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Category: Hobbies & Recreation : Photography
If for nothing else, it pays to have a filter on front of a camera lens to protect the lens from scratches, fingermarks, dampness and any minor damage which might occur.
To replace a soiled or damaged filter is much easier and less expensive than replacing a good camera lens.
For this initial purpose, either a UV (ultraviolet) or 'Skylight' filter is probably most suitable. These filters also act to moderately improve colour response and rendering of monochrome and colour films respectively, so more natural and pleasing tonal and colour rendering are achieved. At the same time, because they are so visually pale, they have basically no effect on the overall amount of exposure.
There are many other filters available to photographers, for correcting, manipulating, or enhancing the end result of your picture. These include contrast filters for monochrome films, colour 'balancing' and colour 'correcting' filters for fine tuning the results of colour pictures. And there are filters which can work equally well on both monochrome and colour films.
Also, there are filters to achieve special effects: starburst, cross-star, multiple image, graduated density, soft-focus or diffusion filters. I won't say much on these because there are so many to cover.
In an earlier article (Film Types) I noted that films behave differently in different types of lighting. To expand on this we should look at some theories of light and colour. Then we can better understand how filters can be practically applied to manipulating a film's response to these lighting conditions.
Light, as seen by human vision, is composed of different colours of light rays, all evenly mixed together to give the impression of being generally white. The reason for a person or object appearing to be of one colour, is that pigments or dyes in skin, cloth, paint absorb some of those light rays in different amounts, thus reflecting only the remaining rays.
 
Furthermore, some light sources radiate more of some colour light rays than others, and the earth's atmosphere absorbs some colours of light rays but transmits others.
Generally speaking, all films are sensitive to blue light rays. Although most films respond to all colours of light, there are a handful of specialised monochrome films that only respond to blue, slightly to green, and are almost blind to red and yellow. I should also make mention of some films which are actually more sensitive to red wavelengths, and even to infrared wavelengths which are basically invisible to human vision.
While our eyes - controlled by our brain - tend to adjust to and thus accept minor variations in colour of different lighting, film being a simple recording medium doesn't have this ability. Film reacts to and renders light exactly as it is: if there is a minor change in the colour of light, this will be very evident on colour film. Such situations might include as the sun moves through the sky after rising or before setting, or in overcast or wet weather, or when different light types (daylight vs artificial light) are used.
Monochrome films simply render the different colours of a subject as different shades of grey, but even they will render the greys differently depending on the light source.
COLOUR TEMPERATURE
At this point it might be a good idea to introduce a term you may hear occasionally. I have said that different light types radiate different colour of light. There is a general term relating to the colour of light, and the measurement of that colour. This is Colour Temperature
Natural daylight has a distinctly different colour quality (colour temperature) than that of artificial (tungsten) light. The colour temperature of daylight is higher than other light sources, and that of flash light is very close behind daylight.
In this context, artificial light is typified by electric tungsten filament globes used in many homes, and halogen floodlights used in photo studios, sports arenas and theatres. These have a lower colour temperature.
Fluorescent and sodium / mercury vapour lamps could also be classed as artificial lamps, but they emit a different quality of colour yet again. It is very difficult to categorise the colour temperature of these lamps because they are an oddity of sorts. One usually needs to experiment and try varying corrections for them.
It could be argued that electronic (strobe) flash units emit light artificially, because that sort of light is not naturally generated. However, in relation to colour of light, electronic flash is usually grouped together with daylight, because the colour of flash light is perceived by colour film to be very similar to that of daylight.
FILTER FACTORS
If your camera has a built-in light meter, which measures light passing through the lens, then you shouldn't have to worry about filter factor. This is more relevant if you have an older style or simpler camera without a light meter, or one in which the light meter measures light from outside or separately to that passing through the lens.
The filter factor is an adjustment factor that takes into account the colour density or darkness of the filter. Because most filters (except for the UV / Skylight and Close-up filters) noticeably absorb light, an increase needs to be made to the exposure when using them.
Filter factor will often be expressed as . . .x (times), eg, 2 x or 1.5 x or 4 x etc.
An easy way to remember this factor value is that it works in the same way as multiplying 2 to the power of another number (maybe you remember some of your squares and cubes from school maths), for example:
2 multiplied to the power of 1 = 2, 2 squared = 4, 2 cubed (to the power of 3) = 8, 2 multiplied to the power of 4 = 16, and so on.
Thus the amount of f-stop / shutter speed increments needed for a filter factor is the power to which the number 2 is multiplied to get that factor number.
A (filter) factor of 2 x = 1 increment
A factor of 4 x = 2 increments
A factor of 8 x = 3 increments
A factor of 1.5 x = 1/2 increment
Some filter manufacturers supply a little chart or index with their filter to indicate the common filter factors and appropriate exposure adjustments for them.
The next article on filters will detail many of the different types of filters available.
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David Barry |
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Photo enthusiast |
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Other Articles in this Category
How To Understand Your Camera and Film 9: Filter Types
by David Barry
How To Understand your Camera and Film 2: Shutter Speed
by David Barry
How To Understand your Camera and Film 4: Putting it all together
by David Barry
How To Understand your Camera and Film 3: Lens Terms
by David Barry
How To Understand your Camera and Film 1: Film Speed
by David Barry
How To Understand Your Camera and Film 6: Lens Focal Length
by David Barry
How To Understand Your Camera and Film 5: Film Types
by David Barry
How to Understand Your Camera and Film 7: Depth of Field
by David Barry
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