I have to wear strong prescription glasses to correct my short-sightedness. When people with normal vision try them on, their vision becomes distorted. However, when I hold my glasses over the lens of my digital camera, the photographic image comes out crystal clear. Why is this so?
Either the camera has 鈥渢hrough-the-lens鈥� focusing, or the glasses are being held in front of an infrared sensor on the front of the camera. In either case, the camera鈥檚 focusing system works by adjusting the camera lens until the image on the digital sensor is in sharp focus. In doing so it compensates for any lens placed in front of the camera.
Alastair McDougall, Warrington, Cheshire, UK
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鈥� The human eye has evolved with the power of accommodation: that is, the potential to reshape the living lens inside your eye to focus light onto the retina, producing a sharp picture. It can adjust the lens to focus images of objects both far and near, but its ability is limited. You only need to consider how many people wear corrective glasses or contact lenses. These external lenses begin the process of refracting, or bending, light before it hits your eye, allowing the much less powerful lens in your eye to make the final fine adjustments.
鈥淭he eye can adjust to focus both far and near, but it is limited. You only need to consider how many people wear corrective glasses or contact lenses鈥�
This ability is relatively powerful in many animals, and our sense of sight is actually quite keen, but it pales in comparison with the focusing capability of a digital camera, especially one with autofocus capabilities. Many of these cameras use a digital feedback system to produce the sharpest image. The camera鈥檚 microprocessor measures the contrast between pixels along a strip running across the centre of the image field. It then moves the lens back and forth, finding the lens position that produces maximal contrast, since this is usually the clearest image.
This has a much higher accommodative power than the human eye, because the camera physically moves the lens to and fro. Some cameras even feature a multiple-lens set-up.
Steve Minear, Marietta, Georgia, US
鈥� The questioner does not give his exact prescription, but taking -5 dioptres (units of refractive power) as a fairly strong correction, placing such a lens in front of the camera will effectively bring the position of infinity in to 20 centimetres. That鈥檚 well within the range of a digital camera 鈥� mine, for example, can focus down to 2 centimetres.
However, long-sight correction (which uses lenses with positive dioptres) takes the viewed scene out 鈥渂eyond infinity鈥�, and the camera would not be able to cope with that. Nor could it correct for astigmatism.
Tim Gossling, Cambridge, UK