see also:

• photography main index
• photographic colour temperature (white balance)
• https://fstoppers.com/originals/review-kase-filter-system-and-why-i-stopped-using-lee-filters-347291

• although one can achieve many effects in image editing programs such as Photoshop that simulate the effect of many of these filters, it is usually better to get the image right in the camera without need for image manipulation in the computer which tends to degrade the image in other ways.
• some effects cannot be easily reproduced in Photoshop such as polarising or infra-red
• even more important is the problem that digital cameras have limited dynamic range and it is very easy to burn out highlights and irreversibly lose detail in them. Thus graduated optical filters become even more important in digital photography if one wishes to retain cloud details in a scene, or perhaps the brightly lit end of a room or corridor. There is still the problem though of objects that pass from dark to light areas potentially revealing the position of the graduated filter cutoff line.
• One thing that no amount of Photoshop can fix is shooting into the sun, if you have the sun on the horizon and shoot straight into it you will end up with flare and a general washout of colour, use a ND Grad to block that light and the tonal range is captured again. 
• adequate lens hoods are imperative to reduce lens flare, particularly when shooting into the light source.
• prices are approximate from Vanbar 2005 to give you an idea of comparisons.
• what would I use for digital photography?
  • UV filter to protect lens - consider removing it to take critical photos esp. those into a light source when flare needs to be controlled. Not much point spending $A70 on a filter to protect a $A200 lens though, and no point taking photos through a $A20 filter when using a $A1000 lens. So if you are going to remove the filter to take your photos, you can buy a cheap one, otherwise buy a multicoated expensive one.
  • circular polariser filter to darken blue skies, give more saturated foliage colours and reduce reflections on water or glass
  • ND400 circular ND filter to allow longer exposures such as needed to produce flowing water motion on sunny days
  • perhaps a ND8 filter to allow fill in flash on sunny days and still get wide apertures for shallow depth of field (DOF)
  • ND graduated filters of the square design (eg. Cokin, Lee) to prevent the sky getting blown out. Don't bother with circular ones as you can't adjust them up and down for horizons, etc.
    • most commonly used are 0.6ND soft or hard as well as 0.9
    • an option for sunsets is the “reverse grad ND” which is darkest in the middle where the horizon lies
  • maybe an R72 infrared filter if you want to try infrared digital photography.

• screw-on filters:
  • these are the usual ones available
• slide in filters:
  • these are usually square glass or resin filters which slide into a holder which has been screwed onto your lens filter thread.
  • this enables the vertical or horizontal position of the filter to be adjusted to suit which is critical when using graduated filters that are used to darken the sky, etc.
  • these systems usually incorporate lens hoods that attach to the adapters
  • Cokin have 4 filter systems:
    • “A series”:
      • 67mm filters for lenses with filter threads up to 62mm and focal lengths >= 35mm;
      • cost $A20ea for basic, to $A30 for graduated & up to $A50-60 for some special effects
    • “P series”:
      • 84mm wide for lenses with filter threads up to 82mm and focal lengths >= 28mm;
      • filters cost $A25ea for basic, to $A35 for graduated & up to $A50-60 for some special effects
      • adapter available for A-P $A12
      • see also http://www.earthboundlight.com/phototips/cokin-p-versus-lee-grad-nd-filters.html 
    • “Z Pro”:
      • 100mm wide filters for lenses with filter threads up to 96mm and focal lengths >= 20mm;
    • “XPro”: 
      • 130mm wide filters for lenses with filter threads up to 118mm and focal lengths >= 15mm;
      • professional series. $A90 basic; $A136 grad; 
  • Singh-Ray filters:
    • Galen Rowell 120x84mm graduated soft-step & hard-step ND filters (fit Cokin P)
    • Daryl Benson reverse ND graduated filters with max. ND at horizon for sunset then gradates less for sky.
    • Gold'n'Blue Polarizer $US179 - changes the colour in polarised light only, thus quite natural
    • colour intensifiers
  • other systems:
    • Lee filters:
      • Lee glass filters 84mm to fit Cokin P
      • “Foundation kit” filter holder $A143 & lens adapter ring $A47
      • bellows lens hoods: $A220-440
      • 100mm resin /105mm glass- $A130 basic;  $A400 linear polariser; $A700 circular polariser;
      • 150mm - $A176 grad;
      • 100mm polyester: $A44 colour compensation; 
      • 150mm polyester: $A85 colour compensation;
    • Sinar 
    • Irix filters
      • 100mm wide, 2mm thick glass filters with adapters
• lighting filters:
  • filters that slide in front of professional light sources such as monoblocs
  • Lee:
    • various filter packs $A55/pack

