Canon AL-1 QF

The Quick Focus (QF) system was a new technology, and with Canon, reliability is always the key feature, which means making small steps during introduction. Fast and accurate focusing has always been a problem with photographers, especially if they had poor eyesight or wore glasses, and spending the whole day turning the focusing ring hunting for the best focus can be an eye-straining job even for those with good eyesight. What was required was an electronic "assist" system, which detected the sharpest focus and conveyed that information in a simple way by lighting an LED. They called it QF, and incorporated its power switch into the first stage of the two-stage shutter release switch. The owner simple composed the shot, turned the focus ring until a greed LED lit, and took the shot. It was not perfect and there were caveats for when it would fail to work, however when it did work, it was excellent and a step forward.

The principle of measuring focus electronically is quite simple, and as any boy scout will tell you that when you use a lens to focus sunlight on a spot, then at best focus, that spot will have the highest intensity of brightness and contrast. You can measure this light intensity using an electronic sensor known as a charge-coupled device (CCD).

As you turn the focusing ring, the light slowly comes into focus, and if you keep turning the ring in the same direction, then it eventually goes back out of focus. Therefore, all that is required is a microcontroller running a clever algorithm, which compares current light intensity to the previous value. As the intensity increases, the image is coming into focus, and at peak intensity, it is in focus. The viewfinder displays this information in the form of three LED lights. There are two red illuminating arrows indicating whether light is coming into focus or going out of focus, and therefore the direction to turn the focusing ring. When the light intensity reaches peak and then just starts to go down again, the microcontroller lights the centre green LED indicating perfect focus when you should take the shot.

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