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Beyond the Camera - Filters
A filter is simply a piece of glass or plastic that fits in front of the lens and has some effect on the image. In general, they are used to:
- Change image colour balance or contrast
- Decrease the amount of light getting to the sensor (allowing to you change the exposure parameters)
- Create some visual effect (some useful like dealing with unwanted reflections, some cheesy like creating stars out of lights).
There are two types
- Circular screw on. Convenient, especially if it is to stay on the lens. The downside is that you have to match the filter diameter to the lens diameter, and if you have more than one lens, it could well mean that a particular filter may only fit one of them. The cost of larger diameter filters can be considerably more expensive than smaller ones.
- Square filters. The square filter slides between a standard size holder which is then attach to the lens using a converter ring. For each lens you own, you just need to buy one converter ring, and all your filters can then be used on that lens. This type of filter was made popular by Cokin who now have four established standards called 'A' (67mm), 'P' (84mm), 'Z-PRO' (100mm) and 'X Pro' (130mm). The 100mm system has also been adopted by other manufacturers such as in the Lee Photographic Filter System.
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The use of filters is a lot less popular than it was in the days of film photography since many of the basic effects can now be simulated in software (e.g. colour tints, soft focus, filters for black and white photography etc.) However there are some effects that must be applied at the capture stage.
Below we will look at some of the more popular filters still in fairly widespread use. |
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UV Filter: Designed to reduce haziness created by ultraviolet light. It is transparent to visible light and so has a negligible effect on most images although it is possible to buy stronger UV filters that can impinge of the visible spectrum.
Many people buy this filter to leave permanently on the front of their lens to provide a form of protection for the front lebs element. However read the section below on this before you consider doing the same.
Polarising Filter: Can be used to darken bright skies, increase colour saturation and reduce reflected light from non-metallic surfaces (e.g. water or glass) (the last of these can't be corrected in your photo software afterwards).
Reducing reflections off water or glass can be useful in enabling the photographer to shoot through windows or beneath the surface of water by eliminating (or strongly reducing) reflections that would otherwise obscure the subject on the other side of the glass / water.
There are two types of polarisers: linear and circular. For a digital camera, you MUST use a circular polariser otherwise the metering and autofocus on the camera will not function correctly. In this context the word circular has nothing to do with the physical shape of the filter. Don't assume a polariser is circular just because it is round. These can be rather expensive for what looks like a simple piece of glass, but in terms of quality, you get what you pay for.
The theory of polarising filters should indicate that 50% of light will be removed by the filter - thereby requiring you to increase the exposure by one-stop (+1 EV). However, in reality, more than 60% of the light is usually blocked, therefore you will probably need to increase exposure by about 1.5 stops (see the guide than came with your polariser).
They also tend not to work as well on very wide angle lenses as the amount of polarisation can vary across such a wide angle of view and become obvious in the picture.
One other think to watch out for. You rotate a polarising filter when it's on the lens to get the effect that you want. However on many lenses, the front element of the lens rotates during focusing. Therefore the effect of the polarising filter will change. If you use one of these filters a lot, then you should look to getting a lens with internal focussing (the front of the lens does not rotate as focus changes).
Lastly, polarising filters aren't cheap if you want a decent one.
For a detailed explanation of polarisers, check out the Wikipedia Article which gives a nice scientific / mathematical explanation.
Neutral Density (ND): These are the sunglasses of the filter world. They reduce the amount of light reaching the sensor. Although you can obviously shut down the aperture, there are times when this is not possible or undesirable.
Here's a couple of examples of when you might want to use a neutral density filter:
- It is extremely bright and you still want a slow shutter speed to capture motion. Even the smallest aperture might not be sufficient to reduce the light. Lenses are also not going to produce their sharpest image at the smallest apertures.
- You want to create a shallow depth-of-field and therefore require a large aperture, but the shutter speed won't go fast enough.
