by Gerald Boerner
“Generally speaking, I’m not one to adovcate photographic rules, but I do think it helps to take off the lens cap.”
— Catherine Jo Morgan
“The whole thrust in my life right now is spinning my assignments around and making them work in a more personal way (…) I wanted to go back and do the original thing: one camera, one lens, one film. You really have to put yourself in a position of danger to be creative.”
— David Alan Harvey
“Minolta makes the best bodies, Nikon makes the best lenses, Canon makes the best compromise.”
— Author Unknown
“Of what use are lens and light to those who lack in mind and sight?”
— Author Unknown
“You didn’t compromise on your camera. You shouldn’t on your lenses.”
— Author Unknown
“It’s not the size of the lens. It’s how you use it.”
“I am always surprised when I see several cameras, a gaggle of lenses, filters, meters, et cetera, rattling around in a soft bag with a complement of refuse and dust. Sometimes the professional is the worst offender!”
— Ansel Adams
Using Filters: Part 3 — Special-Purpose Filters
There are several types of special-purpose filters that are used in photography. These typically produce special effects in the image (film or digital) that cannot be replicated fully with digital image editing tools. These filters include both diffusion and diffraction filters. Close-up filters are also included in this category although they may also considered to be lenses as well.
“But don’t make the mistake of thinking that photography will always be a relaxed process. The photographer who always sees himself as calm & sedate, as nothing but a slow absorber of a place, will more than often miss the meaning which the place is trying to offer him. Rush to get it if you have to, moving as quickly as nature often does. The quick movement, the sudden noticing of something perfect but passing in it’s perfection – that too must be part of the photographers’ vocabulary, which goes far beyond the boxes of lenses and filters. It is a quality of mind. The shutter in the camera will move in tiny fractions of a second. You must not lag behind.. . . it is that instantaneous, disappearing nature of the beautiful moment that of course makes it all the more precious. To capture that is something only photography can do and is above all – even speaking technically and chemically – a response to what is there in front of you. The light is going, it’s beauty is there for no more than a few seconds, it is fading before your eyes, and you cannot ignore it.”
— Charlie Waite
Diffraction (Cross Screen) Filters
A diffraction/cross screen filter, also known as a star filter, creates a star pattern, in which lines radiate outward from bright objects. The star pattern is generated by a very fine diffraction grating embedded in the filter, or sometimes by the use of prisms in the filter. The number of stars varies by the construction of the filter, as does the number of points each star has.Colors seen in a spider web are partially due to diffraction,
according to some analyses
Diffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Similar effects are observed when light waves travel through a medium with a varying refractive index or a sound wave through one with varying acoustic impedance. Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, x-rays and radio waves. As physical objects have wave-like properties (at the atomic level), diffraction also occurs with matter and can be studied according to the principles of quantum mechanics.
While diffraction occurs whenever propagating waves encounter such changes, its effects are generally most pronounced for waves where the wavelength is on the order of the size of the diffracting objects. If the obstructing object provides multiple, closely-spaced openings, a complex pattern of varying intensity can result. This is due to the superposition, or interference, of different parts of a wave that traveled to the observer by different paths (see diffraction grating).
The formalism of diffraction can also describe the way in which waves of finite extent propagate in free space. For example, the expanding profile of a laser beam, the beam shape of a radar antenna and the field of view of an ultrasonic transducer are all explained by diffraction theory.
A diffusion filter (also called a softening filter) softens subjects and generates a dreamy haze (see photon diffusion). This is most often used for portraits. It also has the effect of reducing contrast, and the filters are designed, labeled, sold, and used for that purpose too. There are many ways of accomplishing this effect, and thus filters from different manufacturers vary significantly. The two primary approaches are to use some form of grid or netting in the filter, or to use something which is transparent but not optically sharp.
Silk sheets can also be used in this manner, and in fact were until the invention of translucent plastics. "Opal" is a common translucent or opalescent diffusion.
Both effects can be achieved in software, which can in principle provide a very precise degree of control of the level of effect, however the "look" may be noticeably different. If there is too much contrast in a scene, the dynamic range of the digital image sensor or film may be exceeded, which post-processing cannot compensate for, so contrast reduction at the time of image capture may be called for.
