Nikon Owner Issue 16
THE FACTS OF FILTERS Part III by Simon Stafford
Simon Stafford concludes his series on the art of good filtration.
In the first part of this article I examined the purpose of filters, the various systems that are available, discussed some practical considerations in respect of choosing which type to use, and concluded by discussing probably the most useful filter of all – the polariser.
In the second instalment I covered neutral density, including graduated types, and colour correction filters. Now, in this, the final part of the series, I will take a look at some specialist filters. It is worth noting at this stage that all of the filter effects mentioned in this article can be emulated by digital techniques either by selecting an option on the camera, or using appropriate software to manipulate the image with a computer. However, that is not to say that in the case of the infrared, fluorescent, diffusing, and soft filters that these do not need to be used with a digital camera.
INFRARED
Film-based black and white infrared (IR) pictures are typically associated with course grain, glowing white highlights and solid black shadows. Such results can be emulated quite successfully using a digital camera, although the technique does require some after-work using image manipulation software on a computer.
Whether you shoot on film or digitally you will need an IR filter, which is designed to exclude most if not all the visible light. These are available with a variety of different ‘cut-off’ points (the wave length below which they do not transmit light), from a number of manufacturers (see table). Since exposure times will be relatively long, even in bright sunlight, because these filters are either semi-opaque, or totally opaque to visible light, the other essential piece of equipment is a tripod!
The level of IR light will vary significantly depending on the daylight conditions; a bright sunny day is most favourable, although it is still possible to shoot IR pictures under an overcast sky. To compensate for the fact that IR light is brought to focus in a different plane to visible light, you will need to adjust focus slightly. Manual focus Nikkor lenses have an IR focus index mark, which can be used to adjust the focus point with some degree of accuracy. However, many modern AF Nikkor lenses lack this feature, so stop the lens down to a small aperture (f/11 – f/16) and allow the depth-of-field to account for any small differences in the focal point.
Generally, it is best to use an IR filter with a low cut-off point, in the range of 695nm to 720nm, because they transmit sufficient visible light to allow most TTL metering systems to provide at least an approximate exposure value (it is not possible to rely on the TTL meter reading due to the restricted spectrum of light available to it). As it is impossible to assess the amount of IR light present in a scene by eye, and it can vary enormously according to the prevailing weather conditions, I recommend, strongly, that you bracket your exposures. Increments of 1/2-stop are sufficiently small and a range of +/- 2-stops should provide you with at least one good exposure with a full range of tones.
Furthermore, it is likely that you will be able to see very little, if anything, through the viewfinder, due to the amount of visible light the IR filter cuts out. So composition is best done before you fit the IR filter – there is another good reason to use a tripod!