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PHOTOGRAPHIC FILTERS INTRODUCTION To
understand filters in colour photography, it is necessary to understand light
and how we perceive it.
COLOUR CORRECTION FILTERS This
forms the basis for colour correction filters. The same principles are used
when manufacturing film to compensate for certain conditions where light from a
particular part of the spectrum may be predominant. For instance, when
photographing indoors with a daylight balanced film, under tungsten lighting,
the resultant image will have a predominant yellow or orange tint. To correct
for this condition, tungsten film uses a blue layer in its mask, which blocks
some of the overabundant yellow light to give more natural results. The
reverse is also true. If you use tungsten film outdoors or with a flash, which
produces light close to 5500° Kelvin, the resulting images will have a bluish cast,
due to the built-in filtration in the mask of the film that removes too much
yellow light, leaving unbalanced amounts of blue and green. The
reverse conditions to tungsten lighting exist outdoors sometime during the
winter. The presence of a bright blue sky combined to the very white and
reflective snow, increase the amount of visible blue
light, making it necessary to filter out the excess blue with a filter of a
colour from the opposite side of the colour wheel, in this case yellow or
amber. FILTER TYPES Filters
are available in a variety of forms, materials and sizes. The most common are
either glass or plastic. Glass
filters are commonly mounted in a brass ring (thermoplastic is used in less
expensive makes and models) to make the mounting and removal of the filter
easier. They are typically fitted by screwing them to the front of the lens,
although in some cases, they are inserted into the body of the lens, in the
light path before it reaches the film. A good
quality filter will be distortion free and will allow light to go through it
without introducing any unwanted aberrations. For those filters being inserted
into the body of the lens, it is essential that those filters be part of the lens
optical equation. Typically, inserted filters of this type are limited to
neutral density, which reduces the amount of light reaching the film. They are
mostly used in fixed aperture lens, like a reflex lens, also called a mirror
lens. Whenever the neutral density filter is removed, a “clear filter” is
inserted in its place, to complete the optical equation of the lens. POLARIZING FILTERS Polarizing
filters are now very commonly used for two main reasons: first, they add
neutral density to the light passing through the lens, reducing the amount of
light reaching the film. This effect makes it easier at times to control
excessively bright situations; second, they “polarize” the light reaching the
film. Typically, light being reflected off objects is scattered in all
directions, and for that reason, the “pollution” washes out visible colours. By
polarizing the light, this filter only allows light travelling in a specific
direction to pass through the lens, eliminating all other light beams scattered
in the other directions. This has the effect of increasing the relative
brightness of the other colours, eliminating reflections off bright and not so
bright objects, including the surface of water. Blue skies will appear bluer,
green leaves will be more vivid, and reds will be more striking. Polarizing
filters come in two varieties: linear and circular. Linear filters should only
be used with a manual focus lens. Their design does not allow auto focus
systems of modern cameras to work properly, making it necessary to fine tune
focus manually before taking pressing the shutter release, a minor or major
inconvenience, depending on the circumstances. For this reason, circular polarizers
are recommended for any camera with automatic focusing systems. Polarizing
filters have one additional feature, which is a rotating ring. This allows the
photographer to adjust the amount of polarization to the desired effect.
Maximum polarization is attained when the subject stands at right angle to the
sun and the coloured pointer or finger knob is pointing directly at the sun. No
polarization is possible with the sun directly behind or in front of the
subject. Other
types of polarizers are also available from Cokin and other manufacturers. For
instance, Cokin offer a blue-green polarizer where the overall colour of the
image can be biased towards one colour or another, in this case, blue or green.
This effect can be used to advantage when trying to intensify the effect of a
colour cast or to render a particular mood to the scene. The big
plus of glass filters is the fact that they are much more resistant to
scratching and damage of any kind. A major drawback of these filters however is
directly related to their size, price and weight. The larger the size, the more
expensive they are. It is usually recommended to purchase the filter size that
fits your lens with the largest front element. Faster lenses have larger front
elements to allow more light in, and therefore require larger filters. For that
reason, many photographers purchase only those filters they most often will
use, foregoing special effects filters and many of the colour correcting ones. Glass
filters are usually a better investment than those they came to replace: the
wratten gelatine filters. Although rarely used other than by professional
photographers, these filters still provide much more accurate colour
correction. They are however easily damaged, cannot be cleaned easily, and need
to be cut to size to fit inside the correct mount and adapter for your lens. COKIN FILTERS As an
alternative, the Cokin filters are made of plastic, which offer better
resistance to damage than gelatine, but can still be more easily damaged than
glass filters. Cokin
filters come in two sizes, the A System for amateurs or small diameter front
element lenses, and the P System for professionals or large diameter front
element lenses. Overall, the P System is more versatile, although more
expensive. The adapter ring to attach the filter holder in front of the lens
can sometimes block a sensor on the camera body, situated close to the lens.
