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Lens Focal Length

Lens Focal Length may be calculated after determining DISTANCE to SCENE (DS) from Camera and the WIDTH of SCENE (WS) required at that distance.

Focal Length of Lens = (DISTANCE to SCENE × 4.8) ÷ (WIDTH of SCENE).
All measurements in mm [equation for 1/3" Sensor, 4.8 mm wide × 3.6 mm high]

Focal Length of Lens = (DISTANCE to SCENE × 3.6) ÷ (WIDTH of SCENE).
All measurements in mm [equation for 1/4" Sensor, 3.6 mm wide × 2.7 mm high]

Focal Length of Lens = (DISTANCE to SCENE × 6.4) ÷ (WIDTH of SCENE).
All measurements in mm [equation for 1/2" Sensor, 6.4 mm wide × 4.8 mm high]

For example if DS = 1500mm, WS = 2000mm, Sensor = 1/3" equation is:
(1500 × 4.8) ÷ 2000 = 3.6.

A 3.6 mm focal length lens will provide an on-screen image of a scene 2000 mm wide at a camera distance of 1500 mm. In practice results may vary ± ~15% due to component tolerances, variations in image sensor dimensions and monitor over-scan.

 

Other transpositions of equation:

To Find:

Width of Scene = (DISTANCE to SCENE × 4.8) ÷ (Focal Length of Lens).
All measurements in mm [equation for 1/3" Sensor, 4.8 mm wide × 3.6 mm high]

Width of Scene = (DISTANCE to SCENE × 3.6) ÷ (Focal Length of Lens).
All measurements in mm [equation for 1/4" Sensor, 3.6 mm wide × 2.7 mm high]

Width of Scene = (DISTANCE to SCENE × 6.4) ÷ (Focal Length of Lens).
All measurements in mm [equation for 1/2" Sensor, 6.4 mm wide × 4.8 mm high]

 

To Find:

Distance to Scene = (Width of Scene × Focal Length of Lens) ÷ (4.8).
All measurements in mm [equation for 1/3" Sensor, 4.8 mm wide × 3.6 mm high]

Distance to Scene = (Width of Scene × Focal Length of Lens) ÷ (3.6).
All measurements in mm [equation for 1/4" Sensor, 3.6 mm wide × 2.7 mm high]

Distance to Scene = (Width of Scene × Focal Length of Lens) ÷ (6.4).
All measurements in mm [equation for 1/2" Sensor, 6.4 mm wide × 4.8 mm high]

 

Lens Selection Guide for Cameras with 1/3 Inch Image Sensor

Lens focal length: mm	Horizontal angle-of-view degrees	Scene width in mm @ 1 metre	Scene width in mm @ 3 metres	Scene width in mm @ 4 metres	Scene width in mm @ 5 metres	Scene width in mm @ 7 metres	Scene width in mm @ 9 metres	Scene width in mm @ 10 metres	Scene width in mm @ 15 metres
2.1	108	2 286	6 857	9 143	11 428	16 000	20 571	22 857	34 285
2.9	88	1 655	4 965	6 620	8 275	11 586	14 896	16 551	24 827
3.5	73	1 371	4 114	5 485	6 857	9 600	12 342	13 714	20 571
3.6	72	1 333	4 000	5 333	6 667	9 333	12 000	13 333	20 000
3.8		1 263	3 789	5 052	6 315	8 842	11 368	12 631	18 947
4.0	62	1 200	3 600	4 800	6 000	8 400	10 800	12 000	18 000
4.3	58	1 116	3 349	4 465	5 581	7 813	10 046	11 162	16 744
5.0		960	2 880	3 840	4 800	6 720	8 640	9 600	14 400
5.5		873	2 619	3 490	4 364	6 109	7 855	8 727	13 091
5.7	46	842	2 526	3 368	4 210	5 894	7 579	8 421	12 631
6.0	45	800	2 400	3 200	4 000	5 600	7 200	8 000	12 000
6.5	44	738	2 215	2 954	3 692	5 169	6 646	7 384	11 076
8.0	34	600	1 800	2 400	3 000	4 200	5 400	6 000	9 000
8.5		564	1 694	2 259	2 824	3 953	5 082	5 647	8 470
9.0		533	1 600	2 133	2 667	3 733	4 800	5 333	8 000
11.5		418	1 252	1 670	2 087	2 922	3 757	4 174	6 261
12	23	400	1 200	1 600	2 000	2 800	3 600	4 000	6 000
15		320	960	1 280	1 600	2 240	2 880	3 200	4 800
16	17	300	900	1 200	1 500	2 100	2 700	3 000	4 500
18		266	800	1 066	1 333	1 866	2 400	2 666	4 000
25	11	192	576	768	960	1 344	1 728	1 920	2 880
39		123	369	492	615	861	1 107	1 230	1 846
50	5	96	288	384	480	672	864	960	1 440
69		69	208	278	348	487	626	695	1 043

