Use decent filters, not tricks
Because many beginning PJs spend so much money on quality lenses, they're temped to protect the lens. They often use an ultraviolet (UV) filter or a "sky filter" to protect the lens from mud, blood and other junk. This is fine and actually a good suggestion – as long as it's also the same brand as the lens.
If someone tries to skimp on filter quality, images from the $5,000 lens become $25 filter images (unacceptable). Image quality is only as good as its weakest link.
Here, a cheap filter equals a cheap image. The PJ is best served without a filter than with a cheap filter. There's a risk of scratching the lens. However, if the lens is properly stored, it's less likely to be scratched.
While I am on the subject of filters, avoid "special effects" filters. The world is filled with enough special effects to keep every PJ happy. Most of these trick filters are made of plastic (read as garbage). Sure, they are interesting to see once or twice, but then it gets old. I bought some while I was visually experimenting in college, but they're collecting dust and highly unlikely to change status.
A polarizing filter is an acceptable alternative to a gradient filter (see below) for PJ work. To a purist, it's a neutral density filter. The filter allows the photographer to control reflected light without stepping over any ethical line.
I'll save everyone the quantum physics and suggest the filter "straightens" or limits the waves of light and directs them into the lens from desired angles. Much oblique light and many reflections can be eliminated by harmonizing the direction of light from different subjects.
The end result (what most people want) is minimization of randomly scattered light from reflective surfaces. Since the sky is full of tiny particles with tiny surfaces, the polarizing filter can be rotated to eliminate random light from many of the directions and effectively darken the sky without changing its perceived color.
There's 1.5 to 2 stops of light lost by using a polarizing filter. The costs for high-quality polarizing filters for fast lenses are ... ouch. It's important to get a circular polarizing filter for use with autofocus cameras. It allows the autofocus program to set focus without "hunting" (constantly changing focus) for the subject. Linear polarizers are fine for manual lenses, but it's safest to spend the extra cash to get the circular polarizer.
Popular visual tricks can often be accomplished without a trick filter. Here's how:
The purpose of this filter is to make spectral highlights streak outward from their brightest point. Essentially this is a sky filter which has been deliberately scratched in multiple directions to channel light. It's dependent on spectral highlights (those points of direct reflection of light) to accomplish the desired result.
To accomplish this trick (this is fair game for news photos too), simply stop down to the maximum lens aperture (the highest number –f/22, f/32, f/64, etc.). Since spectral highlights are already a prerequisite, stopping down automatically makes a six pointed star. It may not be quite as dramatic, but the image is actually in sharper focus.
Soft focus filter:
The purpose of this filter is to scatter light to give a younger appearance to someone with wrinkles. It places extra light in the darker areas (usually wrinkles) and increases the size of the circles of confusion.
Obviously, this is the opposite of a quality lens. It deliberately uses poor image sharpness to make someone supposedly look younger.
To accomplish this trick (for a wedding, freelance non-news gig, or illustrations), bend a wire coat hanger into a circle a bit larger than the lens shade. Straiten the excess wire to use as a handle for control from behind the camera or attach to a light stand. Stretch sheer, black pantyhose material over the circle and Superglue it into place. Shoot through the circle. The pantyhose scatters light as it passes through and misdirects the light going through a good lens. This gives the desired effect.
If the light is metered manually, make sure to increase the exposure by a stop or two. If shooting digitally, chimp it until it's right. If on film, bracket exposures and make sure to remember or write down which frames had compensation to know the correct compensation the next time.
There are two other ways to get this effect without filters (for non-news). The first is to use a strobe to sharply focus the subject while giving the background a halo effect.
This is best accomplished in a darker situation (indoors or dusk). Position the subject facing away from the light source or lit area (facing the camera with over the shoulder). Use a flash or strobe and set the lens to the highest depth of field (f/16 for most). Meter the ambient light at one or two stops under correct exposure.
Using a front synch technique, focus on the subject and shoot with the flash then immediately rack the lens out of focus while the subject remains still. This causes the subject to be sharp while the background has an initial sharpness with a halo effect.
The other way is always with the photographer. Use a tripod and frame the subject. Move to the side of the camera and breathe on the lens to make it fog up. Move out of the way and shoot. It has the same effect as real fog without the mess. This technique works best in cooler, wetter climates.
Gradient filters are popular with architectural and some commercial shooters. These filters gradually add extra blue or orange hues to the sky. It keeps a plain sky from blowing out and adds something different and surreal to the image.
The surreal trait is a direct result of cheap, colored plastic. It doesn't look real because it isn't. Everything gets the same pseudo-sky discoloration (trees, buildings, people, etc.). Just avoid it.
Enough for now,