ND filters are graded with a number showing the fraction of light that is reduced (sometimes called the attenuation factor). The table below gives some examples:
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Filter Name |
f-stop reduction |
% light transmitted |
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ND2 |
1 |
50% |
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ND4 |
2 |
25% |
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ND8 |
3 |
12.5% |
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ND64 |
6 |
1.6% |
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ND1000 |
10 |
<0.1% |
A downside of ND filters is that they do not attenuate all frequencies of light by exactly the same amount, and therefore a colour cast may be introduced into the image (especially if you buy cheap ND filters). Many also do not attenuate infra-red light at all, and so looking at the sun through an ND1000 filter may appear safe, but there is still a lot of non-visible and potentially harmful radiation getting through.
A special type of neutral density filter is the graduated neutral density filter. This has a stong ND effect at one side with a gradiated intensity to no effect at the other side. This is useful in landscape photography as it allows the photographer to darken the sky without affecting the land and therefore reduce the extreme dynamic range that landscapes often have. The effect can be replicated in post-processing by taking two exposures (one for the ground and one for the sky) and combining them in Photoshop (or similar).
White Balance: A new type of filter specially designed for digital cameras is the white balance calibration filter (a modern day equivalent to the grey card). It works with the custom white balance mode in all DSLRs to record a neutral image for the current lighting conditions which the camera can then utilise to set a near perfect white balance setting. These are not cheap and to be honest, if you use the RAW mode on your camera, you can set the white balance by eye later.
Diffussion filter: Sometimes called a softening filter or soft focus filter - it is used in portraits to create a dreamy appearance. Rarely used in digital photography as the effect is easily achieved in software later.
Diopters: Sometimes called close-up filters. The allow objects close to the lens to be brought into focus and are considerably cheaper (though not as good) as a dedicated macro lens or extension tubes.
Convertors: Not really a filter at all but lets just pretend. Their purpose is to effectively alter the focal length of the lens. Some are designed to give a wider angle of view (e.g. a 0.8x wide angle convertor); others are designed to increase the focal length such as a x2 teleconvertor - the latter often attaching between the lens and the camera rather than on the front of the lens.
The downside of filters
With any filter you are adding an extra piece of glass into the lights route from the outside world to the cameras image sensor. Common sense will probably tell you that the more things the light has to travel through, the more 'quality' will be lost - be it image resolution, sharpness, contrast etc. Of course the effect of the filter is often more important than the minor loss in quality - but the message here is to only use filters when you really need them. Before you use any filter on a lens, think about what it is you want it to change in the image, and decide if this is important to the image.
Use of filters as lens protectors
As mentioned earlier, many people will use a filter to protect the front glass element of the lens in case the camera is knocked or dropped - after all it's cheaper to replace a filter than buy a whole new lens. You can also clean the filter more frequently without worrying about damaging the lens coating. But let's stop and think for a minute - if you've just spent a small fortune on a top quality lens - do you really want to be putting a £6 ($US 12) piece of glass in front of it? Would you buy a Ferrari and then put budget / retread tyres on it?
Many people use a UV filter or skylight filter as they are fairly inexpensive and supposedly have negligible effect on the image. The reality is that these filters can have a significant effect on the final image and not in a good way.
One effect that is often attributed to a cheap UK / skylight filter is ghosting. These are bright patches of light that start appear in places where there wasn't any light in the scene. It is actually a ghost image of a light somewhere else in the image which has reflected off the sensor, back of the inside of the filter and onto the sensor again in a completely different position. If you get these, try removing the filter for a while and you will probably see them disappear for good.
If you really must have a filter on the lens for protection, you can buy a clear protective filter (Canon call it a "Regular Filter"). This has no filtering effect at all and it's purpose is to just protect the lens. It is also made of high quality multi-coated glass to cut down stray reflections to a minimum. The cost for these starts around £25 (nearly $US 50) depending on filter thread size, so you decide if this is worth it.
So how should you protect your lens?
One of the best ways is to use a lens hood - as well as the protection these offer, they can also provide a definite positive effect on the final image quality.
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