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EDUCATION Lenses can’t be that complicated - can they? (Part 2) How do we measure a good lens? Graham Reed As camera technology moves forward so fast then many cameras are coming to the market from ‘cheap’ ones to those that costs many thousands of pounds. So how is the programme maker going to choose which one to use when nearly every week a new camera comes out which is better then the last. Many of the cameras come with an integral lens or with a lens which is part of the package, but is this lens any good? A camera with an attached lens which may cost around £4K but compare this to a large OB ‘box’ lens will cost around £80K. Yet they may all produce a great image. When a lens produces an image this is focused onto the sensor which then turns this image into an electronic signal, this image is then edited, post produced, distributed then viewed. All through this programme chain the image produced at the sensor can only be degraded, changed, yes, but if it was not very sharp on the sensor it will be only worst when viewed. This image is produced by the lens, so a good lens is essential in producing a good image. But what is a good lens and how do we know how good a lens is? At every air/glass surface light is lost, this light can then be scattered within the lens and produce ‘flare’. This loss of light can lead to a lens being not very ‘fast’ that is, have a small ‘f’ number, e.g. f3.5. Another issue with ‘cheap’ zoom lens is that they can ‘ramp’, which means as you zoom in the lens will iris down, for example from f3.5 to f4, so that the image will become under exposed as the lens zooms to a longer focal length. A lens is basically a light collector so it needs to collect enough light for this not to happen and all the light it collects needs to end up at the sensor not lost at the air/glass surfaces! A lens also needs to resolve fine detail and to be able to handle high contrast across the whole image plane and throughout the whole zoom range. To achieve all this is hard for lens designers made even harder with short focal lengths, long zoom ratios and large apertures. And now with 4k the task has become even more challenging. How can we test whether a lens is any better then another and will a lens made for HD be OK for 4k? A great lens being used on a poor sensor will of course produce a poor image, where as a poor lens with a great sensor will also produce a poor image. So we have to think of a sensor/lens combination. Consider this photo of some railings on a bridge. But as we get further away the railings become invisible. 38 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 109 JANUARY 2016