lens filter and its uses

by damen Tue Jun 29, 2010 8:37 pm

Filter Factors

Many filter types absorb light that must be compensated for when calculating exposure. These are supplied with either a recommended "filter factor" or a "stop value." Filter factors are multiples of the unfiltered exposure. Stop values are added to the stop to be set without the filter. Multiple filters will add stop values. Since each stop added is a doubling of the exposure, a filter factor of 2 is equal to a one stop increase. Example: three filters of one stop each will need three additional stops, or a filter factor of 2x2x2= 8 times the unfiltered exposure.

When in doubt in the field about compensation needed for a filter that you have no information on, you might use your light meter with the incident bulb removed. If you have a flat diffuser, use it, otherwise just leave the sensor bare. Aim it at an unchanging light source of sufficient intensity. On the ground, face up at a blank sky can be a good field situation. Make a reading without the filter. Watch out for your own shadow. Make a reading with the filter covering the entire sensor. No light should enter from the sides. The difference in the readings is the compensation needed for that filter. You could also use a spot meter, reading the same bright patch, with similar results. There are some exceptions to this depending on the filter color, the meter sensitivity, and the target color, but this is often better than taking a guess.

Filter Grades

Many filter types are available in a range of "grades" of differing strengths. This allows the extent of the effect to be tailored to suit various situations. The grade numbering range can vary with the effect type, and generally, the higher the number, the stronger the effect. Unless otherwise stated, there is no mathematical relationship between the numbers and the strengths. A grade 4 is not twice the strength of a grade 2. A grade 1 plus a grade 4 doesn't add up to a grade 5.

CAMERA FILTERS FOR BOTH COLOR AND BLACK-AND-WHITE

Ultraviolet Filters

Film, as well as video, often exhibits a greater sensitivity to what is to us invisible, ultraviolet light. This is most often outdoors, especially at high altitudes, where the UV-absorbing atmosphere is thinner; and over long distances, such as marine scenes. It can show up as a bluish color cast with color film, or it can cause a low-contrast haze that diminishes details, especially when viewing far-away objects, in either color or black-and-white. Ultraviolet filters absorb UV light generally without affecting light in the visible region.

It is important to distinguish between UV-generated haze and that of air-borne particles, such as smog. The latter is made up of opaque matter that absorbs visible light as well as UV, and will not be appreciably removed by a UV filter.

Ultraviolet filters come in a variety of absorption levels, usually measured by their percent transmission at 400 nanometers (nm), the visible-UV wavelength boundary. Use a filter that transmits zero percent at 400 nm, such as the Tiffen Haze 2, for aerial and far-distant scenes; the Tiffen Haze 1, transmitting 29% at 400 nm, is fine for average situations.

Infra-Red Filters

Certain special situations call for the use of black-and-white or color infra-red sensitive films. For aerial haze penetration, recording heat effects, and other purposes they are invaluable. Their color and tonal renditions are very different, however, from other film types (consult film manufacturers for further details). Various filters are used to reduce unwanted visible light. Red, orange, and yellow filters, as used for panchromatic black-and-white film can enhance contrast and alter color. Total visible light absorption transmitting only infra-red, as with the Wratten #87 or #89 series of filters, can also be useful. The results will vary with film type and other factors. Prior testing for most situations is a must.

Neutral Density Filters

When it is desirable to maintain a particular lens opening for sharpness or depth-of-field purposes, or simply to obtain proper exposure when confronted with too much light intensity, use a neutral density (ND) filter. This will absorb light evenly throughout the visible spectrum, effectively altering exposure without requiring a change in lens opening and without introducing a color shift.

Neutral density filters are denoted by (Optical) Density value. Density is defined as the log, to base 10, of the Opacitance. Opacitance (degree of absorption) of a filter is the reciprocal of (and inversely proportional to) its Transmittance. As an example, a filter with a compensation of one stop has a Transmittance of 50%, or 0.5 times the original light intensity. The reciprocal of the Transmittance, 0.5, is 2. The log, base 10, of 2 is approximately 0.3, which is the nominal density value. The benefit of using density values is that they can be added when combined. Thus two ND .3 filters have a density value of 0.6. However, their combined transmittance would be found by multiplying 0.5 x 0.5 = 0.25, or 25% of the original light intensity.

Neutral density filters are also available in combination with other filters. Since it is preferable to minimize the number of filters used (see section on multiple filters), common combinations such as a Wratten 85 (daylight conversion filter for tungsten film) with a ND filter are available as one filter, as in the 85N6. In this case, the two stop ND .6 value is in addition to the exposure compensation needed for the base 85 filter.

Color-Grad® Graduated ND Filters

Often it is necessary or desirable to balance light intensity in one part of a scene with another, in situations where you don't have total light control, as in bright exteriors. Exposing for the foreground will produce a washed-out, over-exposed sky. Exposing for the sky will leave the foreground dark, under-exposed.

Color-Grad ND filters are part clear, part neutral density, with a smoothly graded transition between. This allows the transition to be blended into the scene, often imperceptibly. A ND .6-to-clear, with a two-stop differential, will most often compensate the average bright sky-to-foreground situation.

These filters are also available in combination colors, as where the entire filter is, for example a Wratten 85, while one half also combines a graded-transition neutral density, as in the 85-to-85N6. This allows the one filter to replace the need for two.

