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Home → Technical Support → Sensor

Sensor
2017-6-15
How to select a lens
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How to select a lens:

There are approximatively 20-30 optical parameters limiting or defining the optical lenses. Some of those are more obvious and instinctive, we includes all those parameter in our product datasheet in PDF. Some others require professional knowledge to understand, if you estimate your application needs more technical support with those parameters, please don’t hesitate to ask us.

 

  1. Sensor Size & lens Image circle

Sensor size precisely its active area is a decisive factor for lens selection. In order to cover whole sensor active area without any dark corner, you need select a lens with its image circle at least equal to the sensor active area diagonal. In order to adapt some mechanical mounting tolerance, usually the lens image circle should be slightly bigger than sensor active area diagonal by 0.1-0.2mm.

For example 1” sensor with 16mm active area diagonal needs a lens with its image circle 16.1 or bigger. 1/2.3” sensor with 7.9mm diagonal needs a lens with minimum image circle 8.0mm.

Sensor format with its active area diagonal reference here for your convenience. Please refer you to your sensor datasheets to make sure it

   Pls. note for some applications particularly fisheye and multiple image stitching, the lens image circle could be smaller than sensor active area diagonal depending on your purpose.

   

      

  1. Sensor resolution and lens MTF matching

The Sensor resolution is defined by its pixel size while a lens MTF ( Modulation transfer function ) describes the lens resolution capacity over its image circle therefore sensor active area by black & white line pairs per millimeter.

For example a sensor with pixel size 1.6 micron needs a lens capable of affording minimal MTF at 312.5 line pairs/mm. ( 1000/1.6*2 = 312.5 lp/mm ).

In order to simplify the integration engineer work, quite often lens suppliers just give the lens resolution for example “3M lens” instead of MTF number for a determined sensor active area.

Please remark even the sensor resolution and lens MTF matches, it could still have big differences in the image resolution from one to another because of MTF actual value or other conditions at the matched line pair. The lens quality is strongly depends of the design, manufacture capability and cost for a given lens resolution.

 

  1. Sensor CRA matching

The sensor CRA ( Chief Ray Angle ) indicates the optimal incident angle of the rays hitting the sensor. The actual CRA is a property of the lens you select and conditions the sensor illumination way. Sensor suggested CRA should match with lens CRA within 3 to 5 degrees. The mismatching between sensor and lens could have negative impact on the image color rendering and illumination uniformity at image corners.

Nevertheless the sensor and lens CRA are not precisely matching parameter. Usually a big CRA sensor can use a short and compact lens often with mobile phone camera application while small CRA sensor requires longer and bigger lens applied in machining vision industry. There is case where CRA matching is less important.

 

  1. View angle & focal length

By definition: Field of view = 2*tan-1(h/2f)

h is sensor height

f is lens focal length

Generally speaking the horizontal view angle is more instinctive instead of diagonal one. You don’t need know the focal length of the lens once you know the sensor size and view angle.

For a very large view angle lens, the field of view vs. focal length and sensor size can will be deviated from the above formula due to the excessive large distortion at the image corners.

      

 

  1. F/# and lighting condition

It is the ratio of the optical focal length to the aperture diameter of the lens. It’s a lens parameter measuring brightness of optical system. The number usually increases by multiples of √2 which decreases the light throughput of the lens by a factor of 2.

f/# impacts more than just light throughput, it is related to lens resolution and depth of field. It is one of the most critical parameters of lenses limiting whole optical system performance.

 

  1. TTL & BFL

TTL is Total Track Length from the first lens vertex to the sensor active surface, please don’t confuse with the lens physical dimension from first to the last lens surfaces. Sometimes your camera or system dimension could limit the lens TTL selection, especially for a compact system.

BFL is Back Focal Length from the last lens vertex to sensor active surface. This space matters with how you mount your lens to the sensor and how many additional optical or opt-mechanical items inserted between lens and sensor for example blue glass filters or shutters.

 

  1. Other parameters

Lots of other optical parameters could be assessed with your specific demands. For example the optical, F-theta and TV distortions, color aberration, thermal propriety of the lens, Flare and ghost analysis etc.

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