A Polarising filter.		A Graduated Filter in it's frame. The filter can be raised/lowered or rotated to perfectly adjust it for any given shot.

Using Polarising filters

A polarising filter basically eliminates or reduces reflections from certain surfaces and in certain circumstances. They can work well with glass and metal painted surfaces and they can be used to darken the sky or remove reflections from water. A polarising filter rotates and it's effect changes as it is rotated. They have the effect of increasing colour saturation in some circumstances by reducing unwanted reflections such as the sky, clouds or glare. They can also be indispensible when photographing metallic paints which need to 'pop' by having a hard light source reflecting in the paint surface. The polarising filter is used to tame the reflection of the bright light source allowing the metallic paint to appear vibrant and saturated without the glare of the light source itself. Polarising filters are potentially very useful when photographing cars but they also have some disadvantages. The images below show some typical uses for polarizing filters in car photography.

The Polarising filter was set to minimise the light reflecting in the side panel of the car. Notice the effect of the polarising filter on the side windows of the car where patterns have become visible. These effects are due to the types of glass used and in this case 2 different types of glass have been used so the effect is not the same for front and rear door windows.		Polariser set to minimise reflection in front windscreen and bonnet so had no effect on the side of the car.

Polarising filters can also create odd colorful reflections in plastic head lights, which can easily be fixed in post processing by selectively desaturating the offending effect. These can be considered an advantage or a disadvantage depending on your point of view. Polarising filters can also create similar reflections in glass which can look a little like oil stains on a wet road.

Using Polarising filters with flash

Polarising filters can be used to reduce or eliminate glare or reflections which might otherwise mask the effect of the flash. Removing or reducing glare gives stronger colour saturation. Of course this also requires more power from the flashes themselves as the Polarising filter will generally reduce an exposure by about 1.5 stops.
 

Here the Polarizing filter has been set to eliminate reflections on the side of the car so that the flashes will work most effectively and not have to compete with any bright reflections in the panels, see next image. However the reflections in the windscreen are prominent and distracting. The glare in the windscreen can also be eliminated but the Polarising filter has to be set in a different orientation as can be seen in the subsequent image. The 2 images are later combined or 'composited'.		Here the Polarizer has been set to eliminate reflections from the windscreen. Note how the background, a painted wall, also becomes brighter and less saturated as does the side panel of the car.
It only takes a few minutes to composite the 2 images together to get the best of both worlds. In this example it was easiest to use the first image as a background (because only the windscreen in that image needed alteration, instead of everything except the windscreen) and selectively darken the windscreen with a Layer Mask of the second image with the darker windscreen and the appropriate blending mode, 'Darken'.		This is the lighting setup for the above 2 images. Only the Polarizer was altered in the above 2 images. All RAW processing setting are identical.

Graduated Filters

Graduated filters are usually a square or rectangular filter which is held on the front of a lens with the appropriate frame or filter holder. The filters themselves have a section which is clear and another which is neutral (grey) or a colour such as amber, blue or magenta to name just a few. The filter can be slid up or down within the frame and adjusted to darken any part of the frame, usually a part of the sky. Graduated filters are commonly used in landscape photography to control the dynamic range in an image by darkening brighter parts of an image. Many car images are simply landscapes with a car so this filter technique is a valuable one when shooting cars. Graduated filters can be combined with polarising filters as their effects are different.
 

A single stop ND (Neutral Density) Graduated filter was used to darken the sky. The effect of the ND Grad is often quite subtle and certainly doesn't have to be gaudy and obvious.		A 2 stop ND Grad was used to darken the sky significantly as the lens was pointing in the direction of the sun, ie the clouds where backlit.
These 2 almost identical images, shot at the same location, have had 2 different graduated filters used. In this case an amber/orange/yellow grad was used because this colour was already evident in the sunset so the filter only added to it's saturation and did not try to create an effect or colour which wasn't already present. This resulted in a natural effect.		In this case the sunset had a magenta colour cast so a Magenta grad was chosen to exaggerate it  even further. Again, the filter was not used to create a colour that wasn't already there so the effect is relatively natural.

In both of the above examples a Graduated Neutral Density could have been used instead of coloured Grads but may not have been as effective or dramatic It would still have darkened the sky and therefore kept more colour saturation than if no Grad had been used at all. Graduated filters are probably the most commonly used filter in car photography and are normally used in very subtle ways rather than to dramatically darken the sky as in some of the examples above.

Graduated filters are available in many variations such as strength (1, 2 or 3 stop), colour (Sunset/Amber, Neutral, Blue, Magenta etc) and rate of gradation (how quickly the change takes place from completely clear to filtered). A medium strength is most practical and a selection of 1 and 2 stop filters in Neutral, Sunset/Amber/Yellow and Magenta (or FLW) are most commonly used.

Graduated filters are also available in many sizes or filter 'systems' such as Cokin P or Z, Lee, or a range of other sizes from Tiffen and other manufacturers. A collection of filters can be quite expensive so it's best to buy into a system which is likely to last rather than to out grow a cheaper or smaller system and have to start all over again. The larger the filter the easier it is to use with wide angle lenses but they are also much more expensive and cumbersome to carry so a compromise is often necessary. The Cokin P system is quite cheap, flexible and effective but is not really suitable for very wide angle lens where a larger Lee 100mm or Cokin Z style of filter would be preferable. Note that systems are not always compatible as even the thickness of the filters themselves can some times vary between brands of filters.

Unfortunately many of the systems use a form of plastic filter which although quite cheap is also fairly soft and therefore easy to scratch when cleaned. Cleaning marks or fine scratches also increase the likelihood of flare which is a very important consideration for any filter facing a bright light sources, a typical use for a graduated filter. Tiffen is one company which offers glass graduated filters in various sizes and these perform extremely well and are very easy to keep clean with much less likelihood of scratching and therefore flare. However they are very expensive.