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What Focal Length Lens is required to Fill Up your sensor

This page has a CALCULATOR below to help you determine the focal length of the lens you need to order from us to fill up most of your CCD (or other image sensor types). We assume a C mount lens...

Background: determine the focal length of the lens

CCDs (and other image sensors) are usually rectangular with a 4:3 aspect ratio. Lenses are usually circular . When you put a circle over a rectangle - you must "waste" either part of the rectangle or the lens. Usually you waste part of the lens - making a circle of light larger than, and concentric to, the CCD. This is a given. CCDs are measured in "sizes", such as 1/3, 1/2, 2/3, and 1 inch. These sizes are not the diagonal length of the CCD - they are sizing standards based on the "old video tube" sensors that the CCD replaced.

Focal Lengths:

As rays of light enter a camera through its lens they converge to a point of focus some distance behind the lens. Objects closer to the lens will focus at a point farther behind the lens than objects more distant from the camera. For other than the most simple of cameras, a means of obtaining a sharp focus for subjects at various distances must be provided; however, as the subject becomes relatively distant from the camera, perhaps fifty feet or more, the entering rays become parallel for all practical purposes (similar to a point at an infinite distance) and the point of sharp focus becomes stationary no matter how much farther away the subject is from the camera. That stationary distance between the lens and the point of focus is known as the focal length (FL) of that particular lens. One source defines the focal length as “The distance between the optical center of a lens and the plane where parallel light rays from an object at infinity come to a sharp focus (Bruce:38).”

Applying The Lens Formula:

At first glance, when using the above optics formula the parameter to determine the distance from the lens to the CCD seems to be missing - But this "missing" distance is not required since your standard "C-mount" adapter determines this distance. Since the aspect ratio of lenses is 4:3, the optimum Focal Length to completely fill the CCD in Horizontal or Vertical directions is different . Which direction do you optimize for? Whichever direction is important to you. If you have to measure something 2 mm wide, you optimize for the Horizontal. If your objective is to measure is 1 mm tall, you optimize for Vertical.

Calculator Inputs:

Parameter 1 Requires you to provide the working distance between lens and object to be imaged in mm.

Parameter 2 Requires you to provide the size of the object you are trying to image in mm measured either in H or V direction.

Parameter 3 Requires you to provide one of the bold parameters below determined by the size of your camera sensor and the direction you chose to optimize..

For 1/3-inch CCDs: H = 4.8 mm, V = 3.6 mm
For 1/2-inch CCDs: H = 6.4 mm, V = 4.8 mm
For 2/3-inch CCDs: H = 8.8 mm, V = 6.6 mm
For 1-inch CCDs: H = 12.7 mm, V = 9.5 mm

You will then insert these 3 Parameters in The Lens Calculator

Inch to Metric Conversion Tool

Since all entries in Calculator must be in millimeters (without commas) we provide this conversion tool! (If the calculated FL is"NaN," that means it is "not a number" in JavaScript - and the answer is too small for a typical single lens - 3 mm or less.)

-To Go from: Millimeters Inches (to) Millimeters Inches


Type the distance you wish to convert in here (decimals):

Lens Focal Length CALCULATOR

Param1. Enter the distance to the object you wish to image
Param2. Enter the size (largest of either width or height-See discussion) of the object you wish to image
Param3. Enter the size of the CCD area the image should fill, in mm (see above - Lens Size)
Result: Your lens Focal Length should be:

Microscope and lens basics


Java coding courtesy of Philip Stripling, his web site is at



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