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FAQ Objective Lenses Frequently Asked Questions (and their answers)

What is the focal length for a “standard lens” for my imager?

The focal length of a "standard" lens is usually approximated by the length of the diagonal of the imaging area at the focal point. In most cases, this gives an image size that approximates the image seen by the unaided human eye (that is, roughly the size of image formed o the camera's "detector" is similar to the size of the image formed on a human retina by the eye’s lens). Using this estimate, a 35mm camera with image size of 24mm x 36mm has a diagonal of about 43.2mm, so a 50mm focal length lens is therefore the "standard" lens for this image size. Similarly, the rule applies to the following Electrophysics products:

Product	Image Size "Standard" Lens	Focal Length
WAT902	4.8 x 6.4 mm	8 mm
ElectroViewer 7215	17mm diagonal	17 mm
MicronViewer 7290	9.5 x 12.7 mm	16 mm
PV320	11.6 x 15.5 mm	19 mm

How do I determine the magnification of my objective lens?

To determine the actual magnification of an objective lens, one needs to know the focal length that produces a 1X image - and that depends on the image size (see above). Divide the focal length of your objective lens by the “Standard” Lens Focal Length for the imager. Consequently, referring to the table above, a 25mm lens on an ElectroViewer 7215 would yield approximately 1.5X magnification, while the same objective lens on a WAT902 would yield over 3X magnification

What are standard image size formats for CCTV lenses?

Shown below are the industry standard image sensor sizes. For example, a ½” sensor has a 4.8x6.4mm sensor with an 8mm diagonal. Image Sensor Size (units in mm):

How do I determine the lens focal length I need?

To choose the proper focal length lens for a particular application depends on the sensor size, distance from the imager to the object and the field-of-view desired. The focal length can be calculated as follows:

FL = (S x WD)/ FOV

where:
FOV = Field of View (the size of the area to be imaged)
WD = Working Distance (distance from the imager’s lens to the object)
S = Detector Size (the size of the imager’s image sensor).

Be consistent. If you are measuring the width of your object, then use the horizontal detector specifications, etc. If you are working in inches, then do your calculations in inches and convert to millimeters at the end.

Example 1: You have a PV320 with a 19mm sensor diagonal. There is a 30” (76 cm) distance between the object and the front of the lens. The diagonal field of view is 11.4” (29 cm). The desired focal length will be:

FL = (19 x 76)/29 = 50mm

Example 2: You have a MicronViewer 7290 with a 1" sensor (16 mm diagonal). There is a 1.5mm distance between the object and the front of the lens. The desired field of view is 1.0x1.3mm (1.6mm). The desired lens focal length will be:

FL = (16 x 1.5)/1.6 = 15mm Magnification = FOV/S = 10X


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