Optics

Error-free depiction of the field of view

Lens aperture

The aperture of the lens is the opening in the lens through which light can fall onto the sensor. It is therefore directly responsible for the brightness of the image. The size of the aperture is achieved by a circular arrangement of overlapping blades, similar in function to the pupil of the human eye.

This is achieved by radially trimming the optical path from the edge of the lens.

Changing the aperture has an influence on the

A higher f-number results in a longer exposure time, but a greater depth of field; a wide open aperture results in shorter sensor exposure times, but a smaller depth of field.

Aperture, light incidence and depth of field

The diaphragm blades reduce the aperture diameter of the lens by a factor of 1.4 (square root of 2), which is equivalent to halving the aperture area.

1,4 * 1,4 =2

2 * 1,4 =2,8

2,8 * 1,4 =4 etc.

The result is the International F-stop range:

f/1--f/1,4--f/2--f/2,8--f/4--f/5.6--f/8--f/1--f/16-f/22...

The absolute diameter of the aperture depends on the focal length of the lens. A 12.5 mm aperture for a 50 mm lens has the same effect as a 25 mm aperture for a 100 mm lens. If you divide the diameter of the aperture by the focal length, you get 1/4 in both cases in our example, regardless of the focal length.

The aperture is therefore often given as a fraction of the focal length, rather than as an absolute value in millimetres; this is known as the f-number. 1/4 is also written as f/4, F4 or 1:4 in lens specifications.

F-stop number vs image intensity

The international aperture scale is arranged so that each step halves or doubles the exposure time.

Effective f-number

The numerical values of the f-number k specified on the lens are normally calculated for an image taken from infinity. However, when images are taken from close up, which is more common in MV applications, the image appears darker. This effect is particularly noticeable in macro images.

k_effective=k* (1 + β)

Reproduction scale (magnification)

The magnification β is calculated as follows

β= Image / Object

An image of 144mm ('postcard size' as object field) on a 1/3' camera chip with 4.8mm gives β= 1/30, which is therefore of little importance as a factor in the above formula. With very small object fields on relatively large sensors, the effective f-number can change more. The image will appear much darker.

Important for Machine Vision

By cutting off marginal rays (artificial vignetting of the optics), certain imaging errors like chromatic longitudinal errors, spherical aberration, coma and astigmatism are reduced when stopping down. In this way the quality of the optic image is enhanced, as focus differences and other blurred imaging forms are avoided.

Optimum imaging is usually achieved with an aperture of 5.6 to 8 on entocentric lenses. Closing the aperture further increases the effect of light diffraction at the mechanical slit of the iris, which can lead to a reduction in image sharpness.

Stopping down the lens: reduction of spherical aberration

Tip:

An online wizard for the calculation of the effective f number can be found in the "Service" section of this website.

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