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ACQUISITION Circles of Confusion by Graham Reed W e seemed to have gone from SD to HD to UHD in a very short time but has lens technology kept up? Most camera manufactures now produce 4K cameras but they often look very similar to HD cameras, are they just the same with a 4k chip and also have they added 4k lens? They publish very little information about the lens. It is still a common held belief that a SD lens is ok for HD and a HD lens is ok for UHD but this is not the case. Another held belief is that if a 4K camera has the same sensor size as an HD camera there will be no change in the depth of fi eld. But is this the case? One of the most important jobs for a cameraman is to get the image in focus. But when is an image in focus? At what point does an image become ‘out of focus’ and how do we calculate this point? We must fi rst agree on the viewing distance of the image. Of course, the nearer you are to the image/screen the more ‘blurred’ the image will become. If you are very near to the screen you will see the pixels that make up the display device. Think of the large screens at a music concert, at the back they look good but in the front you can see that the picture does not appear sharp. The best viewing distance to a display device is approximately 1.2 times the diagonal distance. Once we have decided on the viewing distance we can then decide when an object in the picture becomes blurred or ‘out of focus’. 50 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 106 OCTOBER 2015 With a fi xed lens and camera position a moving image will become blurred at two points. These will be as the object moves away from the camera and then again when it becomes closer to the camera. The distance between these two points when the image is sharp is called the Depth of Field (D.o.F.) of the lens. The D.o.F depends on the aperture and focal length of the lens, image size, focus distance and the diameter of the permissible circle of confusion. But what is the permissible circle of confusion? Think of it this way. Remember when you were young and you lit a fi re by focusing the sun with a magnifying glass onto some paper? Let’s consider that this image of the sun as a infi nitely small image. The better the magnifying glass the smaller the image of the sun you could produce and quicker the fi re started. As you moved the magnifying glass away from the surface the round image of the sun became blurred and bigger and the paper wouldn’t burn. The smallest image of the sun that you make could be called the ‘circle of least confusion’ for that lens. Now imagine that a friend was holding the magnifying glass and you were an observer, you may not be able to see the point when the sun became blurred because you were standing too far away to see. By moving nearer, at a certain point you could actually see the image of the sun becoming blurred. At this viewing distance the diameter of the image of the sun could be called the ‘permissible circle of confusion’.