- When & why do we take photographs with polarising filters?
- How does a polarising filter work and what effect does it have?
- What are circular polarising filters (CPL filters)?
- Particular features of Rollei‘s CPL polarising filters
- What should you look for when you are purchasing a CPL filter?
The use of polarising filters in photography
For what should a circular polarisation filter be used and applied? How can an invention, which dates back to a kind navigation aid of the Vikings, be important for modern photography & system cameras/single lens reflex cameras? Polarising filters are used mainly for photography, if mirror images and reflections on non-metallic objects and surfaces should be minimized. Because there is still no image editing programme that could take over this unique work performed by a polarising filter. In addition, a CPL filter increases colours & contrasts and absorbs complementary polarised light.
Primarily, areas of application include:
- product photography
- architectural photography
- landscape photography
For example, a polarising filter causes the filtering out of unwanted light reflection on:
- glass (e.g. shop windows)
- and other non-metallic surfaces (e.g. grass) of your pictures
Because these surfaces primarily reflect light of a certain polarization, in particular with an angle of reflection of from 30° to 40°. How strongly the polarisation filter works, depends above all on whether, how and on which materials and/or surfaces the incident light in a subject occurs. Therefore you can also turn polarising filters, in order to adapt them to the particular angle of incidence, the desired look and the reflection strength. They are always used if a photographer wants no reflections at all in the picture and would like to insert them purposefully later orfor example, if the colour tones of grass or the like ought to be intensified while blue reflections are filtered out by the CPL filter used. Besides ND filters the CPL filter is one of the few photo filters, without which we also won’t be able to manage in the future, despite image editing software. We may be able to control many of the effects such as the filtering out of mist and the increase in saturation also via RAW development by software. But the reduction of mirror images and reflections is possible only with polarising filters.
By putting a spiral shape on the light on the subjects of your choice, they thus filter out reflections on subjects such as cars or windows and facades, in the same way as those on water surfaces for example. In this way, large surfaces are intensified in colour and the colours of an image create a much warmer effect. At the same time the desired subject moves into the foreground, e.g. a duck on the water. While a photo without a CPL filter would depict a subject in the way a human eye captures it, images with polarising filters look more natural and clearer. Colors and contrasts are intensified. Thus, modern single lens reflex cameras need circular polarisation filters like the CPL filter for optimal colour representation. Thus the CPL filter is part of the basic accessories for motivated amateur & professional photographers. When it is in use it’s simply screwed onto the camera lens and it’s best to use it with a camera standin twilight or when it’s dark.
How does a polarising filter work?
A CPL filter is thus a circular polarising filter. Basically, there is a distinction between linear and circular polarisation filters. In the case of linear polarisation the radiation that is let through is polarised in the direction of the filter which the remaining intensity reflected or absorbed.
Circular polarisation builds on the linear. With circular polarisation light initially strikes a linear polarisation filter and afterwards hits a λ/4 retardation layer. The optical axis of the retardation layer is rotated up to +45° or −45° −In relation to the linear filter. The result is a polarised light that is rotating to the left or the right.
What effect does a CPL filter have on photos?
- The green of leaves and grasses is intensified because the polarisation filter primarily absorbs or “swallows” reflected blue light from the sky. A polarising filter can hold back a majority of the light of a cloudless bright sky so that the sky in the final result is illustrated as darker and stronger in colour. At the same time, white clouds emerge more clearly due to the stronger polarised contrasts.
- Thus a polarising filter doesn’t only let the light that is on the so-called polarisation level of the filter through. The light, which leaves the CPL filter, is thus always polarized.
- The quantity of light is reduced by approx. 1 f-stop.
- The colours of the rainbow can be intensified or also reduced.
- Even mist can be reduced a little.
