FILTER FACTORS
All coloured filters absorb a some light, depending on the density of the filter
This means that when you use a filter you need to increase the exposure
The increase is dependent on a factor applied to the filter, such as 11/2x yellow,
2x orange, 3x red, etc.
For example if a normal exposure or meter reading would indicate 1/125 sec at f-11,
the exposure with the
11/2x yellow filter would be 1/125 sec at f-9, (midway between f-11 and f-8) or, with the
3 x red filter, 1/125 sec at f-6.3 (midway between f-8 and f-5.6). Note that the filter
factor is the multiplication of the exposure, not the number of stops to increase the
exposure.
Note: Alternatively, if you are using a particular f-stop to control depth of field then you
could change the shutter speed by an equivalent factor, i.e. for 2x orange 1/60 sec at f-11.
If your camera has 'through the lens' (TTL) metering facilities, you will not usually need
to calculate the exposure because the filter on the lens affects the meter reading.
Nevertheless, it is advisable when using the more deeply coloured filters to check the
reaction of the meter by taking readings both with and without the filter.
The spectral sensitivity of meters varies, and may not match the characteristics of the film.
OTHER FILTERS
Polarizing filter: -
This type of filter can be used for both black-and-white and colour film.
The Polarizing filter makes use of the special properties of polarized light.
Light travels in a wave motion, with the waves radiating in all directions at right angles to
the line of travel. In some circumstances, however, it can become plane polarized,
i.e. instead of radiating in all directions, the waves radiate in one plane only (horizontal,
vertical, diagonal, etc) but still at right-angles to the line of travel.
Light can be polarized by passing it through a special filter, which can be thought of as
resembling a slotted screen, cutting off all radiation except that in one particular plane,
which can be varied according to the orientation of the filter.
Most shiny surfaces (except metal) will polarize light that strikes them at a certain angle,
generally at about 55deg to the normal. Such surfaces are glass, water, heavily polished
wood or other material, gloss paper etc. Thus, if you take a photograph at about 35deg to
such a surface (which is 55 deg to the normal), any reflections you pick up from the surface
are highly polarized.
If you place a polarizing filter over your lens in reflected light, you can rotate it until the
reflections, as seen in your viewfinder, disappear, leaving only the random light from the
surface or objects behind the surface to affect the film.
Note: - The maximum effect is only be obtained when the reflections are fully polarized
and that only occurs, as we have seen, when the light strikes the surface at about 55deg
to the normal or perpendicular. This means that any reflections can only be totally
eliminated when the shooting angle is at about 35' to the reflecting surface. At other
angles the light is only partially polarised and reflections will not be completely eliminated.
Colour photography:-
Because light is reflected from a clear sky at right-angles to the direction of the
sun's rays it is frequently polarised to some extent. By suitably orienting a
filter over the camera lens, you can remove some of the skylight and deepen the blue of
the sky.
Polarising screens are neutrally coloured and have no effect on other colours in the scene.
However, they have some density, and absorbs light quite heavily. The filter factor varies from
about 2x to 4x, depending on the characteristics of the individual filter.
Neutral density: -
Also used with Black & White and Colour film.
In a situation where the light level is outside the exposure range of the camera and likely to
lead to over exposure, such as with a bright sky, and fast film, a neutral density (ND) filter
can be used. An ND filter is a neutral grey tone and therefore does not affect the tonal range
of the subject, Its purpose is to reduce the amount of light reaching the film. The amount of
light reduction depends on the density of the filter (filter factor).
Ultra Violet (UV) filters: -
The UV-absorbing filter(UV) is occasionally necessary because photographic emulsion is
sensitive not only to visible light but also to the invisible ultra-violet radiation.
On colour film it can sometimes cause a noticeable blue cast. This happens where the
atmosphere is clean and free from the dust and other particles that scatter short-wavelength
radiation and prevent it from reaching the earth. These conditions are
usually found at high altitudes and near large tracts of water.
The true UV filter is colourless, but some filters have a faint yellow or pink tinge, these
would be more accurately described as haze or skylight filters. They absorb both UV and
some visible blue. They can be useful in reducing the bluish haze over distant parts of a
landscape or and the blue cast caused by an excessively blue sky.
UV and haze filters generally require no increase in exposure.
Colour correction filters:-
These are pale coloured filters mainly used with colour film.They are used to correct for colour casts,
or for special effects.
A 'warm up' filter. this will correct for blueish lighting and make the image appear 'warmer'.
A graduated sky filter is partly tinted to tone the sky without affecting the landscape.
Various colours can be used to create special 'in camera' effects.
Special effects: -
Other filters are the so-called 'special effects' filters, these include, Starburst, soft focus,
multiple image and filters with shaped holes or clear areas to create a vignetting effect.
Most of these, apart from the soft focus filter, could be considered as gimmicks and of
limited use to the serious photographer.
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