Do I need to use filters when taking photographs? Surely I can do anything I need with photographs in post processing using Photoshop or Lightroom or other photographic imaging software.

It is possible to replicate the effect of most filters in Photoshop or other software but some photographs cannot be replicated in post processing. Filters are made of either Resin or Glass. Glass filters have a consistent density and opacity and is used for the Big Stopper and Little Stopper Filters. Resin is used for the Graduated Neutral Density Filters

Fllters are produced in three different sizes. 75 mm, 100 mm (standard) and 150mm (wide angle). The ones shown are the standard 100 mm filters. The circular polarising filter is 105 mm.

Why use filters? Well, firstly we need to look at what filters do. They can reduce glare and reflections, reduce the amount of light to permit long exposure images of moving water, clouds etc. They can balance exposure in images so that post production is not necessary where there are extremes of light and darkness in images. They can protect lenses from dust, dirt, grease and damage. They can help create images with a WOW effect. They are mainly used for landscape photography but can also be used for portrait photography. This is particularly the case where a combination of daylight and flash is used. Neutral Density Filters can allow a wide aperture to be used on bright days to throw the background out of focus or to allow you to set the correct flash synchronisation speed.

Long exposures tends to create a glass / silky effect which can be very artistic if a little unnatural. By using a tripod and taking two photos - one with a short exposure and one with a long exposure it is possible to blend the two photos in Layers in Photoshop to create an image that has both a natural and unnatural look.

Lenses have filter thread sizes. Buying a separate filter for each lens can be very expensive, particularly if you have lots of different lenses with different thread sizes. You would need to buy a separate filter for each different thread size. Alternatively you could buy step up rings which will allow you to use filters with one thread size on a lens with a different thread size. Most opt for a filter system. Lee Filters, Cokin, B&W, H&Y, Marumi, NISI, Hitech, etc.

Filter systems work by using a holder which fits to the camera lens by using a thread adaptor ring the size of the lens thread. For each different lens size you buy a different thread adaptor ring. This means that you only have to buy one filter and it can be used with all lenses of different sizes provided that you have the correct adaptor ring. Most systems allow you to add filter guide blocks so that you can have up to three slots for filters. This allows you to use more than one filter and use filters of different types. You can also add a guide for a Polarising Filter, letting you use 4 filters in total.

You can slide the filter up and down to get the exact amount of filter to be applied. You can also rotate the filter holder so that the filter affects the exact part of the image that you wish to act on.

There are 4 types of filter. Polarising Filters, Neutral Density Filters, Ultra Violet (uv) filters and Coloured Filters.

Polarising Filters are the only filters that cannot be replicated in Photoshop or other photographic software. The effect of Polarising Filters is similar to that of polarised glasses. The filters reduce the amount of glare, make certain colours such as greens and blues more saturated and can reduce or even eliminate reflections from water. They allow detail to be captured in an image which cannot be added in Photoshop or other photographic software at a later date. A good example is with reflections in water. Sometimes reflections in the water are very desirable and part of the image.

Examples would be leaves on trees in autumn being reflected in the water of a river or lake. But sometimes a photographer wants to take a photo of something in the water, say a fish or stones and wants that detail.

A Polarising Filter can remove the glare and reflections on the water and allow details of the fish or stones, or other contents of the water. Polarising Filters can also be used to enhance the sky and it look bluer or make plants or grass look greener.

According to the angle at which the Polarising Filter is to the sun and the object being photographed the effect is lesser or greater. The best angle is 45 degrees to the surface of the water or glass. The strength of the polarised effect is determined by the amount the filter is rotated. As a result the photographer can choose between no reflection at all to a large amount of reflection, and any stage in between.

Care needs to be taken when using Polarising Filters to increase saturation of blues and greens as sometimes the effect can look unnatural. In strongly lit situations, a blue sky can look almost unnaturally black. With some wide angle lenses the use of a Polarising Filter can make blue skies look patchy.

Polarising Filters work well with bright sunlight. Most cloudy skies do not produce much polarised light so their effect is less pronounced in cloudy conditions.

Polarising Filters are particularly useful when taking photos through glass or windows as the reflections can be reduced or eliminated. A downside of the use of Polarising Filters is that there is a loss of light which can be as much as one and two thirds stops.

