Convert your DSLR to Black & White

There have been a few  attempts at producing a black & white-only camera, but the Leica M Monochrom is perhaps the most famous. This Leica model is based on the M9, but only records monochrome, greyscale images. In theory, any manufacturer could produce a version of one of their cameras to shoot in black & white only, by issuing a version with no colour filter array (CFA) on the
sensor.

WHY CONVERT A CAMERA TO BLACK & WHITE?

When a full-colour image is created, various fine details can be lost. Digital images are usually demosaiced, as without this process images would just be made of red, green and blue pixels of various shades, devoid of any other colours. Demosaicing is a process of interpolation, whereby the camera processor calculates the various hues in an image based on the red, green and blue values of surrounding pixels.

Although the demosaicing process creates a full-colour image, the nature of the process can also introduce a slight loss in definition, and occasionally image artefacts.

CONVERTING A DSLR TO BLACK & WHITE

To convert a DSLR to black & white, the colour filter array must be removed from the surface of the sensor. This isn’t easy, as the coloured filters are bonded to the silicon surface of the sensor. Not only that, but the filters actually sit beneath the microscopic lenses that are positioned above each photodiode.

Removing the micro-lenses and the coloured filters changes the back-focus distance between the lens and sensor, and as such it can lead to slightly soft images, particularly at large apertures. It is possible to adjust the position of the sensor to account for the slight focus loss, usually mechanically by turning some adjustment screws, testing the camera and then making any further adjustments. If your camera has a micro-focus adjustment feature in its camera software, then you may get the result you need using this. However, slight focusing issues are not the biggest problem – that would be damaging the sensor, either in part or completely.

REMOVING THE CFA

Removal of the colour filter array is not for the fainthearted. Unlike the low-pass and infrared filters, which can be carefully prised off, the colour filter array needs a more drastic approach. A quick search on the internet shows a few successful attempts by various brave photographers, and there are two methods. The first involves using an abrasive, such as car polish, while the other involves carefully scratching away at the layer of micro-lenses and then the colour filter array, leaving just the bare silicon.

The real skill here is to make sure that you scratch or rub away the lens and filter layer without damaging the layer of silicon below. Thankfully, the silicon layer that contains the photodiodes is harder than the layers above, so with some light rubbing and patience it is possible.

THE RESULT

After the removal of the colour filter array, there is still one last hurdle to overcome. The camera or computer software will still demosaic the image when shooting JPEG or raw images, but with no colour filter array in place there is no need for this process. Instead, raw-conversion software that doesn’t demosaic, such as the free DCRaw software (https://www.cybercom.net/~dcoffin/dcraw), must be used. The resulting images should have better detail in shadow areas, and make excellent monochrome images.

SUCCESS STORY

Raymond Collecutt, from New Zealand, was one of the first people to perform the ‘de-Bayering’ surgery on a Canon EOS 1000D in 2012. I asked him how he did it, and any advice he had to offer. 

What made you want to give the ‘de-Bayering’ a try?
I’m an astro photographer and live in a built-up area, so using narrow-band filters was the main driving force behind it. These filters cut out most of the light that comes from light pollution, allowing only a narrow band of light from space to get through.

Because of the small amount of light reaching the sensor, you have to get the most from every pixel, so removing the Bayer filters and removing the demosaicing process was an obvious solution. Besides this, it was also a huge saving compared to the cost of a dedicated mono camera for astro photography, while the larger DSLR sensor also offers a much better field of view, which was a big plus.

Did you ever think this idea would work?

I had no idea if it would work, but I did try it first on a webcam and found it was a very easy process,
so I tried it on a Canon EOS 1000D sensor. This wasn’t as easy, but to my surprise it turned out OK.
I did search the internet for advice beforehand, but found nothing – not even a thread on a forum! All I found was people saying it was impossible. 

What advice would you give to those who want to try it?
If you have shaky hands or are unable to see very small things, then I would give it a miss unless you
have lots of spare cameras.

HOW A SENSOR RECORDS COLOUR

A sensor is made up of millions of photodiodes, often called photosites or, confusingly, pixels. Each photodiode records the amount of light that is hitting that particular point, which results in an image map of where the light hits the sensor. Where no light reaches the sensor at all, the photodiode will record no information, resulting in a black pixel in the recorded image. Where the amount of light hitting the sensor exceeds what the photodiode can hold, a white pixel will be recorded. Between these two extremes is a range of grey pixels.

Above each photodiode is a microscopic coloured filter, usually of red, green or blue. As we know, there are various different colour patterns for these filters, known as colour filter arrays (CFAs), but the most commonly used is the Bayer pattern, where there are two green filters for every red and blue filter. The colour filters ensure that only the light of that filter reaches the photodiode – only green light will pass through the green filter, for example.

However, the photodiode below a green filter doesn’t actually record the colour green, it only records the amount of light (green light, in this case) that passes through it. This creates a demosaiced image made of just red, green and blue pixels. All that is needed for a DSLR to create a pure black & white is to remove the coloured filters from above the sensor’s photosites.

Tips & Trick 10:17 AM