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FAQ: Photo Filters

Posted by reedcat | Posted in FAQs | Posted on 09-03-2007

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The photo filters are the “must” for any photographer. 

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Why Filters?

Optical filters function in digital photography much as they do in film work. Properly used, they can condition the light entering the camera in favorable ways—often in ways that post-processing can’t easily duplicate. 

I find it useful to think of optical filters as a kind of image pre-processing. As with post-processing, optical filters can foster maximum photorealism, maximum visual impact or various things in between.
Most commonly used filters serve to match the recording capabilities of the camera more closely to those of the human brain-eye system.
UV cut, polarizing, neutral density, and close-up filters all fall into this category.
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Can the filters be replaced with post-processing?

If you have enough time and experience, you can emulate some effects in the Photoshop or other image editor.

In my opinion, post-processing is no substitute for the filter types listed below.

 Polarizers  Among other things, polarizers can easily save you from fatal white-outs due to bright reflections—not just off water, glass and car paint, but also off foliage. (Foliage reflections are a serious but commonly overlooked problem in landscape work.) The hard fact is, there’s not good post-processing cure for white-outs, especially those complicated by CCD blooming. Polarizers can help you control excess contrast in other ways as well. They also improve color saturation in ways hard to reproduce at post-processing.
Neutral density (ND) filters    ND filters allow you to achieve slower shutter speeds or wider apertures than would otherwise be possible in a given scene. (Note that polarizers make decent 1-2 stop ND filters in the absence of polarized light.)
IR pass and UV pass filters  Even if you could simulate the surreal luminance relationships found in the near IR (NIR) in post-processing—and I suspect that few could do so convincingly—you’d never be able to fake the phenomenal atmospheric clarity found at NIR wavelengths. Nor would many be able to fake the odd world waiting to be discovered at UV-A wavelengths.
UV cut filters  In theory, UV filters add clarity by cutting UV scatter in high UV environments—e.g., at high altitude (over 2000 m) or in long shots over water, but most digital cameras are too UV-insensitive to benefit here. Post-processing can suppress or sharpen a hazy blue channel to good effect, particularly in B&W work, but an effective UV filter (most likely a haze filter) might allow you to improve clarity while preserving blue channel data in your color images.
   

Any disadvantages?

Indeed, optical filters have some problems. Any extra glass in the lightpath between scene and CCD has the potential to degrade your image in various ways—e.g., via internal reflections, dirt or aberration. In practice, the trade-offs are seldom that clear-cut, but the trade-offs will always be with us.

UV-cut, Skylight and Haze filters

UV, skylight and haze filters all block ultraviolet (UV) light to varying degrees. UV filter nomenclature is a bit confusing.

The term “UV filter” by itself usually refers to a neutral (untinted) filter blocking the shorter wavelength UV-B (320-280 nm) and UV-C (10-280 nm) bands while letting a good bit of the UV-A (320-400 nm) through. Haze and skylight filters are UV variants that often carry a tint. Haze filters block more UV-A than regular UV filters but also take a bite out of visible blue. Skylight filters also cut some blue but are no more effective in the pesky UV-A band than regular UV filters.

UV and skylight filters require no exposure compensation, but haze filters may need a small correction. Since uniform coverage of the entire field of view is usually the intent, round UV filters make perfect sense. If your UV filter also serves as a lens protector, rounds are the only way to go.

Skylight filters are slightly colored (usually pink or yellow) UV filters designed to warm the image a bit by removing some blue light along with the UV. They’re generally no more effective at blocking UV-A than ordinary UV filters, but they can be handy in open shade, where blue indirect skylight is the dominant light source.  On digital cameras, skylight filters function primarily as warming filters, but this effect can be easy emulated with post-processing, therefore Skylight is not a need.

Much of the haze seen in visible light film photographs results from the scattering of UV-A by air molecules, water droplets and dust particulates. Haze filters provide stronger UV-A filtration than most of the commonly used neutral UV filters, but at a price: They also block some visible blue and impart a variably conspicuous yellow cast in the process. Tiffen claims that its Haze 1 and Haze 2A filters transmit 29% and 0% at 400 nm, respectively. To block UV-A and the haze it carries effectively, you’ll need something more akin to a Haze 2A—along with a willingness to “get the yellow out” in post-processing.