This works because light spreads out in the form of a wave. This wave is always on one “plane “. The direction of oscillation is vertical, horizontal or at any arbitrary angle. In a normal daylight situation all directions are used evenly, which is defined as non-polarised light. On the other hand, a polarisation filter only lets through light waves that are on its polarisation plane. A circular polarisation filter spins the light in rotation as well. Thus, for example, certain colours intensify.
What distinguishes circular polarising filters (CPL filters) from linear polarising filters?
Let’s compare the linear polarising filter to a fence (Thanks to abenteuerdslrfotografie.de for the demonstrative example). In the case of a linear polarising filter the light strikes a kind of fence, the filter. Now someone wants to put a board, the plane, in which the light moves, through the fence (the filter) and leave it on the other side of the fence or on the camera lens. For that it must run approximately parallel to the fence, or filter, so that it fits through - on the same polarisation plane. Light waves, which do not run parallel to the polarising filter, do not come through. On average a linear polarising filter absorbs approx.25% of the original quantity of light.
By rotating the polarisation filter the orientation is changed so that different light waves come through accordingly and others are “swallowed” again. In this way, the filter can be aligned until the intensity of the reflections corresponds to one’s own taste, or they disappeared almost completely. In addition, the polarising filter is adapted in this way to different surfaces and subjects.
For modern cameras, however, a circular polarising filter (CPL filter) isneeded. Purely linearly polarised light can lead to errors and blurred pictures with modern single lens reflex cameras. Particularly with autofocus and the internal exposure measurement, erroneous readings can occur with simple polarisation filters.
If you can compare the linear polarising filter with a fence, then you should imagine the circular polarisation filter as a kind of rotating spiral. The light is spun by it in rotation, so that it forms itself into a kind thread such as that of a screw.
Thus, nowadays in digital photography circular polarisation filters are used. It is only in analogue photography that you can still work with linear polarisation filters.
Special characteristics of Rollei CPL filters
Rollei polarising filters made from Gorilla®* glass
Glass is unquestionably one of the most sensitive materials and can splinter, break or shatter with minimum pressure or due to extreme variations in temperature. However, in the field of professional filter photography there are still no alternatives to this highly transparent material, which is based on silicate, to be taken seriously. Therefore also no normal “window glass” is used either but special glasses of different quality levels. The high-quality Gorilla®* glass of the American manufacturer Corning Inc. is established in the absolute top range - and will therefore also be used with immediate effect for Rollei’s CPL round filters.
In order to be able to unite normally contrarily facing material properties the glass blanks are exposed to a scalding hot alkaline solution at Corning Inc. in the U.S. Federal State of New York. During the bath there is a complex exchange of ions on a molecular basis. The surface tension of the glass bodies clearly increases clearly and gains in resistance against pressure, scratches, contamination and contact with hard objects. Even with scratches or ruptures the edges do not splinter further and prevent structural failure.
Despite this amazing robustness, Gorilla®* glass has an extremely high degree of light transmission and colour neutrality – the best requirements, in order to assist you while you use Rollei polarising filters for the implementation of your creative photo ideas. In order to protect your filter shots and long-term exposures against unwanted light reflections or distortions, the filter glass is subjected to yet another coating, which was developed in accordance with the unrelenting specifications of the Rollei filter experts. Constant quality controls guarantee the very high quality standard of our CPL round filters of the Extremium & Premium series.
CPL filter glass with the special Luminance Coating
Light very often seeks its own way - and does not comply with the desires of photographers. Aficionados of filter photography and long-term exposure just do not appreciate scattered light, reflections or distortions at all but are not always able to react appropriately by changing the location. Likewise, substantial impairments of the pictures can also result from long exposure times. Rollei immediately provides you with additional safety and flexibility by means of a special coating that was developed by the company itself - not only with the photography techniques that were already listed.
The coating substantially reduces the risk of reflections and distortions without impairing the very good light transmission and colour neutrality of the Rollei polarising filters made from Gorilla®* glass. Thus optical characteristics, which would actually have to be mutually exclusive, are included in the combination of these quality criteria. The greatest resistance to contamination and damage, the best values with respect to luminance and colour neutrality as well as effective protection against reflections and distortions.
