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Choosing an Appropriate Camera Lens - Overview

Choosing an Appropriate Camera Lens - Overview

The two defining parameters for a ine scan camera application are the width of the object and the required resolution. For an object of 80 mm width that must be inspected at a resolution of 20 μm then the camera sensor requires at least 4000 pixels (80/0.020 = 4000). Subsequent criteria would include measurement rate, interface type and spectral sensitivity, which can lead to the selection of one or more suitable line scan camera types. The final selection task is determined by the choice of appropriate lens. Each lens has specific properties and the major determinants are maximal sensor size and magnification range.The maximal sensor size traditionally determines how it is used and thereby the lens category:

CCTV or C-Mount lenses

for shorter sensors (< 22 mm)

These were first used with surveillance (CCTV) cameras. Instead of the actual sensor size, manufacturers still use the old tube diameter specifications. For example, a 2/3’’ lens corresponds to a maximal sensor size of 11 mm, a 1’’ lens to a sensor of 16 mm.CCTV lenses have a C-Mount (threaded connector) and an internal focusing mechanism. Most are designed for distant objects and small magnifications. Macro versions for shorter distances are also available.

Please refer to the link to find specific information on our CCTV lenses.

Photo lenses

for the traditional film format of 24 mm x 35 mm

Since vignetting at the corners was tolerated in many photo lens designs, most photo lenses should not be used for sensor sizes beyond 36 mm. Photo lenses also have an internal focusing mechanism and are also designed for distant objects, i.e. small magnifications.
Please refer to the link to find specific information on our photo lenses.

Scan lenses and macro lenses

These are designed for use with longer sensors and larger magnifications. Unlike CCTV and photo lenses, scan lenses do not have an internal focusing mechanism. Depending on the magnification, they often require a large distance between the sensor and the lens, which is achieved with extension tubes and a focus adaptor for focusing.Within these categories of use, other parameter choices include the focal length of the lens, which determines the working distance and the required space, or the f-number, which determines the signal amplitude, the diffraction limit of resolution and, together with the magnification, the depth of focus. The following collection of optical formulae is intended to help with the design of the imaging system and to provide preliminary information about the performance to be expected.

Please refer to the links to find specific information on our scan lenses or macro lenses.

Imaging Equations and Imaging Parameters

Imaging Parameters

Schematic depiction of the imaging system and definition of variables used.

f = Lens focal length (mm)
S = Sensor length (mm)
L = Length of Region of Interest (ROI) of object (mm)
a = Object range (mm)
a’ = Image distance: Distance from sensor to HH’ (mm)
β Magnification
w = Field angle
OO’ = Distance from sensor to measured zone (mm)
s’A = Flange focal length (mm)
∆s’  = Lens extension (mm)
LT = Tube length 
A = Working distance (mm)
HH’ = Principal point distance (mm) (can lengthen or shorten OO’)
s’K = Camera flange length consisting of focus adapter series FA22 and extension rings series ZR (mm)
LO = Lens length (mm)

Imaging Equation

{!{!{\frac{1}{a}+\frac{1}{a'}=\frac{1}{f}}!}!}

More information on the imaging equation can be found here.

Magnification

{!{!{\beta=\frac{sensor\ length}{ROI\ length}=\frac{S}{L}=\frac{a'}{a}}!}!}
More information on the magnification can be found here.

Lens extension

{!{!{∆s'=f\cdot \beta}!}!}
More information on the lens extension can be found here.

Tube Length

{!{!{LT=s'A+∆s'-s'K}!}!}
More information on the tube length can be found here.

Sensor to object distance

{!{!{OO'=(\beta+\frac{1}{\beta}+2)\cdot f+HH'}!}!}
More information on the sensor to object distance can be found here.

Focal length

{!{!{f=\frac{OO'}{\beta+\frac{1}{\beta}+2}}!}!}
More information on the focal length can be found here.

Field angle

{!{!{w=arctan\left(\frac{S}{2\cdot f\cdot (1+\beta)}\right)}!}!}
More information on the field angle can be found here.

Depth of focus

{!{!{2z=2\cdot ∆y'\cdot K\cdot \frac{1}{\beta}(1+\frac{1}{\beta})}!}!}
More information on the depth of focus can be found here.

Effective F-number

{!{!{K'=K\cdot (1+\beta)}!}!}
More information on the effective F-number can be found here.

Relative signal amplitude

{!{!{R=\left(\frac{{K'}_2}{{K'}_1}\right)^2}!}!}
More information on the relative signal amplitude can be found here.

Edge intensity

{!{!{{\rm Intensity}_{edge} =100\cdot \cos^4w }!}!}
More information on the edge intensity can be found here.

Diffraction Limit

{!{!{∆y'≥2.4\cdot λ\cdot K'}!}!}
More information on the diffraction limit can be found here.