Polarizing filters

These deservedly popular filters, also known as polarizers, use the inherent polarization of atmospheric scatter, glare and other unwanted reflections to remove such photographic pests selectively and prospectively from the light entering your camera. If you shoot much outdoors, the ability to mount a polarizer is reason enough to invest in a filter-capable camera and the required adapters. 

No single filter type will deliver more benefit in routine photography than a properly used polarizer.

Polarizers come in two main types, linear and circular, identical in use and effects and differing only in camera compatibility. Specifically, linear polarizers are incompatible with cameras that rely on split-beam optics for functions like metering (TTL) and auto-focus. When in doubt, get a circular. They are most popular at the moment of time.

Used properly, polarizers can darken the blue of the sky, highlight clouds, suppress unwanted highlights and improve general color saturation by suppressing atmospheric scatter and color-robbing reflections off water, glass, sunlit foliage, vehicles and even bald heads. 

 Neutral filters (ND)

Neutral density filters reduce the light entering your camera without introducing color biases. When the effect is meant to be uniform across the camera’s field of view, round NDs are perfectly adequate.

In bright ambient light, an ND will allow any camera to reach

  • a slower shutter speed to enhance motion blurring, or
  • a larger aperture to enhance subject-background separation or image detail. 

On the digital side, you may be able to accomplish the same thing by selecting a lower ISO setting, with the added bonus of reduced image noise. But when you can’t lower ISO enough, an ND (or stack of NDs) can get you where you need to go.

Is a reason for protective filter?

Looking for answer on this easy question follow a “holy war” in photo world. Here are both sides of the argument. Decide for yourself.

Protective Filter Pros

  • Filters are generally a lot less expensive than lenses.
  • Which would you rather replace, a screw-on filter or a built-in zoom lens?
  • Optically, quality multicoated UV filters have little downside on a properly shaded lens.
  • UV filters generally require no exposure compensation.

Scratches, abrasions, falls and direct blows are obvious mechanical threats to your lenses and filters. Dust, sand, rain and splashes are common dangers as well. 

Not so obvious is the potential for irreversible salt spray damage to optical coatings, especially when the salt is allowed to remain on the glass for some time. Consider an uncoated plain or UV filter for that day at the beach, especially if the surf’s up and the wind is onshore. Of course, there’s always a bit of corrosive salt in the air by the ocean, but an offshore breeze is fairly protective if the surf is calm.

Protective Filter Cons

No matter how expensive or how well coated, filters inescapably degrade the optics of any camera. Filters reflect a small amount of incoming light out of the camera and invite the unwelcome internal reflections known as flare. Flare can be as conspicuous as a series of UFO-like hexagonal blobs of light hovering in the sky or as subtle as a loss of color saturation. With any filter in place, meticulous shading of the lens becomes mandatory. If they’re not perfectly flat and parallel to the image receiver, filters can also introduce aberrations. Stacking filters only compounds these problems, but a purely protective filter constantly swapped out for another serving a real photographic purpose quickly becomes a nuisance.

Light loss and aberrations seldom reach practical levels, but flare’s a fatal image flaw, and a common one at that, especially when the sun’s low in the sky near your subject. Why compromise on quality when careful handling, a lens cap or a rigid lens shade provide adequate protection under most shooting conditions?

How many light filter holds?

All filters, excluding UV hold some part of light. On a filter frame you can found a number looking like “1.4x”. This is a filter factor, which is simply a multiplier for the exposure time.

In other words, a filter with a filter factor of 3x would increase your exposure time from 1/60 sec to 1/20 sec at constant aperture and ISO. When stacking filters, multiply the filter factors together for the final exposure correction required.

To convert filter factors into stops or exposure values (EVs), use the table below.

EV equivalents for common filter factors
filter factor EV filter factor EV filter factor EV
1.2 -1/3 2.5 -1 1/3 5.0 -2 1/3
1.5 -2/3 3.0 -1 2/3 6.0 -2 2/3
2.0 -1 4.0 -2 8.0 -3

What to read about filters else?

On a Web-site of Tiffen  - one of world leaders in this branch you can found outstanding collection of images are made with different types of optical filters.

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