With a rigorous quality assurance strategy we ensure that these three groups of characteristics can be guaranteed in the long term - the coated glasses in the visible light spectrum of 400 to 800nm must allow all colour nuances to pass through precisely and absolutely homogeneously without causing even minimal colour deviations. It is only after successful quality testing that these glasses can be designated as real Rollei polarising filters.
Polarising filters - Engineered and Quality Controlled in Germany
In the course of time different quality assurance philosophies and strategies have evolved in the business. These approaches sometimes differ substantially and are naturally geared to the needs of various sectors and price segments.
With the development of the newest generation of Rollei polarising filters several characteristics, which actually rule each other out and could only be implemented with unusually close tolerance specifications were on the “wish list” of the filter experts. If only one of the strict criteria is not fulfilled, the other characteristics cannot be guaranteed to their full extent either. This particularly challenging quality level thus required a quality assurance strategy, which far exceeds the usual standards of the industry, in order not to lose sight of sources of error during the highly complex production process.
The final quality evaluation procedure was then dubbed "engineered and quality controlled in Germany” and consequently, already starts before manufacturing. After development the first production samples are approved exclusively by the responsible Rollei filter experts. The corresponding agreed upon criteria cannot be amended, reduced or retroactively loosened during the following production steps.
Before delivery to Rollei the individual production lots were then subjected to stringent quality inspections according to the AQL (accepted quality limit) principle. The procedure that is known in the German language as the “acceptable quality limitation level” [Annehmbare Qualitätsgrenzlage] discards the entire manufacturing batch if the quality tolerances of the previously precisely defined sampling quantity do not meet expectations. In the process, the quality of the unchecked products can no longer overturn this decision.
After delivery of the products to Rollei the AQL examination is conducted again. In this way naturally there are considerably higher investments before a Rollei polarization l filter may finally find its way to you. However, you can then be sure that you are really receiving only the very best quality that satisfies your highest requirements.
What should you look for when you are purchasing a CPL filter?
- Which lens size - select from among polarising filters with the following filter diameters: 49 mm, 52 mm, 58 mm, 62 mm, 67 mm, 72 mm, 77 mm and 82 mm
- Tempered filter glass - the glass should be high-quality, because a bad filter glass works like dirty eyeglasses. Our high-quality Gorilla®* glass meets all the requirements of a high-quality CPL filter glass.
Polarising filters are not very suitable for use with wide-angle lenses. Polarising filters darken the blue sky additionally and emphasise the contrast of the white clouds. This works particularly well at a 90-degree angle to the sun. Therefore, through the broad focus of a wide-angle lens it can happen that a part of the sky appears darker, while the remainder shines in a somewhat low-contrast sky blue. That immediately looks unnatural at first sight. Use a grey graduated filterto darken the sky with a wide-angle lens instead.
Sample pictures for comparison - with & without Rollei CPL filters
By now, CPL filters have become standard equipment for many landscape photographers. Yes, nowadays many of its functions can be replaced by Photoshop & other software. But above all filtering out mirror images and reflections still remains the core skill of polarising filters.
So if you want to remove reflections in the water, a CPL filter is very advisable for those shots.
Improving colours and contrasts with a CPL filter
By strengthening contrasts and colours with a CPL filter you will achieve greater separation of the sea from the sky. The blue will look substantially stronger as light of ranges are absorbed.
Summary concerning the use of CPL filters
The use of CPL filters always depends heavily on your choice of subject. If you are out and about in nature or taking photographs of the horizon, a CPL filter is recommended in any case. If you want to suppress reflections appropriately, you should also resort to a CPL filter. Sometimes however reflections are desired. Simply vary the use of the filter.
As soon as you have taken some photos, you will get a feeling for when you contemplate the use or omission of a CPL filter. However, with a little experience a CPL filter will improve your results tremendously.