Most cameras use circular Polarising Filters. This is not a reference to the filter's physical shape but to the way that the filter's crystals are laid out Many circular Polarising Filters are in fact square. Slimline Polarising Filters can be to a focal length of 16-17 mm with a full frame DSLR camera.

With all filters, care must be taken to keep them clean and free from dirt and grease.

Neutral Density Filters are filters which reduce the amount of light. This is not always necessary. Low light around dawn and sunset can achieve the same effect as neutral density filters. Why is low light desirable?

There may be times when for artistic effect it is desirable to use a low shutter speed without using a very narrow aperture which may create images of inferior quality. The artistic effect may be a long exposure of water in rivers, the sea or fountains or of clouds moving in the sky. Reducing the amount of light entering the camera may allow relatively wide apertures to be used as well as long exposures. Another use of Neutral Density Filters can be to take a photo of crowded areas and allow a photo to be taken without any people (provided of course that the people move during the exposure). An example might be a photo of Trafalgar Square. It would be virtually impossible to take a photo of Trafalgar Square in London without any people in it but if the exposure could be, say, 30 minutes or longer anyone in the photo who moved during the time of exposure would not appear in the photo.

What types of Neutral Density Filters are there? Well Neutral Density Filters are essentially darkened glass which blocks out light. Welder's glass is effectively a type of Neutral Density Filter. A welder uses it to darken the effect and protect his or her eyes from the intense light from welding. A piece of welding glass could be put in front of a camera to reduce the amount of light entering the camera and registering on the sensor of the camera. But there is a very good reason why photographers tend not to use welding glass - Neutral Density Filters have very defined strengths to give photographers control over the amount of light entering the camera. And neutral density filters can create colour casts. This is not an issue with black and white photos. And welder's glass could in theory be used for black and white photography if the photographer measured the strength of the glass and the amount of light reduction created by using the glass. But with colour photographs colour casts can be a significant problem.

Filters come in many shapes and sizes. They also come at very different costs. Some use cheap filters. Some use expensive filters. What is the difference? Well, the first thing depends on the quality of the lenses being used. If they are poor quality and cheap, then perhaps using a lower quality filter is not such an issue. But, if the lenses used are very high quality and expensive, the question needs to be asked "Why spend all that money on an expensive, high quality lens, and then put a cheap, poor quality bit of glass in front of the lens?". Filters are ostensibly bits of glass. Not surprisingly the more expensive filters produce less degradation of image quality than the cheaper ones. And they also produce less, or even no colour casts. Those are the main reasons for the huge differences in cost.

I use Lee Filters. They are the most expensive filters but, in my view, they are also the best. On a workshop in Umbria with David Noton, I met the MD of Lee Filters who gave me a tour of their factory in Andover, Hampshire. I saw for myself the processes to create the filters (all made by hand by skilled and experienced crafts people) and the machines used to test their quality and the amount of colour casts. If the filters do not meet the extremely high standards, they are thrown away. That is what you are paying for with Lee Filters. Professional photographers such as David Noton, Joe Cornish, Jeremy Walker, Tom Mackie, Charlie Waite, John Gravett, David Ward, Mark Denton, Mark Bauer, Jonathan Critchley and Paul Gallagher all use Lee Filters. There is a very good reason - they are very high quality.

Neutral Density Filters (or NDs as they are known) come in different strengths. The Big Stopper reduces the amount of light by 10 stops. The Little Stopper reduces the amount of light by 6 stops. There are Neutral Density Filters that reduce the light by 3 stops. And recently the Super Stopper Filter, a Neutral Density Filter has been created by Lee Filters which reduces the amount of light by 15 stops. It is possible to use 3 Neutral Density Filters combined so that the amount of light could be reduced by 31 stops (15+10+6). With a Polarising Filter added which reduces the amount of light by one and two thirds stops, it is possible to reduce the amount of light by 32 and two thirds stops. In practice, such an amount of light reduction is never required.

I mentioned above that low light exists at the beginning and end of the day when Neutral Density Filters are probably not necessary. Their use tends to be during the day when there is a lot of light. A good example might be a fountain, say in Trafalgar Square where even at the lowest ISO level of 100 creates a shutter speed of 1/125th second at say, f/9 or f/11. A shutter speed of 1/125th second would freeze the droplets of water mid air and an aperture of f/11 might create too much depth of field.

