Nikon Owner Issue 14
The Facts of Filters Part I by Simon Stafford
The first part of a two-part series on filters for both film and digital
Introduction
As a consequence of the exponential growth in digital photography, an increasing number of Nikon Owner subscribers have questioned the worth of traditional optical filters. They argue the case that with powerful image-enhancement applications such as Adobe Photoshop and a little skill, it is easy to emulate their effect. This may be so in some situations but, regardless of whether you engage in photography as a profession or a pastime, ask yourself the question: would you rather spend your time behind a camera or in front of a computer monitor?
Getting as much right in camera remains an imperative; equip yourself with a few key optical filters and not only will you find this task relatively straightforward but you will also save yourself precious hours that would otherwise be spent trying to correct and enhance your pictures with a computer.
If you shoot your pictures on film the arguments for using filters are even more compelling since they are often the only means by which you can control light to record your vision on this medium.
Why use a filter?
It is pointless using a filter just for the sake of it, because at the risk of stating the obvious a filter will change the way that a film, or sensor records a subject. Therefore, it is important to consider, in the same manner as you would choose a focal length or lens aperture to convey your perception of a scene to the viewer, whether filtration is appropriate, and if so which filter(s) is required.
A filter can do one or more of the following,
- Control exposure to all or part of the frame area
- Change the overall colour or tone
- Adjust the colour balance
- Reduce or increase contrast
- Create a visual effect
Photography is all about making decisions and it is essential that in amongst these you analyse what if any aspects of the scene you are photographing you would like to change and why. In other words, you have to decide what it is that you are trying to achieve. To this end I have always believed the art of good filtration is that its use should not be obvious. Used well, filters should enhance and compliment your photography, not detract from it!
For example, it is not uncommon for a scene lit by bright, direct sunlight to exhibit a dynamic range, the difference between the darkest and lightest areas, which is equivalent to a difference of approximately twelve stops. Our eyes are particularly good at coping with such situations as they are able to adapt, swiftly, to changes between these values. At a span of around eight stops the dynamic range of colour negative and conventional black and white film is somewhat more restricted, as are most digital sensors, which can manage about seven stops difference. Modern highly saturated colour transparency films can handle even less, typically no more than about five stops, and furthermore some such as Fuji Velvia have a propensity toward dense blocked shadows. Consequently, without the aid of appropriate filters, whenever the dynamic range in a scene exceeds that of a film or sensor you will be faced with having to compromise your exposure by either letting the shadows block up or the highlights burn out. Yet, with the help of graduated neutral density filters it is possible to reduce the level of contrast by controlling the amount of light from the brightest areas of the scene to a point where the film or sensor can retain detail in both the highlight and shadow areas.
So should you always use a filter of some description? All filters degrade optical performance. That’s right, all filters degrade optical performance. Regardless of quality, a filter introduces another two air-to-glass (or resin) interfaces in to the path of the light passing through the lens, which will inevitably affect to some degree the level of image contrast due to the effect of refraction. Granted with the use of high quality multi-coated filters this effect is unlikely to be perceived but it is still present, and in some situations it may be worth avoiding the use of a filter altogether. For instance when you shoot directly into a light source, such as a sunset, light can be reflected from the front element of the lens and bounce back off the inner surface of a filter, which contributes to the formation of flare spots.
A question I hear regularly concerns the use of ultra violet (UV) or Skylight filters, and whether they should be left on a lens at all times. The issue of reducing the effects of UV light, particularly prevalent in bright sunlight, and at high altitude, which can cause a ‘cool’ blue colour cast to occur is no longer relevant since most modern films and digital sensors have very little if any sensitivity to UV light. The other justification given for keeping such a filter permanently attached to lenses is their value in protecting the front element from physical damage. If you shoot under difficult conditions, for example in abnormally high levels of dust or moisture, you are likely to clean the front of the lens far more often, which increases the risk of inadvertently damaging the front element, so in these circumstances I would consider keeping a filter in place. At all other times I am content to let a lens hood protect the front of my lenses – after all that is what they were designed for.
This last point leads us neatly in to the next frequently asked question – what sort of filters should I use?