark. However, they have little or no effect on digital sensors and the clarity slider in Adobe Raw is actually more effective. They are usually sold with lenses to protect the front element, but in reality, in many of the lenses they are sold with, the front element is deeply recessed, so the chance of damage is actually quite low. The biggest problem is though, that most of the UV filters sold in this way are of low optical quality, so will reduce the image quality in a photograph. There are exceptions of course, as some of the high end UV filters will probably not have a noticeable effect on image quality, although they will still increase the risk of flare slightly. They are quite expensive, but if you have a high quality lens which doesn’t have a deeply recessed front lens element, then they may be worth considering, as a £100 filter is still a lot cheaper than replacing front element following damage.

Polarising Filters

Polarising filters come in two flavours, linear and circular. Linear polarising filters should never be used on digital cameras though, as they interfere with the metering system. Circular polarisers on the other hand allow light to enter normally and allow the meter to take an accurate reading. In addition, circular polarisers can be attached via the screw mount on the front of the lens or as part of a square filter set, where they slide into a specially designed holder and can be used in conjunction with graduated filters.

Circular polarisers are probably the most important filter for any photographer to own. They reduce glare and as a consequence increase saturation in a way that can’t be reproduced in Photoshop. Sunny days are never ideal for photography, but a polariser may reduce glare enough to rescue an otherwise unusable shot. A macro photo of an insect is likely to result in glare from the wings or from a shiny body, but a polariser may reduce that enough for it not to be a problem. Likewise, other reflective surfaces, such as water and shiny vehicles at shows for example benefit from polarisers in any light.

The biggest disadvantage of polariser is that they result in the loss of between 1-2 stops of light (depending on the polariser). In low light, this can be an issue, but it can also aid creativity. Sometimes you may want to produce some motion blur, such as rivers and waterfalls, but if light is too bright, they can be used to reduce the amount of light to enable the increase of shutter speed required. Another problem with polarisers occurs when used with wide-angle lenses. Because the field of view is so wide, the angle from the light source varies considerably across the frame, resulting in uneven polarisation. Also something to be aware of, polarisers tend to give a cool cast, but that can easily be adjusted if you shoot in RAW, although it can also aid creativity if left. Alternatively, some companies (e.g. Singh-Ray) produce a warming polariser, which can give more pleasing results than adjusting the white balance.

Polarisers work best when the light source (either reflected or direct) is at 90 degrees to the sensor, but they will have some affect at other angles too. They consist of two pieces of polarising glass (a bit like in sunglasses), which filters the light in two different directions, basically ordering the light photons into nice orderly rows, instead of scattered all over the place pointing in different directions. In order to polarise the light into these nice rows, the outer ring must be turned and as the light is optimally filtered, the image gets darker through the viewfinder.

Neutral Density Filters

Just like in polarising filters, there are two main types of neutral density (ND) filters, solid ones (usually screw mounted, but square filters for filter sets are also available) and graduated filters, which are usually only available for use in filter sets (although a few screw mounted filters are available). ND filters are neutral grey (hence the name), with the aim of not having any colour cast, although this varies according to their strength and the manufacturer. Some of the cheaper filters are known for causing an undesirable colour cast (usually pink) and also, some of the very strong filters can cause a cast.

ND filters are measured in terms of the amount of light reduction they cause in whole stops. Confusingly, it isn’t always obvious without a bit of research, how many stops of light they reduce and to make matters worse, there are two main scales used. The table below lists some of the more common  strengths of ND filters, the relative light compared to no filter use and the two main labelling conventions, based on logarithmic and neutral density scales.

Number of Stops	Amount of Light Relative to no Filter	Logarithmic Scale	Optical Density Scale
1	1/2	ND2	0.3
2	1/4	ND4	0.6
3	1/8	ND8	0.9
4	1/16	ND16	1.2

Both solid and graduated filters are measured in the same way.

