In this final part of a series of articles dealing with filters, I look at what is perhaps the most complicated of the lot – the polarising filter. The physics behind the workings of the polarising filter is complex and, in truth, my understanding of all the phenomena involved is less than complete. With that said, the overarching aim of such a filter is to restrict the light passing through it to only that which exists on a specified plane. In other words, this filter allows light of one polarity to pass and not the other.
The first thing to remember when trying to wrap your head around this sometimes-mystifying filter is the difference between emitted and reflected light. Objects like the sun, candles and even cell phone screens are all in the business of emitting light. They effectively take a source of energy and through various mechanisms and reactions create light waves that never existed in the first place. For most light sources, these waves have no polarity; in fact, the distribution of the light waves and indeed the planes on which they travel are entirely random.
By contrast, the overwhelming majority of the natural world is all about reflected light. Most natural entities take the light created by the relatively small number of light sources and reflect it such that the bodies themselves become visible. Plants, for example, take sunlight and reflect the green part of the light spectrum, making them visible and giving them their characteristic green colour.
Interestingly, when light is reflected from a flat and highly reflective surface, like a pond or a pane of glass, the act of reflecting polarises a significant portion of that light along the same plane as the reflective surface. Said another way, light that we see as glare comprises mainly light travelling along a single plane, and it is this characteristic that makes the polarising filter useful.
By placing a filter in front of a lens, photographers have a way to reduce the glare present in a scene substantially. The decrease in glare is achieved by orienting a filter so that it rejects any light travelling along the primary plane of the glare, while simultaneously allowing light from alternative planes to pass. Furthermore, with the light intensity of the principal plane reduced, it is possible for a camera to resolve more detail than usual in the high glare areas of a scene. Light travelling along the minor planes is less likely to be overpowered by the primary reflected light, effectively making the detail that it carries easier to perceive.
Strangely, as irrefutable as the above explanation may sound, the act of limiting light to only that of a particular polarity can have wildly varying effects on the final photograph. Under certain conditions, using a polarising filter can have virtually no impact at all while the results can be striking in other situations. Unfortunately, there are no hard and fast rules dictating the degree to which using such a filter will impact on an image; except to say that if there is water or glass involved, the impact is likely to be the most pronounced.
Now, it is worth pointing out that the human eye does not, in anyway, polarise light. Consequently, using a polarising filter in photography is a step in the direction of art and away from truth. When used correctly, these filters produce images with more saturated colours and improved tonal balance. The sky, in particular, is inclined to take on a richer and darker blue colour as any atmospheric glare is eliminated. Also, as suggested above, under certain circumstances, these filters can even reveal detail in a scene that would otherwise be completely invisible.
Streams often illustrate this uncloaking effect well. Using a polarising filter when photographing a watercourse frequently allows the photographer to show subsurface features such as rocks and logs that would ordinarily be obscured by reflections. As with most things in photography, the opposite is also true; setting the filter to a different angle can have the effect of accentuating reflections, further obscuring objects that were initially less obscured.
It is the ability of the filter to fundamentally change what is and is not visible in a scene that brings to the fore one of my personal bugbears. Some commentators within the social media sphere – who consider themselves authorities on all matters of photography – submit that the results of a polarising filter can somehow be reproduced during post-processing. This is simply not the case. During post-processing, it is impossible to recreate detail that the camera did not capture at the outset. If the camera didn’t resolve a rock beneath a body of water but instead resolved a reflection, there is no way to make that rock magically appear in the final photograph, no matter how skilled you are with a mouse.
All gripes aside, despite the ability of the polarising filter to unmask hidden detail, like all filters, it has its set of drawbacks. Chief amongst these shortcomings is what are known as hotspots. Hotspots manifest as areas of the final image that become unduly dark or saturated when the filter is in place. These undesirable variations in colour and tone are most evident in uniform regions of a photo such as a sky. There is little that one can do to avoid the presence of hotspots other than to reduce the degree of polarisation present in a scene. More than anything, hotspots are the product of polarising itself and, as a result, will exist no matter how expensive the polarising filter you own. The trick is to be mindful of their presence and to be careful not to be heavy-handed with the effect.
When it comes to the point of whether or not I would recommend that other photographers buy a polarising filter, I am a distinct fence-sitter. There is no doubt that polarising filters allow photographers to capture detail in scenes that would otherwise be impossible to capture. The problem that I have had was with finding compositions that permitted the full use of this capability. Naturally, my difficulty with finding suitable applications for the filter can also be attributed to a lack of skill, but not in its entirety. Thus, my solution to this rather ambiguous conclusion is that if you intend taking a significant number of photographs with water or even dew-covered grass, then a polarising filter is a worthwhile investment. If, however, this is not your intention, then I would suggest that the money is better spent elsewhere. I carry a polarising filter and I use it, but I wouldn't describe it as a must-have.
The Introduction to Filters Series
This series of articles takes a detailed look at the four most common filters available on the market today: the Ultraviolet, Neutral Density, Graduated Neutral Density and Polarising filters.