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Lens Technology

Carl-Axel Alm

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> Degree in electronics from Lund University (M.Sc.E.E. in 1989)

> Employed by Axis since 1993

> One of the inventors of the network camera

> Presently working with innovations regarding optics, electronics and mechanics for new concepts

Carl-Axel Alm

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> Visible light cameras: 400-700nm

> Near Infra Red (NIR) cameras: 400 - 1,100nm

> Thermal cameras: 8,000 - 12,000nm (8-12µm)

Radiation

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Lens parameters

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> M12 – Small inexepensive lens, used in e.g. AXIS M31-series

> CS-mount – 12.5mm from camera edge to sensor

> C-mount – 17.5mm from camera edge to sensor

> SLR type – For large sensors

> Built in types – Solves a lot of problem for high resolution cameras

Lens mount

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Viewing angle

Tele f~10 mm 22º horizontal Wide f~2.8 mm 80º horizontal

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Sensor size

Vidicon, photo from Wikipedia

> Initially, sensor size was defined by the outer diameter of the television vacuum tube Vidicon.

> With image sensors made of silicon, ”inch-dimensions” do not make sense.

> Some years ago, all sensors in surveillance cameras were 1/2”, 1/3” etc. Today, ”all” image sizes exist.

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> The viewing angle depends on the focal length of the lens in combination with the active area on the sensor.

> The projected image from the lens must cover the image sensor.

Sensor size / Viewing angle

Example. Wrong lens gives dark sides/corners.

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Iris steps

> F no. = Focal Length / Entrance Pupil

> Iris steps: 1 1.4 2 2.8 4 5.6 8 11 16 22 32 ....

> One iris step changes the light gathering area with a factor of 2, causing

the amount of light to change with a factor of 2.

> The exposure algorithm adjusts the average light level in the image. If the iris is decreased one step, this needs to be compensated by doubling the shutter time or increasing the gain with a factor of 2.

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Iris – depth of field

> The depth of field changes with the iris opening

Example on how the sharpness can be perceived on a certain lens.

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Iris – diffraction

> Waves passing a small hole divert. This phenomena was documented by the Italian scientist Francesco Maria Grimaldi in 1665.

Information and images from Wikipedia.

Waves divert. Amplitude versus distance. Resulting image.

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Iris – depth of field

> Low F no.

> Short depth of field

> Optimal F no.

> Medium depth of field

> High F no.

> Large depth of field

> Diffraction problem

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> No iris

> Manual iris

> Video iris, very uncommon today

> DC iris, no absolute position

> P-iris, the iris position can be set to an exact position

Iris alternatives

Lens performance example

A camera with P-iris uses the optimal iris position as much as possible

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> The lens resolution can be seen in an MTF chart

Resolution/MTF

High MTF Low MTF

Example for a HD lens

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> An MTF test instrument can also measure geometrical distorsion

MTF measurement

Image from Trioptics.

Barrel distorsion is common with wide angle lenses

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> Lenses used with IR light must be IR compensated, i.e. they must have good resolution in both visible light and IR light at the same focus position.

> The camera must have a mechanism that can remove the IR cut filter

D/N – IR compensation

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> All lenses are compromises!

> Some design parameters considered by lens designers: – Focal length

– Aperture (F no.)

– Resolution

– Distorsion

– IR performance

– Aberations – distorsion, ghost, flare, color, relative illumination...

– Mechanical quality/tolerances

– Size

– Price

Lens design

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> It is difficult to make a good camera-lens match

> Important parameters: – Mount, e.g. CS

– Image size, make sure the lens matches the sensor size

– Focal length, to give proper viewing angle

– Iris, P-iris if possible, low F no. if used at night

– IR correction, D/N lenses are needed with D/N cameras

– Resolution, make sure to use Megapixel lenses for HD cameras

– Quality, well known brands are recommended

Lenses - conclusion

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> A paper with a Siemens star is a very good help for adjusting focus on cameras

> A Siemens star indicates focus independently of the focal length or the distance

Siemens star

Siemens star test chart sold by Imatest.

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> A dome acts as a lens element

> A dome causes a focus shift (the focus point is moved away from the camera)

> Dome cameras with motorized focus are recommended

Dome – focus shift

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> A dusty/dirty dome decreases the dynamics of the camera (the image gets hazy)

> Dome cameras with protective shields (dust/dirt, intense light) are recommended

> Very high quality PC or PMMA domes are needed for HD cameras.

Dome – dust/quality

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> A good lens is not enough for making good image quality

> Sensor

> Mechanics holding the lens/sensor

> Mechanics for temperature management

> Analog electronics

> Image processing

> Compression algorithms

Complete system needed

Integrating sphere for sensor tests. Photo from Labspere.

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Thank you! network video

innovation

glob

al

HD

TV

partn

er n

etw

ork

leader

camera

protect outdoor

image usability

leader

safe

ther

mal

easy installation

inte

llige

nt

open

integration ease of use

competence

megapixel

environment

worldwide

H.264

focus

Axis

video encoder

conv

erge

nce

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