Solid ND Filters

Solid ND filters most commonly screw into the lens mount. Their main purpose is to reduce the amount of light. On very bright days, it can be difficult to use the aperture you need to get the creative effect you are looking for and in some cases, even with the highest possible shutter speed, over-exposure is inevitable. The original use of solid ND filters was in outdoor portraiture. Often a very wide aperture is required to blur the background, but at such wide apertures, light entry is very high. Without a filter, the photographer would need to stop down, risking the intrusion of the background, making isolation of the subject difficult. Typically a filter of 2-3 stops would be used. However, recently, filters as high as 10 stops have been designed. These have a purely creative use and are used mostly by landscape photographers for excessive blurring of water motion in daylight. Normally, this would only be possible in low light, but use of these very dark filters has opened up a new style of photography. The biggest downside of such dark filters technically, is that it is impossible to see through them, so it is impossible to compose, focus and meter. Focussing and composition must therefore be set first. The exposure is then either calculated or corrected by trial and error.

Graduated ND Filters

Graduated ND filters are as the name suggests, they start off clear at one end and gradually darken towards the other end. Most commonly, they are used as part of a filter set, which consists of an adaptor ring, attaching to the lens screw mount and an adaptor that slides over the top of the ring, allowing the square or rectangular filters to be slotted in. The filters are then turned on the ring and moved up and down to position them as required. Screw mount graduated filters are available, but are of limited use, because they cannot be adjusted, so the scene has to be composed with the edge of the graduation in the centre, thus breaking one of the main compositional rules.

Graduated filters are further differentiated into hard and soft graduations. Soft graduated filters are a gradual transition, evenly distributed. Hard graduated filters on the other hand are clear on the bottom half, without any graduation, then at the halfway point, the graduation starts suddenly, before gradually getting darker as in soft graduated filters. Hard graduations are usually used when there is a definite horizon, such as in coastal photography, while soft graduations are useful when there are hills or mountains blocking the true horizon. One company called Singh-Ray, in conjunction with a well known photographer called Daryl Benson, have created a special hard graduated filter, designed for sunsets, where the brightest part of the sky is on the horizon, instead of higher in the sky. They call this filter a reverse graduated filter, as the darkest part of the filter is in the centre, gradually decreasing towards the top, while the bottom is completely clear.

The purpose of graduated filters is to selectively darken some areas of a scene, usually the sky (but it could also be a bright reflection). The camera sensor, just like slide and negative film, has much less dynamic range than the human eye, so the filters are used to even up the exposure differences, enabling the sensor to capture detail in both the shadows and the highlights. To some degree, gradients applied in RAW editing software can mimic the effects of graduated filters, but usually, some detail is lost and some strange colour casts can sometimes appear. This is particularly true of many of the high density crop sensors, which generally have less dynamic range than full frame sensors.

Graduated ND filters are probably the most complex to use and take the most practice. Hard grads are easier to position than soft grads, as they can be seen in the viewfinder quite easily, but it can be quite tricky to position soft grads correctly, until more experience is built up.

Other Filters

There are many other types of filters, but many have limited use in digital or are mainly for specialist uses. Most of these are coloured filters. Some people use coloured grads to add colour to sunset, but I tend to practice the principle “less is more”, I apply it to processing and filter use. I’m probably a bit of a traditionalist, but I think coloured filters can look fake and awkward. Other filters rarely used in digital include warm up filters, for certain film types and lighting conditions and red, blue and green filters, which were used in black and white photography, which is the principle used in Photoshop when applying the channel mixer or using the sliders in a black and white layer (where yellow and infra-red are also available).

And Finally…

If used judiciously, filters can take your photography to a new level. While Photoshop can reproduce some of the effects, I find it isn’t as satisfying as getting it mostly right in camera. Filters can be a challenge, but they can produce some very effective results and save huge amounts of time in post production. Why spend hours in front of a computer, when you could be out there getting more photos?

Related articles

• Video – Polarising Filters (photocreativeness.wordpress.com)
• ND Filters – What’s All The Fuss? (photofocus.com)
• Conversion to Black and White – Filters (photocreativeness.wordpress.com)