Scene width is directly proportional to Lens - Object distance, for example a 3.6 mm lens (1/3" Sensor) at 1.5 metres captures a scene width of 2 metres, at 3 metres width is 4 metres, at 6 metres width is 8 metres, etc. etc.

Image height is 75% of width.

Figures in tables are calculated values, no allowance has been made for manufacturing tolerances, variations in image sensor dimensions or Monitor 'Over-Scan'.

If a common Video Monitor or TV is used to display images, up to ~ 15% of the image may be lost in 'Over-Scan' and be unseen.

If images are displayed on a PC Monitor using either our VGA-CONVERT or a Capture Card & Computer 100% of the image will be displayed.

 

Lens Selection Guide for Cameras with 1/4 Inch Image Sensor

Lens focal length: mm	Horizontal angle-of-view degrees	Scene width in mm @ 1 metre	Scene width in mm @ 3 metres	Scene width in mm @ 4 metres	Scene width in mm @ 5 metres	Scene width in mm @ 7 metres	Scene width in mm @ 9 metres	Scene width in mm @ 10 metres	Scene width in mm @ 15 metres
2.1		1 714	5 142	6 857	8 571	12 000	15 428	17 143	25 714
2.9		1 241	3 724	4 965	6 206	8 689	11 172	12 414	18 620
3.5		1 029	3 086	4 114	5 143	7 200	9 257	10 286	15 429
3.6		1 000	3 000	4 000	5 000	7 000	9 000	10 000	15 000
3.8		947	2 842	3 789	4 737	6 632	8 527	9 474	14 211
4.0		900	2 700	3 600	4 500	6 300	8 100	9 000	13 500
4.3		837	2 512	3 349	4 186	5 860	7 535	8 372	12 558
5.0		720	2 160	2 880	3 600	5 040	6 480	7 200	10 800
5.5		654	1 963	2 618	3 272	4 581	5 890	6 545	9 817
5.7		632	1 895	2 526	3 158	4 421	5 684	6 316	9 474
6.0		600	1 800	2 400	3 000	4 200	5 400	6 000	9 000
6.5		554	1 661	2 215	2 769	3 877	4 984	5 538	8 307
8.0		450	1 350	1 800	2 250	3 150	4 050	4 500	6 750
8.5		423	1 270	1 694	2 117	2 964	3 811	4 235	6 352
9.0		400	1 200	1 600	2 000	2 800	3 600	4 000	6 000
11.5		313	939	1 252	1 565	2 191	2 817	3 130	4 695
12		300	900	1 200	1 500	2 100	2 700	3 000	4 500
16		225	675	900	1 125	1 575	2 025	2 250	3 375
18		200	600	800	1 000	1 400	1 800	2 000	3 000
25		144	432	576	720	1 008	1 296	1 440	2 160
40		90	270	360	450	630	810	900	1 350
50		72	216	288	360	504	648	720	1 080
69		52	157	209	261	365	470	522	783

 