In photography, soft focus is a lens flaw, in which the lens forms images that are blurred due to spherical aberration. A soft focus lens deliberately introduces spherical aberration in order to give the appearance of blurring the image while retaining sharp edges; it is not the same as an out-of-focus image, and the effect cannot be achieved simply by defocusing a sharp lens. Soft focus is also the name of the style of photograph produced by such a lens.
Zeiss manufactures a widely noted Softar diffusion filter which is made of many tiny globs of acrylic deposited on one surface which act as microlenses to diffuse the light. In some versions the globs are on the inside of the filter (facing the photographer) while on others they face outwards (towards the subject). In various versions the globs vary in number and diameter, from approximately 97 to 150 globs each 1 mm to 3 mm wide.
Homebrew approaches to transparent diffusion filters are generally based on modifying a clear or UV filter by placing various materials on it; the most popular choices are petroleum jelly, optical cement, and nail polish. Transparent filters are more commonly used for the "dreamy" or "misty" effect than for contrast reduction.
Various widths, colors (often black or white), and grid shapes (typically diamonds or squares) and spacings of netting, usually made from nylon, are used to provide diffusion effects. These are used both for the "dreamy" look and for contrast reduction. The homebrew approach to this sort of effect is generally to stretch a piece of pantyhose material in front of the lens.
While these are not technically filters but accessory lenses, they are sold by filter manufacturers as part of their product lines, using the same holders and attachment systems. A close-up lens is a single or two-element converging lens used for close-up and macro photography, and works in the same way as spectacles used for reading. The insertion of a converging lens in front of the taking lens reduces the focal length of the combination.
Close-up lenses are usually specified by their optical power, the reciprocal of the focal length in meters. Several close-up lenses may be used in combination; the optical power of the combination is the sum of the optical powers of the component lenses; a set of lenses of +1, +2, and +4 diopters can be combined to provide a range from +1 to +7 in steps of 1.
Close-up lenses (also called supplementary lenses) screw into the filter mount on the front of the lens that is fitted to your camera, and bring the focusing range of the camera’s lens closer to the camera. The power of close-up lenses is normally specified in dioptres; higher numbers are more powerful. With the camera’s lens focused on infinity and a +1 dioptre close-up lens fitted, the maximum focusing distance becomes 1 metre, with a +2 it becomes 0.5 metres, and with a +4 it becomes 0.25 metres.
Close-up lenses for 35 mm cameras are commonly available with strengths of +1, +2, +3 and +4, but intermediate and higher strengths are also available. The lenses of digital cameras have shorter focal lengths than those for 35 mm cameras, and so they need stronger close-up lenses such as +7 and +10; these are often of too small a diameter and insufficient quality to be used on 35 mm cameras.
Close-up lenses are not usually corrected for optical aberrations, so you need to stop down the camera lens to at least f/8. The effects on image quality are greater with camera lenses of longer focal length, so better quality (and much more expensive) close-up lenses are needed for telephoto lenses and for roll-film cameras. Two-element achromatic close-up lenses are available: Nikon produce +1.5 and +2.9, Canon produce +2 and +4, and Hoya produce +10. Specially-matched close-up lenses are available for some macro lenses and medical lenses.
You can use two close-up lenses at a time, with the stronger one closer to the camera lens. The effect is additive, so a combination of a +1 and a +2 has the same power as a +3 close-up lens. Combining close-up lenses makes the drop in quality worse.
Close-up lenses are cheap, easy to use, cause no exposure problems, and do not darken the viewfinder, but they cannot match the quality of a macro lens. They are readily available, and are made by camera manufacturers and by independent companies.
A special type of filter is the split diopter mentioned in the Wikipedia reference has just a semicircular half of a close-up lens in a normal filter holder. It can be used to photograph a close object and a much more distant background, with everything in sharp focus; with any non-split lens the depth of field would be far too shallow. These are not generally employed in most photography today, except by selected professionals.
Background and biographical information is from Wikipedia articles on:
Photographic Filters that can be found at…
Diffraction Filters that can be found at…
Diffusion Filters that can be found at…
Soft Focus that can be found at…
PhotoQuotes.com on Photographic Filters…