Color-Grad filters generally come in three transition types. The most commonly used is the "soft" gradation. It has a wide enough transition area on the filter to blend smoothly into most scenes, even with a wide angle lens (which tends to narrow the transition within the image). A long focal length, however, might only image in the center of the transition. In this case, or where the blend must take place in a narrow, straight area, use a "hard" gradation. This is ideal for featureless marine horizons. For situations where an extremely gradual blend is required, an "attenuator" is used. It changes density almost throughout its length.

The key to getting best results with a Color-Grad filter is to help the effect blend in as naturally as possible. Keep it close to the lens, to maximize transition softness. Avoid having objects in the image that extend across the transition in a way that would highlight the existence of the filter. Don't move the camera unless the transition can be maintained in proper alignment with the image throughout the move. Make all positioning judgments through a reflex viewfinder at the actual shooting aperture, as the apparent width of the gradation is affected by a change in aperture.

Color-Grad filters are best used in a square, or rectangular format, in a rotating, slidable position in a matte box. This will allow proper location of the transition within the image. They can be used in tandem, for example, with one affecting the upper half, the second affecting the lower half of the image. The center area can also be allowed to overlap, creating a stripe of the combination of effects in the middle, most effectively with gradated filers in colors (see section on "Color-Grad Gradated Color Filters).

Polarizing Filters

Polarizers allow color and contrast enhancement, as well as reflection control, using optical principles different from any other filter types. Most light that we record is reflected light that takes on its color and intensity from the objects we are looking at. White light, as from the sun, reflecting off a blue object, appears blue because all other colors are absorbed by that object. A small portion of the reflected light bounces off the object without being absorbed and colored, retaining the original (often white) color of its source. With sufficient light intensity, such as outdoor sunlight, this reflected "glare" has the effect of washing out the color saturation of the object. It happens that, for many surfaces, the the reflected glare we don't want is polarized while the colored reflection we do want isn't.

The waveform description of light defines non-polarized light as vibrating in a full 360 degree range of directions around its travel path. Polarized light is defined as vibrating in only one such direction. A polarizing filter passes light through in only one vibratory direction. It is generally used in a rotating mount to allow for alignment as needed. In our example above, if it is aligned perpendicularly to the plane of vibration of the polarized reflected glare, the glare will be absorbed. The rest of the light, the true-colored reflection, vibrating in all directions, will pass through no matter how the polarizing filter is turned. The result is that colors will be more strongly saturated, or darker. This effect varies as you rotate the polarizer through a quarter-turn, producing the complete variation of effect, from full to none.

Polarizers are most useful for increasing general outdoor color saturation and contrast. Polarizers can darken a blue sky, a key application, on color as well as on black-and-white film, but there are several factors to remember when doing this. To deepen a blue sky, it must be blue to start with, not white or hazy. Polarization is also angle-dependent. A blue sky will not be equally affected in all directions. The areas of deepest blue are determined by the following "rule of thumb." When setting up an exterior shot, make a right angle between thumb and forefinger. Point your forefinger at the sun. The area of deepest blue will be the band outlined by your thumb as it rotates around the pointing axis of your forefinger, directing the thumb from horizon to horizon. Generally, as you aim your camera either more into or away from the sun, the effect will gradually diminish. There is no effect directly at or away from the sun. Do not pan with a polarizer, without checking to see that the change in camera angle doesn't create undesirable noticeable changes in color or saturation. Also, with an extra-wide-angle view, the area of deepest blue may appear as a distinctly darker band in the sky. Both situations are best avoided. In all cases, the effect of the polarizer will be visible when viewing through it.

Polarizers need approximately 1-1/2 to 2 stops exposure compensation, usually without regard to rotational orientation or subject matter. They are also available in combination with certain standard conversion filters, such as the 85BPOL. In this case, add the polarizer's compensation to that of the second filter.

Certain camera optical systems employ internal surfaces that themselves polarize light. Using a standard (linear) polarizer will cause the light to be further absorbed by the internal optics, depending on the relative orientation. A Circular Polarizer is a linear one to which has been added,on the side facing the camera, a quarter wave "retarder." This "corkscrews" the plane of polarization, effectively depolarizing it, eliminating the problem. The Circular Polarizer otherwise functions in the same manner.

Polarizers can also control unwanted reflections from surfaces such as glass and water. For best results, be at an angle of 33 degrees incident to the reflecting surface. Viewing through while rotating the polarizer will show the effect. It may not always be advisable to remove all reflections. Leaving some minimal reflection will preserve a sense of context to a close-up image through the reflecting surface. A close-up of a frog in water will appear as a frog out of water without some tell-tale reflections.

For relatively close imaging of documents, pictures, and small three-dimensional objects, in a lighting-controlled environment, as on a copy stand, large plastic Polarizers mounted on lights aimed at 45 degrees to the subject from both sides of the camera will maximize the glare-reducing efficiency of a polarizer on the camera lens. The camera, in this case, is aimed straight at the subject surface, not at an angle. The lighting Polarizers should both be in the same, perpendicular orientation to the one on the lens. Again, you can judge the effect through the polarizer.

by **maxgoh** Tue Jun 29, 2010 9:14 pm

Thanks for sharing, damen, for ur info, i have moved this topic to "Photonian's Store room (Equipment & Tools Talk) ===> Accessories" damn long story man~~~ hehehehe