Seeing Colour :: Through The Camera Lens

Let’s move onto the more Scientific/Enginering aspects of how the camera captures light of a specific intensity and wavelength. My schematic sketches of the camera cross section is meant to represent both traditional DSLR and newer mirrorless cameras. Both have exchangeable lens elements that focus an inverted image onto the sensor (or in the case of older SLR film). The DSLR however has a mirror that reflects the light up into a prism whose job it is to turn the inverted image up the right way and back through the view finder. So the DSLR user sees optically what the lens is focussing on. Whereas the mirrorless camera (obviously doesn’t have a mirror) and the image is captured on the sensor (often continuously) and this is sent to a graphics processor in the camera where the data from the sensor is converted into a conventional image (a jpeg) and this is displayed in a small screen behind the view finder (or alternatively the LCD screen on the back of the camera). The diagrams do not include the shutter and both systems have a shutter which control the length of time the sensor can capture light to give the required exposure.

The camera sensor (or film) collects the light that falls on it and the more light the closer to white the resulting image will be. In conventional SRGB Black is given the value 0 (zero) and white 255. This is usually called luminance. In a digital camera there are a multiplicity of small compartments each collecting its own luminance value.

How does this create colour, well it doesn’t for each compartment, but instead each set of 2 by 2 compartments have a set of coloured filters (known as Bayer Filters) over them. Each allowing only Red, Green or Blue Light to pass through. In a process known as demosaicing sets of these 2 by 2 colours are combined into single pixels with 3 colour. See the Cambridge in Colour page for a more detailed discussion of bayer demosaicing.

The best way to consider what is happening is to use the CIE chromaticity diagram, Unfortunately I have not been able to locate and camera manufacturers publishing their colour gamut, they seem to prefer to say they have better “colour science”. They all seem to offer a close fit to sRGB colour space and some offer the extra colour space and bit depth of Adobe RGB &/or ProPhoto RGB

I will again hand over to Craig Blackwell, To continue the journey of how the RGB colour space can be used to create colours.

Towards Blue(r) Sky(s)

Original (Polarizing)	Increased Saturation	Decrease Luminance
The light in summer here in Australia is very intense and it is really hard to get a decent blue sky. So I normally have a polaroid filter screwed on the front of my camera from November to April. The filter works a lot like the sun glasses, at different angles they will filter out and cut the intensity of the light. By rotating the filter you can see the darkening. As well as helping darkening the blue of the sky versus gulls or clouds, the polaroid can also work like a mini neutral density filter, to give you a slightly longer exposure on some landscapes.	The common approach is just make the sky bluer by increasing the colours saturation. In Lightroom you’d go to the develop module and slide the saturation slider to the right. Notice the gulls beak is also a more intense yellow	This is a less obvious route, In Lightroom you can go down to colour adjustment and in the HSL tab go into the Luminace panel and hold back on the blue a little. This just darkens the blue only and leaves the tonal range and colours on the gull untouched.

More Low Light HDR

I most admit working with long exposures and movement was fun.

There was a great downpour just before the sunset yesterday, and the lights of city traffic where making great reflections of the wet roadway. So why not try some HDR, the movement of course, makes it very difficult to properly register the images. However my android HDR Camera+ app actually did a good job (see image to the right).  Almalence have developed their own HDR algorithms for precise alignment and moving objects handling.

I have this app settings to keep the three bracketed exposure so I was also able to try out how well luminance and picturenaut could handle the movement. Quiet well it appears (albeit with some distinct colour casts, see images below reproduced here for those that want to tut tut at the lurid colours). Both picturenaut and luminace have auto registration and ghost removal features, and many more tone mapping features but I personally prefer the in phone processed image from HDR Camera+