The photographer may want to use an aperture of say, f/2.8. From f/11 to f/2.8 is 4 whole stops. The shutter speed would increase from 1/125th of a second to 1/2000th second. The water of the fountain would definitely be frozen in mid air. By using a Little Stopper with 6 stops, the shutter speed at f/2.8 could be slowed down by the equivalent of a further two stops to 1/30th second. That might still be too fast a shutter speed.

If the photographer were to use a Big Stopper instead of the Little Stopper with an extra 4 stops of light reduction, the shutter speed could be reduced to 1/2 second. That would create a degree of movement of the water. Adding a three stop neutral density filter would permit the shutter speed to be reduced to 4 seconds. That would create a silk like effect.

The example above is an extreme example of a photograph being taken at midday in bright sunshine. But where the exposure is 1/30th second at f/11, use of a Big Stopper would permit a photo to be taken at a much slower shutter speed of 30 seconds. Use of a Little Stopper as well would increase the length of the exposure to 8 minutes. Reducing the size of the aperture from f/11 to f/16 would have the effect of increasing the length of the exposure to 16 minutes. That would probably allow a photo of a crowded area to be taken without people in it. Most people would have moved in a period of 16 minutes.

The following shows the effect on shutter speeds:

Original Shutter Speed. Neutral Density Filter Stops New Shutter Speed

1/125 second 10 stops. 8 seconds

8 seconds 10 stops 16 minutes

15 seconds 10 stops 30 minutes

30 seconds 10 stops 1 Hour

There is a downside of long exposures - the amount of noise or grain generated is greater than for short exposures. Modern cameras tend to handle noise extremely well and of course modern software also does a very good job in reducing the amount of noise in a photograph. The effect therefore is not significant.

Graduated Neutral Density Filters (or Grads as they tend to be known) are effectively versions of Neutral Density Filters. Instead of all the filter being neutral density and applying a reduction in light across the whole filter, only part of the filter is darkened. Some of it is not darkened at all. Hence, the term graduated. The neutral density part of the filter is only applied to some of the filter, not all of it.

There are two types of Graduated Neutral Density Filters - Hard Filters and Soft Filters. They are used to make all of the image require the same exposure. An example would be to use the Neutral Density Filter to darken the sky so that it requires the same level of exposure as a building or mountain.

Hard Filters have a hard edge between the darkened part of the filter and the clear part.

Soft Filters have a soft edge between the darkened part of the filter and the clear part. They have different uses.

Hard Filters tend to be used where there is a well defined line between a light part of an image and a light part. An example would be the flat roof of a skyscraper and the sky.

Graduated Neutral Density Filters come in different strengths - one stop, two stop and three stop and of course in Hard and Soft versions. The Graduated Neutral Density Filter is positioned at the point where the brightest area of the frame meets the darker area. This is achieved by sliding the filter up and down in its holder, assessing the effect through the viewfinder or live view on the back of the camera, until the correct point is reached.

Vignetting. The more filter guides are used the more likely vignetting occurs. For full frame cameras the minimum focal length for one guide is 16 mm. For two guides it is 17 mm and for three guides it is between 21 mm and 24 mm.

Ultra Violet Filters are used by some to protect lenses from dirt, grease, water and physical damage. They adjust the light and do not let ultra violet light in to the camera. For most the difference is not noticeable. A downside of Ultra Violet Filters is that they can cause lens flare. If this is an issue, they should be removed when taking the photograph.

Coloured Filters are filters which add colour to an image to change it. They are often used in black and white photography to change the contrast of a photograph but blue filters can be used in colour photography to make the image look cooler and orange filters can be used to warm up an image. This effect can be replicated very easily in post processing software such as Photoshop or Lightroom. Personally, apart from the cost saving, I like the flexibility that Photoshop or Lightroom give to make changes. This flexibility is not possible if you use a Coloured Filter at the time of taking the photograph.

There are other filters which can be used to create special effects such as star bursts, soft focus etc.

Lens Hoods. When using filters you cannot use a standard lens hood on the lens to shield the lens from external light and flare. It is possible to buy a Lens Hood (bellows type) either for standard lenses or for wide angle lenses (to avoid vignetting). The lens hoods can be expanded and angled to avoid lens flare. But be warned when working near the sea and in windy environments the lens hood can act like a kite and the wind can cause movement of the filter. This is particularly noticeable for long exposures.