Now that Leica has released a pure black and white camera, the Leica M-Monochrome, I started to think about the advantages a pure black and white camera has compared to a colour camera.
I guess you could remove the Bayer filter, but would this give you higher resolution, better contrast?
I assume there is advantages with a pure black and white camera, since Leica has made one, but what are they?
The biggest advantage is that you get 3X more light sensitivity.
With a bayer filter, every photosite gets 1/3 of the light that falls on it because the filter blocks 2/3 of incoming light to filter for one primary color. So the sensor becomes more sensitive to light. That means that less amplification of the read-out signal to get the same ISO as with a conventional sensor. The end-result is that you get lower noise at each ISO sensitivity.
There is no need for an Anti-Aliasing filter, so you get better sharpness and micro-contrast. As Nikon proved it though, this is not necessary for Bayer-based cameras but is usually the case. When a camera uses an AA filter, it blurs the light before it reaches the sensor to avoid the occurrence of an artifact called moire. Any time you blur something, you reduce contrast because you spread light over multiple pixels. Without an AA filter, the blurring does not happen and you get better contrast.
B&W sensors also obviously do not need Bayer-interpolation. This means that the readout is the image data and there is no question of softness introduced by interpolation (or the AA filter that is not there) and no need to sharpen at the capture level, although you may sharpen when processing for your output medium (print, screen or other).
In addition to Itai's excelent points, it is also worth mentioning that processing the RAW image becomes significantly easier.
With a normal bayer pattern, each pixel on the sensor records only one color (red, green, or blue). The RAW converter has to process this information into an image where each pixel contains full color information. So each pixel in the final image has to be based on at least three pixels from the RAW data. But in reality it is more (the simplest raw converter uses 4 pixels from the RAW data for each pixel in the final image, and this is too simple to produce excellent results). So This conversion inevitable leads to loss of sharpness.
A monochrome sensor shouldn't have this limitation.
I'd like to note that with a B&W sensor, you only capture luminance, so, if you need to apply colour filters you must do so physically with actual optical filters at capture time.
But you can do this in post when you convert an RGB image to B&W, for example by mixing channels or adjusting individual channel properties before converting.
I'm not stating any of these approaches is negative or positive, but they have intrinsic considerations that can impact on budget, technique, workflow and time to final output.
Personally I think a B&W brings back some of the film era experience, so I hope I can fit this camera in my budget some day, but in the meanwhile, I'll take advantage of the post flexibility of channel mixing and individual adjustment.
