Filters for black & white

Filters for black & white

Mat Gallagher investigates the differences between physical filters and digital processing for black & white images using both film and digital technologies

WHEN shooting in black & white, coloured filters provide a simple yet effective way of controlling different wavelengths of light. They allow the user to create better separation between the grey tones or to increase or decrease the contrast. However, with the advent of digital photography many think these filters are superfluous, as the conversion to monochrome can be performed post-capture using all the colours of the spectrum. With careful use of software, colours can be controlled in the b&w conversion to produce results similar to any colour filter at the touch of a button. With this choice left until the processing stage, and numerous combinations of colours and strengths available, it could De argued that digital filters are superior to physical coloured filters placed in front cf the lens. While digital processing is more versatile, the true test as to whether digital filters are better than physical ones has 😮 be in the end result.

For this test we will compare similar images taken with yellow, orange, red and green coloured filters, and with digital filters applied in processing. We have used a Canon EOS 7D DSLR, shooting in raw format to obtain the maximum detail frcm the sensor. The images taken with coloired filters were then converted to monochrome using Adobe Lightroom, while the non--

filtered images were converted and digitally filtered using both Adobe Photoshop CS4 and Nik Silver Efex Pro 2 software. Silver Efex software is the choice of many professionals for black & white conversion. Also during the test we took some images with the EOS 7D in monochrome mode with in-camera filters applied.

In addition to the digital camera we used a Canon EOS-1 V SLR loaded with Kodak T-Max film to shoot similar images, both unfiltered and with each of the coloured filters in front of the lens. These results were scanned into a computer using an Epson Perfection V750 Pro scanner and any required adjustments applied. The non-filtered film frames were filtered digitally using Adobe Photoshop CS4 and Nik Silver Efex Pro 2. The final results were then compared both on screen and in print in a blind test to determine any differences and whether either process offers any benefits.

Wlhat colours do


A yellow filter darkens blues and purples, while lightening green, red, orange and yellow. This is useful for darkening a blue sky and for a clearer image when shooting in haze. It also lightens skin tones.


Orange filters produce intense versions of those achieved by a yellow filter, with darker blues and purples, which is useful for skies and hazy conditions. Definition in foliage is also increased.


A red filter blocks blue and green


This can leave blue skies almost black and create a stromg contrast between foliage and flowers.


A green filter is ideal for any form of plant or flower shot As it lightens the greens and yellows it therefore helps create definition and shows detail among shades of foliage. Green filters can also create pleasing skin tones for portraits. Green wavelengths are useful when using a digital camera, as we will discuss later.


For each series of test shots, as well as real life scenes, a Gretag Macbeth Colour Rendition Chart has also been photographed. This allows us to see how particulars hues are affected by the different coloured filters, whether they are optical or digital.


BY PHYSICALLY reducing the colours that reach a digital camera s sensor you are reducing the amount of light available for the exposure. How much this is reduced depends as much on the colour of filter as it does on the density of the filter. A standard Bayer colour array has twice as many green photodiodes as it does red or blue. For this reason, when choosing a single colour, the green filter is a sensible choice for digital imaging, as green accounts for half of all the photodiodes.




For this test we photographed in raw format without anv phvsical filters in place. The results were converted to black & white using Adobe Photoshop CS4 and Nik Silver Efex Pro 2 software.



The images we captured for the first test were also processed using a range of digital colour filters in both Adobe Photoshop CS4 and Nik Silver Efex Pro 2 to recreate the effects of yellow, orange, red and green filter.


Images were taken in raw mode with Kood Pro colour filters placed in front of the lens. The BW1 Yellow, BW2 Orange, BW4 Red and BW3 Green filters were used in turn. The results were then converted to black & white using a basic process in Adobe s Lightroom 3 software.


The camera was set to its monochrome mode and images were taken in the standard setting with the in-camera yellow, orange, red and green colour filters applied.



For this test we took similar images to those taken with the digital SLR. No physical filters were used. The results were then scanned into the computer using an Epson Perfection V750 Pro.



The images taken for the second test were then taken into Nik Silver Efex Pro 2 and processed with digital filters to produce effects similar to yellow, orange, red and green filters.


Images were taken with the Kood Pro yellow, orange, red and green filters placed over the lens. The results were then scanned into the computer using an Epson Perfection V750 Pro scanner.


INITIAL comparisons of images taken with the DSLR, both with the physical colour filters and digitally applied filters, show changes in the tones of the mono images. The differences between those converted using the colour filters in Nik Silver Efex Pro 2 and Photoshop CS4 are also clear. Photoshop s standard presets have a more extreme effect, whereas Silver Efex produces a more realistic tone that closely matches the results achieved using the physical colour filters. The effects in both programs can be toned down using slightly opaque layers in Photoshop or the density controls in Silver Efex. Nik software works as a plug-in for Photoshop, Lightroom or Aperture.

Comparing the digitally produced coloured filters in Silver Efex with the Kood filters produced more interesting results. The images taken with the physical colour filters had more subtle tone changes, but lost sharpness compared to the digitally filtered files. The use

of filters results in much slower shutter speeds-the red filter, for example, allows roughly 2EV less light to the sensor-and this can increase the risk of camera shake. However, even in very bright conditions, images appear less sharp than those without filtration.

The Canon EOS 7D s in-camera filter effects show differences in tone between colours. The results are darker than the computer-edited files, but this helps retain highlights. The effect is similar to that achieved in Photoshop. For quick shooting this has merit, but the outputted mono JPEG means you can t adjust the results and are stuck with the filter youchosen.

Using the Kood colour filters on the film SLR produced far more subtle tonal changes than even when the same filters were used on the DSLR. To make an impact on your tones youprobably need to use an orange or red filter.

On a digital camera a yellow may suffice.

As the b&w scanned film images contain no colour information, the filter processes used by Photoshop and Silver Efex have no effect. This is also the case for the monochrome images created by digital cameras.


AS A GENERAL rule in photography, it is best to make sure that the image is correct at the time of capture, which usually means the use of filters rather than applying adjustments in software. However, in the case of black & white conversions this appears not to be the case. Those using black & white film must attach a filter in front of the lens, as must anyone choosing to shoot in a monochrome mode-including those using the new Leica Monochrom M camera. Those using a regular digital camera are best placed to capture the full colour information from the sensor using its raw format and convert the image using an advanced black and white program such as Nik Silver Efex Pro 2. This will not only allow the greatest control over the tones of the image, but will also allow the optimum sharpness to be retained.

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