Lens Selection Guide for Cameras with 1/2 Inch Image Sensor

Lens focal length: mm	Horizontal angle-of-view degrees	Scene width in mm @ 1 metre	Scene width in mm @ 3 metres	Scene width in mm @ 4 metres	Scene width in mm @ 5 metres	Scene width in mm @ 7 metres	Scene width in mm @ 9 metres	Scene width in mm @ 10 metres	Scene width in mm @ 15 metres
6.5		984	2 953	3 938	4 923	6 892	8 861	9 846	14 769
8.0	78	800	2 400	3 200	4 000	5 600	7 200	8 000	12 000
8.5		753	2 259	3 012	3 765	5 270	6 776	7 529	11 294
25		256	768	1 024	1 280	1 792	2 304	2 560	3 840
11.5		556	1 669	2 226	2 782	3 895	5 008	5 565	8 348
39		164	492	656	820	1 149	1 477	1 641	2 461
51	7	125	376	502	627	878	1 129	1 255	1 882
69		92	278	371	464	649	835	927	1 391

 

LENS CALCULATOR

Calculate: 2/3", 1/2", 1/3", 1/4" CCD Horizontal and Vertical angles for 2 mm ~ 400 mm focal length lenses.
Calculate: Width and Height at 60 mm to 500 metres for 2 mm ~ 400 mm focal length lenses on 2/3", 1/2", 1/3" or 1/4" CCD Cameras.

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MICROFINE:

Zero Backlash Focussing System for Board Lenses. Eliminates "Float" between thread of Lens and Lens Holder, holds lens more perpendicular to CCD (no skew due to lens lock screw) very fine precise focussing is possible and a lens locking screw is unnecessary.

 

Lens Types

BOARD LENSES: Consist of 3 to 6 (Japanese Optical Glass) Elements, they produce images with good overall focus. These are manufactured in a variety of  thread sizes, our cameras and lenses have a 12 mm x 0.5 mm pitch.

FOCAL LENGTH: We stock 2.1 mm to 25 mm. SPEED: F1.8 to F2.5. The sizes that are suitable for each camera type (fitted with a standard lens mount) are listed under 'accessories' other sizes may sometimes be used if the camera is purchased with a special lens mount such as that used for the MicroFine Focussing System.

PINHOLE LENSES: Consist of a simple single Element, focus and transmittance is compromised for small size, diameter of Lens "Hole" is ~ 2 to 3 mm.

C & CS MOUNT LENSES: The C Mount is widely used in CCTV (also used on some 16 mm film cameras), it is a 1 inch diameter 32 TPI (Threads Per Inch) mount. The Flange-Focal-Distance to the image plane of a CS Mount is ~ 12.5 mm and C Mount ~ 17.5 mm. C Mount lenses may be used on CS Mount cameras by fitting a 5 mm 'spacer ring'. A spacer ring is supplied with C/CS cameras when a CS Mount Lens is not purchased with the camera at 'with camera' price.

IRIS TYPES: Our Board, Pinhole and C Mount lenses are Fixed Iris, the light transmittance can only be reduced by fitting a filter in front of or behind the lens. A Manual Iris lens has an adjustable Iris to control light transmittance, an additional benefit of this is an increasing depth-of-field as the iris is closed. While transmittance can be varied with filters or by varying iris size, the low light sensitivity of the system will be reduced unless the filter is removed or iris opened. An Automatic Iris lens varies iris size based on the level of illumination and may be fully open in low light, user adjustable Level and Metering controls allow tailoring of characteristics to cope with most back-lit and other problem situations.

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Composite Video

The composite video output from Cameras, Modules, Switchers, Quad Processors etc., may be connected to the "Composite Video" or "Audio Visual" (AV) input socket commonly fitted to Monitors, Television Sets, Video Recorders, Computer Video Capture Cards, Transmitters, TV RF Modulator (ie UHF-MOD) etc.

