Digital SLR Handbook. John Freeman
that helps to focus the light properly within each pixel. Any light entering a pixel is converted to a digital signal and sent to the camera’s on-board computer. The captured light is given a value corresponding to its brightness and colour, and all these values added together form the final digital image we see.
This sensor diagram of a Fuji 4th Generation SuperCCD reveals the CCD’s micro lens layer (top), the RGB mosaic filter (middle) and the dual pixels (bottom) used by this type of sensor technology to help capture even finer gradations of highlight in an image.
This diagram of a Foveon X3 sensor shows how the varying colours of light are collected from different depths within the sensor’s silicon.
However, CCDs and CMOS chips are actually colour-blind so to create the colours we see in an image, a filter grid of red, green and blue squares (RGB for short) overlays the pixels and under the micro lens layer. It is this RGB filter mosaic that creates colour in the images and is analogous to the colours created on a TV screen, which use the juxtaposition of red, green and blue pixels to create colour.
In a Foveon sensor, the pixels are embedded within layers of a silicon wafer at varying depths. Foveon sensors take advantage of the fact that the components of white light (red, green and blue light) can only penetrate the silicon to different and specific depths, so each pixel can record each colour at each pixel location depending on its depth, meaning that no extra colour processing is required.
The advantages of one type of sensor over another are debated long and hard, particularly by the manufacturers, but, in truth, the actual quality of an image is dependent on not just the sensor but on the entire DSLR system, such as the metering, white balance control, image processing and, most importantly, the lens. Poor-quality lenses deliver poor-quality light to the sensor, so the better the optics within a camera system, the better the images. No matter how many pixels a sensor houses and no matter how good the processor, if the light is blurred as it hits the CCD, your pictures are compromised.
A CMOS contains millions of tiny picture elements known as pixels. Incredibly, each pixel has a tiny lens that helps to focus the light accurately within each pixel.
Processor
Once the light information has been captured by the sensor, it is passed to the camera’s ‘brain’ – its internal computer, or processor. The processor must chew on the data to get the most from the image, dealing with colour and signal processing, as well as image compression, and controlling the colour display and white balance. Processors are often given names or brands: Canon’s processor, for example, is called a DIGIC processor and is now in its third iteration: DIGIC III. Whatever the camera and processor’s name, they all do the same jobs, helping to make the most of your images.
DSLR image processors are the digital workhorses in any digital camera and come in various guises. This is Canon’s DIGIC II processor, the ‘brains’ behind the image processing performance, helping to get the best from both the camera and the image.
This cutaway diagram shows how the pentaprism in a DSLR works. At the bottom is the angled mirror that reflects light coming in from the lens (at left and indicated by the white line) up into the pentaprism itself. Here, the light is flipped up, round and back out towards the eye (the yellow line), through the viewfinder optics. The 11 focus zones are shown as a ‘head up’ display in the viewfinder – the central (red) AF (automatic focus) point is active in this diagram. The red line shows the focusing information used by the camera to determine the AF point to use and indicate within the head up display.
Viewfinder and pentaprism
The viewfinder allows you to see what you’re about to shoot directly through the lens you have mounted on the camera. Top-end DSLRs have a field of view (the amount of the subject the lens will capture) that represents 100 per cent of the scene before you; less expensive models have, typically, around a 95 per cent field of view.
Shooting information, such as shutter speeds, indication of under- or overexposure, focus area information and confirmation, is usually on show through the viewfinder as well, along with the number of exposures left on a memory card, the shutter speed and aperture in use and if flash is activated or needed. In other words, a DSLR viewfinder can keep you abreast of the camera’s settings without you moving your eye away from it.
The key to all this utility is the pentaprism – a five-sided reflecting prism that catches the light entering the lens, turns it over, then flips it back into the viewfinder for you to see, without inverting the image. Some lower-end DSLRs use a pentamirror set-up instead of a pentaprism. However, they are not so bright as pentaprism viewfinders, which makes viewing more difficult in low light.
The diagram above shows how light travels through the lens to the viewfinder via the pentaprism.
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Equipment care
A DSLR system represents a significant investment, so it pays to look after it, especially as repairing a piece of equipment after the warranty has expired could be extremely costly, sometimes amounting to even more than the purchase price.
Care and common sense
Caring for a DSLR camera and lenses is a combination of care and common sense. Be careful not to drop them, and keep them free from dirt, dust, sand and water, even if you have a high-end DSLR that’s proofed against water and dirt ingress. Sand is particularly destructive, especially if it gets on the front element of a lens. Common sense dictates that you keep your camera in a strong, well-padded camera bag when you’re not actually using it; the same goes for any other paraphernalia, such as lenses. A camera isn’t difficult to clean externally: wipe it down with a lint-free cloth – a microfibre lens cloth is ideal, as the tiny microfibres absorb dirt and finger grease.
When inserting memory cards or electrical leads, such as a USB cable, never use force, and ensure that you put them in the right way round – they will only connect one way. Make sure that rubber caps over external sockets are tightly shut, too. If you store your DSLR for any length of time, place it in a camera bag with a pack of silica gel to keep moisture levels down. Also, remember to remove the batteries, which could leak and cause corrosion.
Protecting the sensor
Many DSLRs have sensors that are not shielded from dust, so each time you swap a lens, dust can fly in and perch on the sensor. There are special cleaning kits for DSLR sensors available but most manufacturers don’t recommend them. Cleaning a sensor is best left to the professionals, although, admittedly, this can be expensive.
It’s important to prevent dust getting on the sensor in the first place. Try changing your lenses in a clean plastic bag, for example, and keep the camera shielded from draughts or wind when switching optics.
If dust does manage to get into the camera, a small lens blower-bulb and a gentle puff of air on the sensor might help. Try this with the camera facing down, and all but the most stubborn dirt should drop out. Make sure that you don’t use pressurized (canned) air for this, as it can damage the sensor and leave moisture marks that will require professional cleaning to remove.