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Rig Shots 11: Filters for rig shots


John Jovic

There are 2 basic types of filters which are recommend for anyone considering rig shots on a regular basis. They are Polarising filters, usually slim versions if available, and a range of Neutral Density (ND) filters ranging from about 3 to about 10 stops. Stacking a Polarising filter with an ND will also allow some fine tuning if needed. If shooting only indoors then there is little if any need for filters at all.

Polarising Filters

Polarising filters are very useful for rig shots, and for general car photography. Polarising filters are normally used to control reflections in panels or on glass, or to darken the sky or reflections on other surfaces in the background. Polarising filters usually reduce light by about 1.5-1.75 stops so they can be used in conjunction with ND's to reduce light intensity and allow a longer exposure. The exposure compensation of ND and Polarising filters are simply added so for example, using a 3 stop ND filter with a polarising filter will require an exposure compensation of approx 4.5 stops.

There are 2 basic kinds of polarising filters, 'circular' and 'linear'. They do the same thing but most modern cameras require a circular polarising filter simply because of the way their metering and/or AF systems work. However if you are not relying on the cameras metering or AF then you can use either one.

Neutral Density Filters

Neutral Density (ND) filters reduce the amount of light which passes through them so they are often used for rig shots because of the long exposures often required. Neutral Density filters are meant to pass all visible wavelengths of light equally so that they don't create a colour cast but in practice most ND filters don't quite do what they are supposed to and colour casts of varying degrees are common, especially with stronger filters. ND filters don't always filter infra red as well as they do visible light so this excess of infra red can lead to a colour cast in cameras with greater infra red sensitivity. Some cameras are simply more sensitive to infra red light than others, hence a stronger colour cast. Colour casts are not always easy to correct and are some times impossible to eliminate altogether.

When selecting filters it's usually best to buy the thinnest filters you can, usually called 'slim' filters, so that you can minimise the chance of vignetting with wide angle lenses. Always check if the filter you are buying has a front thread as filters which don't have a front thread will prevent you stacking another filter on top. It's also a good idea to buy the largest filter that you might need, usually 77mm or 82mm for Canon and Nikon, and then use step up rings if the filters need to be used on lenses which require a smaller filter. Filters are expensive, step up rings are very cheap.

The table below is an example of the kind of choice you might find in ND filters. This table shows the filters available in a 77mm thread from B+W, Heliopan and Hoya in 2009 and is not meant to accurately represent B+W, Heliopan or Hoya's products. See each manufacturers web sites for accurate and up to date product information. Other filters may be available in other thread sizes.

F stops B+W
(filter designation)
Heliopan Hoya
0.3 2x 1 (101) Available NDx2
0.6 4x 2 (102) Available NDx4
0.9 8x 3 (103) Available NDx8
1.2 16x 4 NA NA NA
1.5 32x 5 NA NA NA
1.8 64x 6 (106) NA NA
2.7 400x 9 NA NA NDx400
3.0 1000x 10 (110) NA NA

Optimising Aperture and Shutter speed combinations with filters

The aperture and shutter speeds that you select for a rig shot will impact your final image significantly. Choosing an appropriate shutter speed is one of the most important factors in a rig shot and is discussed fully in
Rig Shots 9: Selecting the optimum shutter speed for rig shots.

Selecting the most appropriate aperture is straight forward
. In most cases we want the aperture to allow the car to be sharply in focus from front to back so this is usually the main consideration. The aperture and ISO can be used to fine tune the exposure to allow longer or shorter shutter speeds to be used. However, stopping the lens down dramatically, for example F16, F22 or F32 can noticeably reduce image quality as most lenses perform optimally 2-3 stops from their maximum aperture or wide open and image quality falls off as the lens is stopped down. For this reason alone it is better to keep an aperture somewhere between F8 and F16. One way to do this is to use mild ND filters, possibly with polarising filters, simply so that you get the best performance from your lens. High quality filters will have negligible adverse effects on image quality (as long as they are clean and in good condition) and will often give a net improvement in image quality (sharpness) compared to shooting at very small apertures!

Shooting in full sun

he Sunny 16 rule is a simple way to gauge exposure in full sun. It tells us that shooting at F16 our shutter speed is the reciprocal of our ISO. For example, at ISO 100 the exposure should be F16 at 1/100th of a second, or 1/125th for simplicity. Also, at ISO 400 the exposure should be F16 at 1/400th of a second, or 1/500th for simplicity.

The Sunny 16 rule is just a guide but it is close enough to use as a staring point and for the examples in the table below which are based on an ISO of 100. Three columns show the shutter speeds in full sun at F16, F11 and F8 depending on how much exposure compensation is used with filters.

exposure compensation
in F stops
F16 F11 F8
0 1/125 1/250 1/500
1 1/60 1/125 1/250
2 1/30 1/60 1/125
3 1/15 1/30 1/60
4 1/8 1/15 1/30
5 1/4 1/8 1/15
6 1/2 1/4 1/8
7 1 1/2 1/4
8 2 1 1/2
9 4 2 1
10 8 4 2
11 16 8 4
12 30 16 8

The above table shows the range of exposures possible in full sun with a range of exposure compensation values. For example, a 9 stop ND filter will allow a 4 second exposure at F16, at ISO 100 in full sun. Other combinations can easily be estimated using the above table.

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