By fitting a UHF-MOD (CCTV-TV/VCR Interface Module) in coax cable between the TV Antenna and Splitter, TVs and VCRs connected to the antenna system may be tuned to receive Video and/or Audio, several UHF-MODs may be used on different channels with multiple cameras.

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Automatic Electronic Shutter

Our CCD Cameras utilise an Automatic Electronic Shutter (AES) system, this varies the period (shutter speed) during which an electrical charge is accumulated by the CCD Detector Elements, period varies (depending on type) from a maximum of 20 milliseconds to a minimum of 10 microseconds 2000 Times.

The AES will cope with most normal interior lighting conditions, over exposure is likely in brightly lit areas with our Hi-Sensitivity cameras and with OUTDOOR DAYLIGHT scenes. Over Exposure may be reduced with a Manual Iris Lens, Automatic Iris Lens, Neutral Density, Polarising and some other Filters.

THE IDEAL Over-Exposure SOLUTION:
For daylight or high levels of illumination if  'Low Light' sensitivity is to be retained, an 'Automatic Iris' Lens and C/CS Mount Camera will produce the best result.

An automatic Iris Lens controls exposure by mechanically varying it's 'Iris' size in response to the light intensity at CCD sensor. Other benefits from using an Auto Iris lens are reduced vertical smear (vertical streaks from bright areas in the image) this CCD characteristic becomes more noticeable at higher shutter speeds, the 'Depth-of-Field' is also increased when the Iris closes.

Our CS Mount Japanese Lenses produce Sharper Images and pass more than 2 X more light than most common C Mount Fixed Iris Lenses, use a CS Mount Japanese Lens for 'Crisper' images, to improve Low-Light performance of Camera / Lens combination and for Lower-Noise images.

COMPROMISE Low-Cost Partial Over-Exposure SOLUTIONS:

Variable Density or Polarising Filter: A variable density filter can be constructed with two Polarised Filters (FIL-Polxxx), rotating the filters with respect to each other will vary light transmission.

In some situations (low sensitivity Colour Cameras) a single Polarised Filter may be adequate.

Manual Iris Lens: The Iris can be manually opened and closed to control exposure.

The solutions above will resolve problems due to high levels of illumination, however the combined Camera / Lens sensitivity is reduced considerably, results at night or in low light will be quite poor unless the Filter is removed or Iris is fully opened, the Infrared filter solutions below partially address this problem when using a monochrome camera.

InfraRed Pass Filter: The camera (Monochrome) may be operated in the Infrared Spectrum only, this can be achieved by fitting a Visually Opaque 670 nm or 730 nm Infrared Pass Filter (FIL-IR670) or (FIL-IR730) over the front or rear of the lens, or the Image Sensor.

InfraRed Cut Filter: The camera (Monochrome) may be operated in the Visible Spectrum only, this can be achieved by fitting an Infrared Cut Filter (FIL-IRCUT) over front or rear of lens, or the Image Sensor.

The Infrared solutions above allow the characteristics of different Lamp types to be used to advantage:

The majority of the radiation from Filament type lamps (Common Spot Lamps, Tungsten, Halogen, etc) is Infrared, therefore little of this energy will be attenuated by an IRP filter.

The majority of the radiation from Cold type light sources (Fluorescent, Mercury/Sodium Vapour, etc) is Visible, therefore little of this energy will be attenuated by an IRC filter.

Infra-Red 'Pass' and 'Cut' Filters will also attenuate transmission of the 'desired' spectrum, reducing the overall 'sensitivity' of camera.

Long term over exposure (Daylight and Artificial) will cause deterioration of CCD Image Sensor, this occurs regardless of whether the camera is powered on or off.

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Focus Infra-Red (Monochrome Cameras)

A combination of Infra-Red and Visible illumination degrades overall focus and image quality, Resolution, Sharpness, Contrast and Colour-to-Grey Conversion will be improved, with an Infra-Red 'Cut' (FIL-IRCUT) filter or Lens with an Integral IR-CUT Filter.

