A look at the technological reasons that should underlie your choice.
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A look at the technological reasons that should underlie your choice. On the basis of new cameras announced in the past couple of months we can state with confidence that by the middle of this year there will be at least five 6-megapixel DSLRs and five 8-megapixel, non-interchangeable compact-type digicams on the Australian market. This creates a quandary for photographers who plan to invest in a new digital camera: which type to buy? Going on the pixel count alone, the 8-megapixel digicams seem to have a significant advantage. But high pixel counts aren’t everything: you need to check ‘beneath the bonnet’ to understand the pros and cons associated with both camera types before you can make a well-thought-out decision. The best place to start is with the sensor. Sensor Parameters As most keen photographers know, digital camera sensors consist of an array of light-capturing photodiodes that produce an electric charge when exposed to light. Microprocessors in the camera convert this charge into the pixels that make up the digital image, maintaining an exact correspondence between the number of ‘effective’ pixels in the sensor and the largest output image size. For photographers, the physical dimensions of the imaging area are a major determinant of potential picture quality. The diagram below shows how much smaller the sensor from an 8-megapixel camera is than a typical under-$3000 DSLR sensor – and how both compare with a 35mm film frame. (The table at the end of this article lists the sensor sizes of a range of current digicams and DSLRs.)  You can fit roughly 16 of the 2/3-inch (11.08 mm diagonal) sensors into the same area as a 35mm film frame – or 2.56 of the Canon sensors. Naturally, in a 2/3-inch type, 8-megapixel sensor, the photodiodes have VERY small surface areas, which means their light-capturing ability will be limited. In dim lighting, this sensor will be struggling to collect photons and the resulting signal will need considerable amplification to produce an image. (And everybody knows that when a signal is amplified, the associated noise is also boosted!) Because the sensors themselves are much larger, the photodiodes used in DSLRs have greater surface areas that can gather more light, leading to an increase in the sensor’s effective sensitivity without the inevitable noise associated with an increase in gain in the system. The downside is that they cost more to manufacture and take up more space in the camera, which means camera bodies have to be bigger. Output Parameters The sensors in the 8-megapixel cameras produce a maximum file size of 3264 x 2448 pixels, whereas the 6-megapixel DSLR sensors deliver 3072 x 2048 pixels. If you print out a high-res 8-megapixel file at 300 dpi, the maximum print size is 10.88 inches (276.35 mm) by 8.16 inches (207.26 mm), whereas a 6-megapixel DSLR can produce prints that are 10.24 inches (260.1 mm) by 6.83 inches (173.48 mm). While this may appear to be a significant difference, it’s important to note that the 3:2 aspect ratio of the DSLR’s image files fits better onto standard paper sizes than the 4:3 aspect ratio of the 8-megapixel digicam files, which means you get more picture onto your paper (and less waste to trim off). From a practical viewpoint, the difference between the two camera types is, therefore, less than first impressions may lead you to believe. Lens Multipliers Sensor size can also affect the depth of field the camera captures. A detailed explanation of depth of field can be found on the Photo Review website (www.photoreview.com.au) but for the purposes of this exercise it is sufficient to state that cameras with smaller sensors tend to have greater depth of field than 35mm cameras. As a rough rule of thumb, the depth of field of a digital camera at a given aperture is the same as that of a 35mm camera with the lens closed down to the same aperture multiplied by the lens multiplier factor (LMF). (Note: you can calculate the LMF for any camera by dividing the actual focal length of the lens into the 35mm equivalent focal length. We’ve listed some typical LMFs in the table on the opposite page.) Extended depth of field can have useful and undesirable consequences. On the plus side it gives you a better chance of producing pictures in which most elements are sharp. On the minus side, it curtails your ability to throw backgrounds out-of-focus. Thus, the importance of a small LMF will depend on the types of pictures you want to take. A DSLR will give you the same aperture range as a 35mm film SLR because it uses the same (or similarly designed) lenses. Furthermore, its lower LMF will let you use selective focusing over a much wider range of camera-to-subject distances. (Note: many digicams support only a few aperture settings and, although an f2.8 maximum aperture is common, most can’t be stopped down beyond about f8.) Video If video capture figures prominently in your ‘wants’ list, cross off digital SLRs; they cannot capture video at all. DSLRs use full frame transfer sensors, in which the entire surface area of each photodiode is exposed. This further increases their light capturing facilities but means they must be used with a mechanical shutter that controls each exposure (this also means the inclusion of adjustable aperture diaphragms). In contrast, digicams use interline transfer sensors in which lines of photodiodes alternate with shift registers that carry the electric signals from the photosites in a process that’s something like a bucket chain. The sensor’s ‘electronic shutter’ controls when it starts and stops measuring the electric charge. This shifts electrons out into a serial register, from which the signal passes to the camera’s microprocessor. Electronic shutters allow video to be captured and manufacturers often combine them with a mechanical shutter like those found on film cameras – but it’s generally a much cruder mechanism and aperture rings are unnecessary. Viewfinders All photographers want to be able to see exactly what they will capture when they frame each shot and this is one reason why many compact digicam users favour the LCD monitor over the viewfinder. Whereas most digicam viewfinders display between 80 and 90% of the image captured, on an SLR camera, the viewfinder image is virtually identical to the final picture. Furthermore, since full frame transfer sensors can’t support a ‘live’ image display – for the same reason as they can’t support video – DSLR users can only use a viewfinder when composing shots. Compact camera manufacturers can choose between fitting an optical viewfinder and reproducing the picture captured by the sensor via an electronic viewfinder (EVF). To meet demanding photographers’ requirement for framing accuracy, most high-end digicams have EVFs. EVFs have advantages and disadvantages. On the plus side, they really DO show you what the camera will capture – and they also show you all the data that is commonly displayed on the monitor (but may be absent from optical viewfinders). On the minus side, their image quality can be very low and you frequently get broad white streaks across the field of view when composing backlit shots (these streaks are seldom recorded but they can make shot composition difficult). Conclusion High pixel counts are not the be-all-and-end-all in digital camera selection and they won’t guarantee the camera you buy will meet all your requirements. Photo Review’s tests have shown sensors with small photodiodes invariably produce more noise than large-area DSLR sensors. Consequently, while the higher pixel count produced by an 8-megapixel digicam may let you make slightly bigger enlargements before pixellation becomes apparent, it doesn’t guarantee better quality pictures across the board. And because the DSLR’s 3:2 aspect ratio makes more efficient use of standard paper sizes, the apparent numerical advantage of 8-megapixel vs 6-megapixels is limited. When comparing like with like, there’s no question that an 8-megapixel digicam will support more enlargement than a 5-megapixel digicam. However, the latter will deliver better quality night shots from the same-sized sensor, especially if long exposures are involved at ISO 400 or higher settings. But, a DSLR will do much better in these situations because it is better at collecting light. In the end, when you’re investing $2000 or more in a piece of equipment, you need to look carefully at the types of pictures you want to take and how you plan to use them. Then ask yourself whether you would consider a high-end compact 35mm over a 35mm SLR if you were buying a film camera. For somebody who’s serious about photography, it should be clear where the advantage lies. Finally, if you still can’t make up your mind after evaluating all the factors presented above, be guided by the feel of the camera in your hands because if it’s not comfortable to use, it’ll end up in storage and your money will be wasted.
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