Circles of Confusion


Graham Reed TV-Bay Magazine
Read ezine online
by Graham Reed
Issue 106 - October 2015

We 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 field. 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 first 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.

With a fixed 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.

Its very easy to forget that everything we see on TV (and I mean everything!) is carefully planned and structured in a way to give us the best possible viewing experience. Whether it be a promotional advert, a programme sponsorship, an on-screen navigation menu to highlight whats coming up next, or even a bug of the channel logo in the top corner of our screens - the Presentation team are behind it. However, they also manage break patterns, the timing of schedules and any editorial requirements across all Sky channels. It perhaps might not come as a surprise, but it is not an easy job to do. Its an incredibly fast-moving but exciting environment to be in, especially when the pressure is on to meet daily transmission deadlines or make last minute changes if programme schedules have to be altered. Without this team, although we may not realise it, TV would seriously lack in both clarity and continuity. However, a Presentation Scheduler is not the only job in television that goes unnoticed.

I also had the chance to visit Editorial Compliance, a whole team dedicated to ensuring that each programme is suitable for its allocated time slot on the Sky channel it is scheduled for. The latter function in TV is something that Id never even thought of before - but what a great job, I mean who doesnt want to get paid for watching TV all day? But in all seriousness, they are a vital part of the broadcast platform, especially when dealing with acquired programmes. Acquisitions are a huge part of BSkyB, and with Sky Atlantic being The Home of HBO in the UK, they can have their work cut out - providing content notes, recommending classifications, cutting in appropriate breaks and selecting the correct warnings, not only to adhere to Ofcom regulations, but also to protect viewers. Although its a fun and relaxing working environment, achieving a good balance between customer, channel and Ofcom expectation with both acquired and commissioned programmes is not a simple task, and is yet another job in the world of television that lacks recognition.

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 fire by focusing the sun with a magnifying glass onto some paper? Lets consider that this image of the sun as a infinitely small image. The better the magnifying glass the smaller the image of the sun you could produce and quicker the fire started. As you moved the magnifying glass away from the surface the round image of the sun became blurred and bigger and the paper wouldnt 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.
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.

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


Tags: iss106 | education | circles of confusion | d.o.f | depth of field | lens | advice | Graham Reed
Contributing Author Graham Reed

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