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The railings eventually become too small for our sensor/lens system to resolve. We can measure this limit of the lens/sensor with what is known as the Modulation Transfer Function, MTF, of a lens. By measuring the output we can see how the MDF varies across a lens and how it drops off towards the edge of a lens and how also it is worst for wide and a telephoto lens. We can also see how the MDF changes for focusing distance and how much better it is on an HD lens. We can now understand why we need to pay a lot for a good lens and why producing a lens which has a high MDF across the whole image and aperture range is very hard for lens designers to do. If we have a chart of black and white lines which get closer and closer together and then view them through our lens/camera system and we look at the output on a wave form monitor we will see that eventually the system will not be able to resolve the black and white lines, this will be the limit of the MDF for this system. Of course all of this is technical and we must remember that we are producing art so the lens needs to produce the images we are trying to get so a lens with a low MTF may produce the images for which we are looking. KITPLUS - THE TV-BAY MAGAZINE: ISSUE 109 JANUARY 2016 | 39