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Width of picture lines Diameter of Circle of Confusion larger than line width. Image appears out of focus Diameter of Circle of Confusion equal to line width. Image appears in focus Diameter of Circle of Confusion smaller than line width. Image appears in focus Now if the image of the sun was focused on a piece of paper and your friend kept the magnifying glass still but moved the piece of paper the distance between the two points when you could see the sun become blurred would be the Depth of Focus. The nearer you became to the image of the sun that your friend was making, (you reduced the viewing distance) the smaller the distance that the magnifying glass would be able to be moved before you could see the image of the sun become blurred. But if you made the permissible circle of confusion smaller you would be able to reduce the viewing distance before you could see the image become blurred. In other words the definition or ‘sharpness’ of the lens would have been increased. Consider it another way. By making the permissible circle of confusion smaller you would be able to increase the distance the lens could be moved before you could see the image become blurred. That is you have increased the Depth of Field. But if we increased the number of lines in our picture we would have decreased the D.o.F., for example 2k to 4K. We can see the relationship between the viewing distance and the permissible circle of confusion. But by keeping the viewing distance the same we can only reduce the diameter of the permissible circle of confusion to increase the definition of the lens. But there is a limit to how small the permissible circle of confusion can be made for a lens. So by having a high definition image you require a lens that has a small permissible circle of confusion which will have a small Depth of Field. The mathematical calculation for working out the D.o.F. is: D.o.F = 2u 2 NC f 2 u Focus distance N f number: Aperture f Focal length of lens C Circle of confusion (All in millimetres) By having a small Depth of Field focusing becomes more critical and thus its harder to focus when working in HD. Plus by the fact that we have a sharper image and any out of focus image will be more noticeable Depth of field is directly proportional to the circle of confusion, the f number, and the square of the focused distance and inversely proportional to the square of the focal length. Subject distance and focal length have the greatest influence; doubling the value of ‘u’ increases depth of field fourfold, whilst doubling focal length reduces D.o. F (at a fixed distance) by a factor of four. www.grahamreedlightingcameraman.com KITPLUS - THE TV-BAY MAGAZINE: ISSUE 106 OCTOBER 2015 | 51