Fear of Filters
"Snort" went Bill Boffo, the famous equipment maven who rarely takes his precious cameras out to actually shoot pictures, "I would never put a cheap twenty buck filter in front of my $1000 lens". I've heard this bit of twisted logic many times and usually let it pass without comment. The dime store expert who uttered it was either kidding or hopelessly opinionated.
In the first place there are times when filters are just plain necessary, like converting between tungsten and daylight, for color films. There are reasons why filters cost less than cameras that should be obvious to any life form higher than a centipede --- just like a bike costing less than a car, there are fewer parts and less material is involved. The lower price doesn't necessarily mean cheap quality (though some are), or any shortfall of quality that would compromise the excellence of the lens.
My basic comfort with filters goes back to the era of B & W. While a daylight color film exposed in daylight usually renders tone and color correctly, B & W film used for, lets say a scenic, does not show proper tonality without a filter. Unaided, even a panchromatic film (the prevalent type) lets a blue sky go almost white, losing the clouds, and lets the foliage go dark. It was and still is to this day generally recommended that a medium yellow (K2) filter, which darkens the blue sky and lightens the foliage, be used just to achieve normal tones, that a darker yellow to orange (G) might be better and that other colors may do even more for you. Green (B, XI, X2) will lighten foliage even more and red (A-23, A-25) will darken skies and make clouds even more dramatic.
Those filter designations, still in wide use, come from Kodak-Wratten and go back to the time when they were king of the hill in filters. Their product was well made and finished and came in those cute yellow and black plastic boxes but the filters were described with shocking corporate honesty as "gel filter cemented in B glass" (for bigger bucks you got A glass). Have you ever noticed that when you look through really old windows the glass is a bit wavy? When you look through sideways at a very shallow angle this waviness is exaggerated to a wild scramble. Similarly, while the view through the B glass looked perfectly okay seen straight on, if you swung the disk (like a door) so that the near and far rims almost line up, giving you a narrow view at a very acute angle, you could see some waviness and distortion in the glass.
This was apparently still good enough to cause absolutely no loss of quality for any use except extreme telephoto or extreme wide angle (at the edges). Since the eye could evaluate optical flatness with this simple test I began to check every new filter I came across. If the view coming through at around an 88 degree angle is still clear and undistorted, perfect optical flatness is assured (anyway it's far better than B glass) and the observed filter simply is not going to take anything away from a perfect image.
At first, say 25 years ago, this test turned up many dogs among the cheapies, but in the last 10-15 years I have never found less than perfect optical flatness in even the cheapest filters. All are vastly superior to the old B glass filters. The differences that I do see are in construction. Spiratone filters may have perfect "glass" but they are popped into their mounts and held by snap rings, -- and sometimes snap out; Rolev filters don't seem to get the size right and sometime fall out after being screwed in (someone told me their plastic mounts shrink). Some of today's lower priced lines like Hoya, Vivitar and SP have a good combination of excellent glass, fairly good mounts and coatings.
Single coatings are a near necessity for filters. I've occasionally gotten ghost images of light sources reflected from a lens element to the back surface of an uncoated filter. I'm less sure that multicoatings are better. Wh
e a single coating is a fairly hard mineral (magnesium fluoride) some of the multicoatings may actually be plastic and they are easy to scratch. Not such a good idea for filters which are more exposed to the world than the lens they cover.
Of course not all filters are gels sandwiched between glass. There are solid glass filters, premium items but well worth the extra cost. The glass stays perpetually clear and is never affected by cleaning liquids dribbling into the edge of the mount. (With ordinary gel-between-glass filters damage can be caused if fluids flow past the edge. That can lead to clouding or separation, in fact some deterioration may occur after many years with even the best care.)
It seems that solid glass was commoner 30 or 40 years ago. They were great for the solid yellow, orange, red, etc., colors used in black & white but glass can't be made in the carefully calibrated colors and densities used in color work, like an 81 a or CC.30 M (magenta). Cemented gels took over from solids in a great wave, in fact most of the fussy "CC" filters (for precise color control in printing and duplicating) can only be had as plain, naked, square gels. Their surfaces are optically smooth but the filters simply aren't flat. It is claimed that they don't hurt the image because they are so thin but I've found that when the slight curve turns to wavy and wrinkly they do indeed degrade the image. I've had several cases of unsharp areas in photos, corresponding to wavy spots on gels (the only degradation I've seen from any filters), so I use them with reluctance --- but if you need a CC40G on your lens to get proper color on a dupe, what choice do you have? ( FYI, that CC40G means "color compensation,.40 density, green).
Well there is a new choice after all. Solid, optical grade acrylic filters offer one virtue of solid glass (no separation), they are optically flat to the highest standard and yet the plastic offers a controllable medium in which color can be precisely calibrated. There is a mistaken notion that plastic "diffuses" the image a bit. The fact is acrylic
is clearer and whiter than the best glass but it is easily scratched and many people "diffuse" the surfaces with poor cleaning practices. In essence you never want to touch the surface dry. Better to wash them in water plus Photo-flo, or even Joy detergent, with a dab of wet cotton, rinse and drain on their edge like dishes. I've kept a set of Polycontrast printing filters (acrylic) in good condition for 30 years this way.
Cokin filters are a common example of acrylic filters. They're okay optically but that maker just doesn't have enough colorimetric control to produce precise color values like CC's (they're more into "fun filters"). It is left to other sources like Lee, Hitech and Calumet to produce the ideal, acrylic CC filters, unfortunately at triple the cost of Kodak's gels.
Another objection some people have to filters is that the fat markup on a single large filter can often be as much as the markup on a lens or camera. This is often quite true. Tight competition produces distortions like that (like auto dealers making more on service than on the sale of the car --- they have to get it somewhere). About the only way to beat that is to avoid expensive, camera brand filters and substitute cheaper off-brands. Generally the results will be every bit as good.