Filters and Blurring – Text, week 12.

Filters and Blurring.

There are many different types of filters available for cameras but with the advent of digital photography and software such as Photoshop, Lightroom etc. many are now superfluous. However there are three that are still very much of use to the digital photographer.

UV Filter.

The ultraviolet filter is a must. Although it has little direct effect on the resultant image, unlike that of a film camera, it has a number of benefits.
1) It protects the lens. It is far cheaper to replace a UV filter than a lens.
2) It is easier to remove and clean a filter than a lens, so rain drops, mist and grease smears etc. can be removed with no lasting damage.
3) It protects the lens from adverse climate effects, such as misting, raindrops, etc.
4) It prevents sand and dust adhering to the lens and getting in the lens filter grooves.

One thing to be careful of is not overtightening the filter. This can inadvertently happen when cleaning the filter. The filter should be cleaned with a cloth in the same direction as the filter is screwed on, to ensure it remains secure. However this can cause the filter to be overtightened. It can pay to have a filter wrench to loosen a tight filter and they can be purchased for less than £5.

There are a couple of potential disadvantages, but they are minor.
I) They can cause or accentuate glare.
2) They can cause vignetting when used in conjunction with additional screw type filters. It is advisable to swap the lenses or be aware of the limitations.

Overall a UV filter is a necessary piece of kit. It should be pointed out that a quality filter is advisable as cheaper versions may cause image quality reduction.

Polarising filter.

What is a polarising filter and what does it do?

Light is transmitted in wavelengths of varying but equal oscillations. However they can scatter when they hit a reflective surface such as glass or water. A polarising filter is two filters in one. Both have a linear surface fixed in one direction. By rotating one against the other through 90º the amount of unpolarized light allowed to pass through is reduced as the light waves are straightened. As the filter is rotated it adjusts the linear unpolarized light into polarized light as the image shows below.

However there is a point to bear in mind when using one, and that is the time of day and the angle and position of the sun. For maximum efficiency the photographer should be standing at 90º to the sun. A trick I read is to use ‘the pistol trick’ to calculate the optimal angle.
Facing the sun, point the index finger at the sun. Cock the thumb at 90º to the finger. This will give the maximum angle of polarisation. So at Noon GMT when the sun is overhead, the sky would be polarised horizontally, giving an equal appearance to the sky in all directions.
If a polarising filter is used during the blue or golden hour it can cause a darkening in the centre of the image, with a gradual lightening towards the edges. This is particularly noticeable with a wide angle lens. It may be advisable to use a zoom ot telephoto to reduce this.
As the polarising filter is rotated it will be noticed that the image becomes darker, so an adjustment will have to be made to the f-stop, shutter speed or ISO to allow for this. It also allows the photographer to choose how much polarisation to use as it will also saturate the image.

Neutral Density filters.

What is a Neutral Density (ND) filter and why use it?

A ND filter is a glass filter that has a certain darkness to reduce the amount of light entering the camera’s sensor. A good comparison would be like me going out in bright sunlight. Normally I would have to screw my eyes up (reduce the aperture) to see better. Or I can put on sunglasses (add a filter). It allows the photographer to slow the shutter speed and/or create a shallower depth of field. Its use is most often seen where a photograph has been taken and the water has become ‘milky’. To do this with a narrow aperture the quality would be greatly reduced. Filters can be added together to reduce the light. There are two types of filter, a screw type filter or a square filter with a filter holder and a holder ring. The former tends to be a fixed density so a number of filters are required, or a variable filter used that darkens as it is rotated. The latter has a holder that can hold up to 3 filters.
There are also graduated filters. These are often used where the sky needs darkening, but it needs a flat visible horizon, otherwise it darkens the landscape as well, such as the tops of mountains.

Here is a table showing how each filter reduces the amount of light allowed onto the sensor, and how many f-stops the aperture must be stepped down.

ND Factor Fraction Lens area Density % light F stop reduction
ND 0 100
ND 2 ½ 0.3 50 1
ND 4 ¼ 0.6 25 2
ND 8 1/8 0.9 12.5 3
ND 16 1/16 1.2 6.25 4
ND 32 1/32 1.5 3.13 5
ND 64 1/64 1.8 1.56 6
ND 128 1/128 2.1 0.78 7
ND 256 1/256 2.4 0.39 8
ND 512 1/512 2.7 0.20 9
ND 1024 1/1024 3 0.10 10

There are stronger filters like Big Stopper and Mega Stopper but these are usually used by professional photographers. They are often used for landscape photography, but can be used where the amount of light needs to be reduced or a longer shutter speed needed.

Abstract photography using blurring.

Fuzzing, blurring or creating movement or abstract images in a photograph can be done with a number of different techniques.

Once simple way is to move the camera fairly fast in a straight line. This creates a blur that obscures the original image. Another is to quickly rotate the camera so skewing the edges but keeping the centre static. The third is rapidly zooming in/out. The effects will depend on the shutter speed/aperture/ISO.

This is best explained below by the Holy Trinity, or photographer’s triangle, to show how each of the above interact with each other.


There is one word of warning when using filters. It is advisable to only use one filter at a time if using a screw-in type filter. Using two or more of these filters can result in vignetting when the zoom is used, and the aperture setting may need adjusting as the amount of light admitted through the lens can be reduced. Changing filters regularly can damage the threads and also require both the lens and filters needing frequent cleaning.|
See Filters and Blurring – Photographs for my examples.

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