When using Infra-Red Illumination only, lens setting for sharpest focus will differ from best setting for Visible or mixture of Visible and Infra-Red illumination.

An Infra-Red 'Pass' filter (FIL-IR670) or (FIL-IR730) will allow adjustment of focus in Day or Artificial Light, readjustment is then unnecessary with Infra-Red Illumination only.

Infra-Red 'Pass' and 'Cut' Filters will also attenuate transmission of the 'desired' spectrum, reducing the overall 'sensitivity' of camera.

The Relative Response (Spectral Sensitivity Characteristic) of a typical Silicon CCD Image Sensor (SONY) is approximately:

0.5 @ 400 nm
1.0 @ 530 nm
0.35 @ 730 nm
0.025 @ 950 nm

This needs to be taken into consideration when choosing an Infra-Red LED, shorter wavelength types will be 'seen' by the CCD as brighter.

Relative response of a typical CCD sensor to an 830 nm LED is ~ 14%, to a 940 nm LED ~ 4%.

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Lens Selection

Angle of view or width of a scene at a given distance depends upon the focal-length of the lens and CCD Sensor size (1/4", 1/3" or 1/2"). When choosing the focal length of the lens, consideration needs to be given to the purpose of the system, general observation to detect activity/movement, or the ability to identify a person. Also dependent on the purpose is the resolution of any recording and/or display device(s).

For example:
A wide-angle lens may allow observation and detection of activity in a large area, however detail will be limited.

If the lens is chosen to give a scene width of 5 metres at a particular distance, it will be difficult or impossible to identify a person at that distance. A human head is ~ 20 cm wide, with a scene width of 5 metres it will be ~ 1/25 th of the image width. If the camera has a horizontal resolution of ~ 480 lines horizontally there will be ~ 19 picture elements (live viewing) to represent the width of a persons head.

If the image is recorded on a VHS VCR and replayed the situation will be considerably worse, VHS resolution is ~ 320 horizontal lines, this equates to ~ 13 horizontal picture elements to represent the width of a persons head. If a Super-VHS recorder is used (~ 530 horizontal lines) resolution will be similar to live viewing. When viewing video taped images quality will be poorer (than live viewing) due to tape signal-to-noise ratio and other recording limitations.

In a common shop or showroom situation a reasonable compromise may be to use a fairly narrow angle lens on a camera monitoring the entrance, together with other cameras with wider angle lenses to give an overview of other areas. A Quad or Multiplexer may be used to display or record images from all cameras, see application notes regarding limitations of Quads and advantages of Multiplexers.

click here for a High-Resolution Capture solution

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Optimum Results

MONOCHROME:
Use an Automatic Iris Lens and an Infra-Red 'Cut' Filter with 'Automatic Electronic Shutter' (AES) Off, this will ensure SHARPEST FOCUS, BEST RESOLUTION & COLOUR-GREY CONVERSION.

COLOUR:
If  used outdoors, in brightly lit areas or areas where illumination varies considerably, use an Automatic Iris Lens with AES Off.

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Smear

Smear is a CCD Sensor characteristic, strong light entering the CCD sensor will penetrate the photo sensitive layer and create lag. This unwanted image is called smear and becomes more evident as shutter speed increases, the extent that a CCD Sensor can reject strong light is called the Smear Rejection Ratio. See: GEM-II for pictorial representation of 'Smear'.

 

Dynamic Range

Dynamic range is the range between the darkest and the brightest areas of a scene that a camera can cope with and still produce a correctly exposed image with correct colour rendition. Ordinary colour cameras normally have an approximately 3:1 linear dynamic range that is limited by the saturation of Cyan Magenta Yellow and Green filter at 450 mV and a minimum signal level of 150 mV (450 mV : 150 mV = 3:1 ). See: GEM-II for pictorial representation of 'Dynamic Range'.

Higher quality cameras may achieve a wider range with a better CCD, Digital Signal Processing and a special RGB mosaic conversion algorithm, these may achieve 750 mV saturation on Red colour and a minimum signal level of 100 mV = 7:1 dynamic range (750 mV : 100 mV) 2.5 times better than ordinary cameras. For example: MINI-COL-IR.

