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

Read this article in the tv-bay digital magazine
Article Copyright tv-bay limited. All trademarks recognised.
Reproduction of the content strictly prohibited without written consent.

Related Interviews
  • Training and education within the broadcast industry

    Training and education within the broadcast industry

  • Cambridge Imaging Systems at BVE 2014

    Cambridge Imaging Systems at BVE 2014

  • SMPTE on BroadcastShow LIVE at IBC 2013

    SMPTE on BroadcastShow LIVE at IBC 2013

  • Broadcasters mobile phone rack with HD voice from Glensound at IBC 2018

    Broadcasters mobile phone rack with HD voice from Glensound at IBC 2018

  • Glensound show Beatrice the AES67 Network audio intercom beltpack at IBC 2018

    Glensound show Beatrice the AES67 Network audio intercom beltpack at IBC 2018

  • Virgil OB headphone amplifier from Glensound at NAB 2018

    Virgil OB headphone amplifier from Glensound at NAB 2018

  • PARADISO Lite commentary unit from Glensound at BVE 2018

    PARADISO Lite commentary unit from Glensound at BVE 2018

  • Schneider Filters and Cine-Tilt Lens at IBC 2017

    Schneider Filters and Cine-Tilt Lens at IBC 2017

  • Glensound Dante Intercom System at IBC 2017

    Glensound Dante Intercom System at IBC 2017

  • Live Sports Broadcast Commentator from Glensound at NAB 2017

    Live Sports Broadcast Commentator from Glensound at NAB 2017

  • Fujinon 18-55 Lens at BVE 2017

    Fujinon 18-55 Lens at BVE 2017

  • Glensound at IBC 2016

    Glensound at IBC 2016

  • Glensound at NAB 2016

    Glensound at NAB 2016

  • Glensound at BVE 2016

    Glensound at BVE 2016

  • Glensound Expedition at IBC 2015

    Glensound Expedition at IBC 2015

  • Glensound at BVE 2015

    Glensound at BVE 2015

  • Glensound Dante at IBC 2014

    Glensound Dante at IBC 2014

  • Glensound Atomic Copper at NAB 2014

    Glensound Atomic Copper at NAB 2014

  • Glensound Cub at BVE 2014

    Glensound Cub at BVE 2014

  • Glensound on BroadcastShow LIVE at IBC 2013

    Glensound on BroadcastShow LIVE at IBC 2013

  • Glensound with the USB Commentary Mixer at IBC 2013

    Glensound with the USB Commentary Mixer at IBC 2013

  • Glensound: RECCE HD at NAB 2013

    Glensound: RECCE HD at NAB 2013

  • Polecam: Z10 zoom lens at NAB 2013

    Polecam: Z10 zoom lens at NAB 2013

  • True Lens at BVE 2013

    True Lens at BVE 2013

  • Lens Adaptor at BVE 2013

    Lens Adaptor at BVE 2013

  • Glensound at BVE 2013

    Glensound at BVE 2013

  • Glensound at BVE North 2012

    Glensound at BVE North 2012

  • Glensound at IBC 2012

    Glensound at IBC 2012

  • Glensound at NAB 2012

    Glensound at NAB 2012

  • Glensound at BVE 2012

    Glensound at BVE 2012

  • New Filters from Schneider Optics at NAB 2018

    New Filters from Schneider Optics at NAB 2018

  • BeyondHD - Drones at BVE 2015

    BeyondHD - Drones at BVE 2015

  • Polecam Antelope Pico at IBC 2014

    Polecam Antelope Pico at IBC 2014

  • Cooke Optics technology at IBC 2014

    Cooke Optics technology at IBC 2014

  • Cooke Optics talk anamorphic at IBC 2014

    Cooke Optics talk anamorphic at IBC 2014

  • Polecam Toshiba Minicams at BVE 2014

    Polecam Toshiba Minicams at BVE 2014

  • Canon on BroadcastShow LIVE at IBC 2013

    Canon on BroadcastShow LIVE at IBC 2013

  • Zylight on BroadcastShow LIVE at IBC 2013

    Zylight on BroadcastShow LIVE at IBC 2013

  • Cooke Optics on BroadcastShow LIVE at IBC 2013

    Cooke Optics on BroadcastShow LIVE at IBC 2013

  • Cooke Optics at BVE 2012

    Cooke Optics at BVE 2012

  • Cooke at IBC2011

    Cooke at IBC2011

  • MTF at IBC2011

    MTF at IBC2011

  • Bradley at IBC2011

    Bradley at IBC2011


Related Shows
  • Polecam Minicam Show: BVE Day 1

    Polecam Minicam Show: BVE Day 1


Articles
Peli Air 1507 Review
Phil Vinter Originally named after a bird that carries its precious cargo through the skies, it was, perhaps, only a matter of time before Peli released its Air range.
Tags: iss133 | peli | peli air 1507 | trekpak | Phil Vinter
Contributing Author Phil Vinter Click to read or download PDF
OB999 Accelerates Hill Climb Broadcast
Nick Collier Over the last century, The Shelsley Walsh Speed Hill Climb has been attempted by some of the world’s most accomplished racing drivers striving to be the fastest up the 1,000 yard, 1 in 6 gradient track.
Tags: iss133 | ob999 | blackmagic | atem | multicam | videohub | cleanswitch | premiere pro | hyperdeck | Nick Collier
Contributing Author Nick Collier Click to read or download PDF
Ruth Matos Interview - A Career Unfolds
Ruth Matos We first met Ruth when we employed her as a student to help with our live studio at BVE in 2013. We were struck by her determination, passion and ‘I’ll do anything to help’ attitude. We became friends of Facebook and have since seen her career in the industry develop and unfold. We thought it was about time to catch up and share her inspiring story of where she is now, how she got there and the challenges she has faced along the way.
Tags: iss133 | interview | filmming | career | education | Ruth Matos
Contributing Author Ruth Matos Click to read or download PDF
The Pace of Change
Dick Hobbs - new

The youngest human to stand on the moon (so far) was Charles Moss, the lunar module pilot of Apollo 16. Charlie had a wonderful claim: his father witnessed the Wright Brothers’ first flight at Kitty Hawk, and lived to see his son on the moon.

Does anything capture the speed of technological advance better than that? The whole of the history of powered flight in one lifetime.

Tags: iss133 | state of the nation | st2110 | st2110-10 | Dick Hobbs - new
Contributing Author Dick Hobbs - new Click to read or download PDF
Taking on a self employed placement year
Joshua Round The idea of being self-employed or freelancing has always been somewhat terrifying for me. There is a level of uncertainty and responsibility that comes with the freedom of being self-employed, the likes of which makes me wonder why I chose to give it a go for my placement year as part of my university course - (BSc) Television and Broadcasting.
Tags: iss133 | placement year | university | student | education | portsmouth | Joshua Round
Contributing Author Joshua Round Click to read or download PDF