DIGITAL PROCESSING EXPLAINED

 

Your D-SLR might look Like a film camera, but inside is the equivalent of a digital processing lab

Your digital camera does a lot mare wofk to turn what you see in the viewfinder into a finished image than you probably give it credit for. Understanding a little of what goes on deep inside your Nikon s circuitry will help you understand some of the many options that you have to play with.

Perhaps the most important thing to appreciate Is that your image is only converted into digital form well after it has left the sensor The sensor itself is an analogue component, creating an old-fashioned electrical signal that can he tweaked by electronic circuitry to increase the ISO. for Instance, amplifying the signal in much the same way as you crank up the volume on your radio.

How sensors work

The sensor is made up of millions of light-sensitive units, often referred to as pixels, but at this stage more accurately called photosites. These can measure as little asO.004mm across (around 1/16th of the width of a single human hair). Each one

creates Its own electrical signal in proportion to the brightness of the part of the image that it covers.

But these individual photosites can tsee colour-only Luminance.

Picture Control

To produce a full-colour image, each photosite has a miniature coloured filter either red, green or blue, A pixel with a green filter wILL onLy see colours that have some green Light in them. Gut as practically all colours can be made by mixing red, green and blue light together, this still provides valuable information. The clever hit is how the pixels work

together to assign accurate colours to each pixeL in the finaL image.

The green-filtered photosite can effectively  see? red light by using information from neighbouring red-and blue-filtered photosltes, Known as  demosaicing , this interpolation process makes an informed guess about the colour of each and every square that makes up the image.

JPEGs and RAW files

With JPEG pictures, each photosite can registers bits of data, which equates to 256 different shades of brightness. This might not sound like much, but when it s combined with the full colour information generated by demosaicing, this generates an amazing 16.7 million possible colours. RAW (NEF) images offer even more colour detail.

Nikon NEF files store 12 bits of data, equivalent to 4,096 separate shades rather than JPEG s 256.

Advanced Nikon models can also save 14-bit NEF files, equivalent to 16,384 brightness levels that can be recorded for each pixel.

Picture Control

Regular JPEG images will still show a full range of colour and tones, but RAW files have the potential for a wider brightness range and smoother tonal gradations. Whether you choose to record in JPEG or RAW format also affects the way a picture is processed, and the Picture Control you use on your Nikon is a case in point. Changing the Picture Control is a bit like changing the type of film in the old days. Each Picture Control gives a different colour balance, saturation and contrast. Vivid gives you rich, vibrant colours, while

Monochrome shoots pictures in black and white.

If you shoot in RAW, the Picture Control isnapplied to the RAW data, but simply stored alongside it, so you can ignore it completely. However, as youfind out in this monthhomework assignment, even those shooting in RAW should take Picture Controls seriously.

BUFFER BOTTLENECK

For rapid-fire photography, understanding the buffer is vital

A camera needs a great deal of computing power to record images quickly, and this varies from one D-SLR to another. But it also needs somewhere to store them temporarily as it writes the processed images to the memory card. This temporary memory is the .

The amount of processing power will affect how many frames you can shoot in a second, but the camerabuffer controls how many

you can shoot before it can t cope with any more data. Depending on your camera, your image quality settings and memory card speed, a camera may only take six RAW shots before it refuses to take any more pictures, taking a break until it has cleared space in the buffer. But if you switch to JPEG, you ll be able to keep shooting for much longer. See our table on the right for details on different D-SLRs.

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