Likewise, the large filter holder can also block auto focus assist lights on
some models. However,
the Cokin system is usually more versatile and more adaptive to various
circumstances. The filter holder rotates in all directions, and the filter can
be moved up or down inside the holder, making it easier to match a
split-neutral density filter to a given scene. And since the P System will fit
nearly all lenses available on the market with only the addition of a new
Adapter Ring, these filters are much more economical over time. SPECIALTY FILTERS Other
common filters are the UV and the Haze filters. UV filters are usually divided
into UV (0) and UV (1). The first removes some of the ultraviolet light found
in a typical sunny day. The second is better suited for high altitude
photography where UV rays are much stronger. Haze filters help eliminate some
of the “light pollution” visible in the morning near large bodies of water, or
in mountain passes. Besides
polarizing, neutral density, UV and Haze filters, there are all kinds of other
special effect filters available. In fact, filters are probably the single most
sold photographic accessory on the market. Other
specialty filters worth mentioning are the FLD and FL-W filters. The FLD is
normally used with daylight balanced film under fluorescent lighting with a
cold rating light. The FL-W should be used with the warmer type fluorescent.
These filters remove the excessive green cast present in this type of lighting. Among the
special effects are those that produce stars with either 4, 6 or 8 points; others
that create multiple facet images; distort part of the image to create a
seemingly speeding object; diffracting that split the incoming light into its
various colour components – Spectra – Pulsator – Andromeda; create an illusion
of fog, or various degrees of blurriness, or a clear centre focus with
peripheral diffusion; or introduce a special colouring into the image, like
Sepia for instance, to instantly create “old” looking photographs. Some of
the newer filters stem from recent research in optical physics. The Tiffen 812
seems to clean up colours and enhance images taken with the newer films like
Fuji Velvia and Provia. The Hoya Intensifier has a similar but more pronounced
effect, which is very pleasing. The Hoya Green Field enhances greens of any
kind; bringing them out and making them appear to have much more punch. The
Tiffen Ultra Contrast 3 enlivens an image by enhancing the contrast in the
scene, a very desirable effect in mountain photography and fall imagery. FILTER FACTOR An
important discussion when talking about filters is the Filter Factor, or the
amount of light the filter prevents from reaching the film. In a
studio or product photography where handheld meters are used to measure flash
exposure or ambient light, it is essential to know what the filter factor is
and to correct the exposure from the reading on the meter. Failure to do so
will result in assured underexposure. Filter
factors are usually available from the manufacturer of the filter, or provided
with the new filter at the time of purchase. There exist also tables of common
filters used in colour correction work to provide a general approximation of
the filter factor for each type and filter number. When
using the built-in through-the-lens metering system of new SLR cameras, the
filter factor is automatically incorporated into the metering results, and need
not be calculated separately, unless the metering is done through an auxiliary
sensor on the body of the camera. In such a case, there are two possibilities
to obtain a correct reading of the light in the scene: you can either manually
compensate the exposure for the filter factor; or, if not possible, you can
take another reading while holding the filter in front of the sensor; then,
holding the shutter release partly depressed, move back the filter to the front
of the taking lens to complete the exposure. This is best accomplished with the
camera on a tripod. FILTERS FOR BLACK AND WHITE PHOTOGRAPHY Using
filters in Black and White photography is much more complex, because of the
relationship of contrasts and grey tones. Black and
White films are much more sensitive to blue light and ultraviolet rays, which
are invisible to the naked eye. Because of this, tone relationships in a given
scene are often subdued. The image appears different from what the eye
remembers, with less contrast and lighter skies. You can
use three different filters to increase the contrast and intensify the
differences between the various tones in the image: a Yellow (No. 8) for a
minimum effect, a Deep Yellow (No. 15) for a more intense effect, or a Red (No.
