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Article - Technical Term Explained

Protective Filters

Many of us have filters on the front of most of our lenses, not to produce effects but to provide protection to the lens. It is also far easier to clean a filter than it is a lens.

Some lenses however it is not a good idea to have a filter on, these include very wide angle lenses and macro lenses. A few lenses such as fish eyes are shaped in a way that filters cannot be fitted. In each case when first putting on a filter you need to perform a few simple tests to check that the filter itself is not in focus at any point and that it does not affect the image when you are at the widest view. Lets look at what you can do easily.

First before you put on a filter take a photograph of a white piece of paper at the widest angle at the smallest aperture, perhaps something like f22, under flat constant lighting. In theory at least you will get a consistent slightly grey image across the full area. Next put in the filter and repeat.  If the edges are darkening now slightly towards the outside or the corners are clipped off, you will know that the filter is effecting the image at the widest angle. If this is the case you may find you can see this happening in the viewfinder as you go wider or if not perhaps when you do this and then use the depth of field button. If you were to take for example the Nikon 18-200VR lens, you will find that at 18 you can clearly see this occurring in the viewfinder. It is possible to over come this if you desperately want to put a filter on a lens that shows this limitation at its widest angle, using a shim that converts up to a larger filter size, but you will need to experiment with this and be aware that you may then have difficulty getting the standard lens hood to fit over this. In many cases it may be that we operate with these lenses without a filter, and when we do decide to use a filter, such as a polarizer we are careful not to go to its full widest position, or perhaps generally use shims.

Shims are useful for all of us who have a variety of lenses requiring different filter sizes, in that they allow filters of a larger size to be used, allowing us to have just one of each filter type and use them across lenses. Shims are also some times called step up rings.

Assuming that you have discovered no problem with the first test lets look at another. Take off the filter and put on the filter a single hair, or piece of cotton, you can do this so that it sits on the filter when the camera is pointing down or up.  Our preference is with the cameras pointing down as we try not to have the camera pointing up at all as this may cause more dust to fall back onto the sensor. Set the camera again at the widest angle and manually focus on the nearest point it will focus and take the paper again. Depending on the lens you will see nothing if itís a long lens, a slight blur in some and a sharpish item or shadow with others, particularly macro lenses. The purpose of this is to see if you have any dust on the filter if it will or may become visible in the images you take. If you are using a shim to over come the first problem you need to do this with the shim being used as this moves the filter slightly away from the lens and may cause in some case more to come into focus.

The objective here is obviously to be aware of the effects of the filter so you can decide if itís a good idea in your own case and for the use you have for the lens. You may also want to repeat this with the more usual settings you may use. So for example you may have a wide angle lens that is capable of focusing very close but which you only use for taking landscapes as you have a separate macro lens you use for close items, in which case the problem may show in the test hair test at close focusing but not at the sort of distances you will normally use.

Next we come to the question about how much it may affect the image.

This is less easy to test, and we could prove that any filter in some situations is detrimental to the image. The filter is often of far inferior quality to the lens, and often is either not coated or has a far poorer coating than the lens. It is also far more likely to catch flair than the lens, even when a lens hood is used.

If you do comparative tests you find that at some focal lengths  and apertures and in some lighting conditions it is far more detrimental than others. In most cases in practical use it is not a problem. In some cases the effects it creates you may like.  For example if you take a photo of something that is very brightly back lit by the sun, you will often see a halo around the item, be it a bird or a post. You will often find if you remove the filter this effect is not present. It is caused by the way the light is refracting through the filter, and the lens s not affected so much due to the choice of glass types and coating.

Perhaps we should also consider the beneficial effects of filters, mostly the type we are looking at here that live on lenses are protective. They stop the lens getting scratched and they keep the lenses clean. A filter can be easily cleaned, removing dust, spray and pollution such as oil in car fumes. If it is in a bad way you can even wash it in washing up liquid and swill it off, something that would not be wise with our lens.

Most of these  filters are skylight or UV filters and in theory at least cut down the effects of UV light and therefore allow you to see less haze or mist in shots. You would need to try out your camera/lens combination with and without these filters to see if they had any real effect in your case. If you want to make a serious reduction in haze a polarising filter is usually far more effective, and if you are taking black and white then a yellow or orange filter will have a more noticeable effect.

Looking at makes or brands, while some will have particular brands they favour most, the well known ones have developed their filters to a level where you are unlikely to find a great deal of difference between them.  I have to admit that going out with a variety of makes of protective filters to undertake comparative tests is not high on my list of research projects I would like to do, although it might be useful, if undertaken over a range of lighting, weather  and other situations, and was done on a variety of lens. With less well known brands and unbranded economy imports you may be lucky or not. Like supermarket own branded items some may be produced on the same production lines using the same materials.

See also: Filter Section for more articles.


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