• lens protection filters:
  • UV or 1A skylight filter:
    • blocks UV light, reducing bluishness in outdoor shots, while acting as a protector for your valuable lens
  • some people even use a 81A warming filter for most of their photos even in digital
• polarising filter:
  • great for minimising glare/reflections on glass, water & making the blue sky deeper, although the effect is maximal in the sky which is 90deg. from the sun
  • many AF SLR cameras require circular polarisers, while other cameras can use the cheaper linear polarisers.
  • sky effect of polariser filters can be simulated in Adobe Photoshop® see here
  • polariser filters were often used to increase saturation in color films such as Kodak color films but are usually not needed with modern high saturation color films such as Fuji's Velvia & indeed often cause sky to appear a less flattering version of black.
  • circular polarising filters have a filter factor of around 2.5, that is they block about 1 and a 1/3rd stops of light
  • “warm polarisers” have a 81A built-in with the polariser so you don't have to use both which may otherwise cause vignetting with wide angle lenses.
  • see http://www.dpfwiw.com/polarizer.htm 
  • see http://micro.magnet.fsu.edu/optics/lightandcolor/polarization.html 
• colour temperature correction filters:
  • not needed with digital cameras as these can usually be set to correct white balance, but are important in film photography where light source is not that for which the film was designed.
  • for serious colour film photography when colour temperature is important, one may need to consider a colour temperature meter but these are very expensive.
  • all colour conversion filters require an increase in exposure, usually by 1/3rd - 2/3rd stops, although the bluish 80 series require 1-2 stops increase.
  • conversion filters:
    • Film type designed for use in daylight use with 3400K use with 3200K use with clear flashbulb 3800K daylight noon daylight, flash  no filter needed 80B 80A 80C type A  photo lamps 3400K 85 no filter needed 82A 81C type B tungsten 3200K 85B 81A no filter needed 81C
  • conversion filters are often used to create a warmer mood such as use of an 81A
  • for more details see colour temperature
  • some use a Tiffen 812 to warm and reduce green when shooting portraits under a tree with a film camera.
• neutral density filters:
  • useful to allow longer exposure times, eg. to create a sense of water movement in bright sunlight
  • graduated filters can be used to darken the sky to help retain detail in clouds or just create a more moody effect.
• graduated colour filters:
  • filters that enable one to alter the colour of the sky, to make it warmer or perhaps colder.
  • not needed for digital photography as ND probably is a safer bet as can use Photoshop to apply a graduated color.
• special effects filters:
  • fog, soft, diffusion
  • spot, vignetting
  • star effects on specular highlights
  • most of these can be simulated in Photoshop
• infra-red filters:
  • only allow infra-red light through to create special false-colour effects or dramatic, eerie B&W effects
  • see infra-red photography
• lighting filters:
  • filters on flash guns or light sources can create great effects, particularly when used on the background. Combinations of blue & amber lights are often very effective here.

• any of the filters for colour photography may be used
• these colored filters are of little use in digital photography as Photoshop can mimic these see here
• colour filters to alter the gray-scale rendering of the image and thus contrast:
  • yellow:
    • often used to give natural rendition of blue sky, water, foliage & landscape scenery
    • tones down skin blemishes and ruddiness in daylight portraits, and results in soft skin tones as well as intensified blonde hair.
  • yellow-green:
    • often used for portraiture for skin tone rendition esp. outdoor against a blue sky
  • deep yellow:
    • dark water in marine scenes with blue sky
    • diminishes skin blemishes and freckles in artificial light. It also darkens eye colors and lightens lip colors.
    • used for contrast control in aerial IR photography.
    • enhanced texture of stone, sand, etc when sunlit under blue sky
  • red:
    • enhanced texture of stone, sand, etc when sunlit under blue sky
    • dramatic darkening of blue sky making dramatic clouds
    • darkens sky-illuminated shadows considerably increasing contrast
    • darken diffuse reflection from foliage
    • makes daylight scenes appear as though photographed at night
    • very pale skin tones
  • sky brightness rendering of clear blue sky on panchromatic film:
    • no filter ⇒ lighter than correct
    • Wratten 8 (yellow) ⇒ natural
    • Wratten 15 (deep yellow) ⇒ darker 
    • Wratten 23A (light red) ⇒ quite dark
    • Wratten 25 (medium red) ⇒ very dark
    • Wratten 29 (deep red) ⇒ almost black
    • infra-red film with Wratten 25 or infra-red filter with digital camera ⇒ black
  • to darken the following subjects:
    • yellow ⇒ blue filter
    • yellow-green ⇒ blue or magenta filter
    • green ⇒ blue, magenta or red filter
    • cyan ⇒ red filter
    • blue ⇒ red, green, or yellow filter
    • magenta/violet ⇒ green/yellow filter
    • orange or red ⇒ green, cyan or blue filter
  • see also filters for IR photography

• filters often require an increase in exposure to allow for the reduction in light, and this exposure increase required is often expressed as a “filter factor” such that a filter factor of 2x means exposure must be doubled, ie. increased by 1 stop, and filter factor of 4x means 2 stops increase needed, while 8x requires 3 stops.

• polarising filters usually have a filter factor of 2.5, and thus require 1 & 1/3rd stops increase.
• filter factors for color filters in B&W depend on which light source is used, whether daylight or tungsten, as bluish filters will have a much greater filter factor for tungsten light (eg. in this case, double that for daylight film), while the reverse is true for red filters.
  • for daylight, approx. filter factors are: 
    • yellow 1.5; deep yellow 2; yellow-green 4; green 5-6; light red 6; red 8; 25A deep red 16; blue 6; 47B deep blue 8;  61 deep green 12;
• neutral density filters are often given a different system:
  • 0.30 = 50% reduction in light = 2.0 filter factor = 1 stop
  • 0.60 = 75% reduction in light = 4.0 filter factor = 2 stops
  • 0.90 = 87% reduction in light = 7.7 filter factor = ~3 stops
  • 1.00 = 90% reduction = 10.0 filter factor = 3 & 1/3rd stops
  • 2.00 = 99% reduction = 100 filter factor = 6 & 2/3rd stops

• it is important to avoid distorting the filter ring as this may make matters worse
• prevention:
  • don't screw on too tight
  • consider using graphite on thread first
• solution:
  • some shops sell filter wrenches that apply equal pressure circumferentially, alternatively can use:
    • Jens Birch's method: 
      • “I have found those 'cable strips' made from stiff plastic which are intended for collecting bundles of cables very versatile. I have made my own wrench from one with a suitable size. In order to get a good 
        grip, I put a rubber band of the same width as the filter around the filter rim before putting the cable strip around it. I have never failed separating filters since I started with that method.”