25) for a more dramatic effect. On the Colour Wheel, yellow and red are
opposite blue and cyan. The dark
yellow filter will also help to enhance texture in snow, and to darken an
overcast sky, to maintain the mood. Here is a
partial list of filters used in B&W Photography and their recommended uses: Blue Sky: Natural – No. 8 Yellow Darkened – No. 15 Deep Yellow Spectacular – No. 25 Red Almost black – No. 29 Deep Red Night effect – No. 25 Red, plus
Polarizer Sunsets: Natural – No. 8 Yellow Increased brilliance – No. 15
Deep Yellow or No. 25 Red Distant
landscapes: Increased haze effect – No. 47
Blue Very slight addition of haze –
None Natural – No. 8 Yellow Haze reduction – No. 15 Deep
Yellow Greater haze reduction – No. 25
Red or No. 29 Deep Red Foliage: Natural – No. 8 Yellow or No. 11 Yellowish-Green Light – No. 58 Green Outdoor
portraits against sky: Natural – No. 11
Yellowish-Green, No. 8 Yellow or Polarizer Marine
scenes when sky is blue: Natural – No. 8 Yellow Water dark – No. 15 Deep Yellow Architectural
stone, wood, sand, snow when sunlit or under blue sky: Natural – No. 8 Yellow Enhanced texture rendering – No.
15 Deep Yellow or No. 25 Red USING FILTERS WITH FLASH Filters can
also be used with flash photography. Adding a
filter to the flash will add the colour of the filter to the immediate
background covered by the flash light, while the background will retain its
original colour. Applications
of this principle is not so obvious, but still very useful under certain
conditions. For
instance, in a sunset scene where you want to preserve the warm colours of the
evening sky, photographing a person or an object bathed in light coming from
fluorescent lighting in an adjacent shop with a large bay window, may prove
impossible, since the person or object will be rendered with a green cast. To
obtain a balanced exposure, a magenta filter can be added to the flash to
colour correct the scene within the flash range, while retaining the warm
evening light. Mercury vapour lamps produce a similar green cast and are
frequently used in outdoor lighting, making dusk photography much more
challenging. Filtering
the flash can also be used for dramatic effects. Using complementary filters on
the lens and the flash, you can modify the overall colour of the scene, while
retaining the correct colour of the subject within range of the flash.
Complementary colours are those opposite each other on the Colour Wheel, for
example, red and blue. Sample
filters used in television studios and theatres can usually be obtained from
Lee Filters or Rosco. Those are much the same as those used in still
photography. FILTERS FOR INFRARED PHOTOGRAPHY No. 25
Red and No. 29 Deep Red are used most successfully to filter out blue light and
obtain brilliant whites in foliage and water. Adding a
Polarizer to either filter will darken skies even more, for a near nigh time
appearance. CLOSE-UP LENSES Technically
not filters, these optical accessories are typically attached to the front of a
normal lens or short zoom to increase the magnification of a subject. However,
they do not modify the lens optical formula. Therefore, the minimum focusing
distance of the lens will not change. The best
Close-Up Lenses include two elements per lens. WORTHWHILE ACCESSORIES Screw-on
filter types can sometime prove difficult to remove from the front of a lens.
The same happens if you typically store your filters in a stack. To avoid
possible damage to the lens or to the filter, you can use a Filter Wrench to
squeeze tight on the recalcitrant filter and successfully remove it. In a
pinch, a rubber band will also do the trick. However, thin filters may prove to
be a very unique challenge. OTHER FILTERS A Red
exposure filter – usually supplied by the enlarger manufacturer - is used on
B&W enlargers in the darkroom to adjust focus prior to making the exposure.
Since B&W papers are not sensitive to red light, it is also possible to use
this filter while adjusting the position of the easel on the baseboard. Similarly,
special amber filters (Kodak OC) are used in darkroom light fixtures with a dim
15W tungsten light bulb, to provide some illumination in B&W darkrooms. B&W
contrast filters (Kodak Polycontrast or Ilford Multicontrast) are used during
exposure to adjust contrast on multigrade B&W papers. These typically come
in a set of 11 or 12 pre-mounted filters that fit into a special holder that
can be swung beneath the lens, and which is usually included in the filter set. Again in
the darkroom, special effect filters may be used to create the illusion of
texture in an image. For instance, the image could be exposed together with a
specially created “thin” negative to add a fabric, rock, or other texture to
the resulting print. Finally,
colour enlargers use a special type of built-in filtering called dichroic,
which can be dialled in pre-measured increments or units. There are usually
three such filters in the head of a colour enlarger: Cyan, Yellow and Magenta.
To simplify printing and colour balancing, the Cyan filter is usually kept at
“0”, using only the Yellow and Magenta filters. To help
determine when proper colour balance has been achieved in colour printing, a
set of Kodak Colour Print Viewing Filters is available. When flicked in front
of a dry print under daylight lighting (5500° Kelvin),
these viewing filters help identify which colour cast there is in the print if
any, and if so, to what extent the next print should be corrected. CONCLUSION Photography
is all about controlling light and how it is perceived in the images we
capture. Using
filters to modify the behaviour of that light, its colour and intensity,
enhances these images in ways only bound by the limit of the imagination. |
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