An even more dramatic increase in Dynamic Range can be achieved by using a Double Speed CCD, Digital Signal Processing & Digital Memory. For example: GEM-II.

 

Signal to Noise Ratio (SNR)

Most ordinary cameras have a SNR of from 40 ~ 48 dB, better cameras may have an SNR as high as 60 + dB. The SNR figure quoted in camera specifications is usually as measured with the camera's AGC (Automatic Gain Control) off.

At normal to high illumination levels the AGC gain will be quite low, possibly unity (1), therefore the effects of SNR will be less obvious (unless the SNR is very low). At lower illumination levels the AGC gain will increase raising the video signal level, however the noise is also raised and the effect of noise on the image becomes more noticeable (similar to noise on a TV image in low signal conditions).

A camera with a high SNR will produce cleaner images with less noise under all conditions, SNR is an even more important consideration as Digital Video Recording and Transmission (LAN, WAN, Modem, WWW, Telephone Line) becomes more commonplace. With a high SNR compressed file sizes will be smaller and throughput higher, this is because noise which is interpreted as image changes (more data) increases file size.

 

Blemish

Blemish is latent imperfections in the photodiodes (Picture elements = Pixels) in an Image Sensor. A few of the several hundred thousand pixels in an Image Sensor may be imperfect and remain On, Off or somewhere in between producing a White, Black or fluctuating dot on the video image. In most general surveillance applications these are irrelevant and often go unnoticed as a single pixel on a 752 pixel wide image may be imperceptible depending on the image content and level of light on surrounding pixels.

Our Low Blemish cameras are fitted with Image Sensors that typically have 0 ~ 10 imperfect pixels that are usually located outside of the more important central area of the sensor.

Many inexperienced and low volume Camera Manufacturers use low grade (cheaper) Image Sensors that are not acceptable to well established high volume makers of Quality Cameras. These low grade sensors usually have numerous imperfect pixels many of which are located within the more important central area of sensor.

While most imperfect pixels are evident during inspection following manufacture of the Image Sensor Chip, the pixels in some specialised high sensitivity Image Sensors may fail during storage, after installation in a Camera and after delivery to the end user, this process will continue until the latent defects in the photodiodes stabilise.

It is normal for some pixels to fail as part of the normal aging process during the lifetime of a camera.

 

CCIR PAL EIA NTSC SECAM

CCIR = Australian / UK / European Monochrome TV System
PAL = Australian / UK / European Colour TV System
EIA = USA / Japan Monochrome TV System
NTSC = USA / Japan Color TV System
SECAM = France / Russia TV System

!! IMPORTANT note about  TV Systems !!

Although the CCIR/PAL cameras we stock are incompatible with USA / Japan TVs and VCRs they can be used with our PC Digital Video Recording Systems for HIGHER RESOLUTION IMAGES than possible with EIA/NTSC cameras.

For example:

Our GoVideo-DVR4, GoVideo4-TCP/IP, GoVideo-2 and GoVideo-4 PC based Video Recording Systems capture images @ 320 x 240 resolution (76 800 pixels) with EIA/NTSC cameras, with CCIR/PAL cameras resolution is 384 x 288 (110 592 pixels) 44% MORE DATA.

Our GoVideo-1 PC based Video Recording System captures images @ 640 x 480 resolution (307 200 pixels) with EIA/NTSC cameras, with CCIR/PAL cameras resolution is 768 x 576 (442 368 pixels) 44% MORE DATA.

 

Specifications

Are based on information and data supplied by manufacturers.

Due to continual product development and improvement specifications may change.

Additional information and data may be available for some items, if you require additional information, in your request please specify the parameters you require.

All care has been taken to ensure accuracy of specifications etc., however we accept no responsibility for errors or omissions.

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** Information plagiarised from this page may be found on some 'copy-cat' Web Sites **

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