Optics

Error-free depiction of the field of view

Resolution of the optics

In case of measuring applications, the inspection of very fine image details, but also the optical depiction on very small camera pixels, the resolution of the optics is of special interest.

There are clear qualitative differences, which are highly dependent on the glass used, the coating of the lenses and the overall optical design.

In particular, optical aberrations such as spherical and chromatic aberration, coma and astigmatism lead to blurred images that significantly reduce resolution.

Standard lens

Due to various optical errors and low MTF, the images are not very sharp. The optics are unable to depict fine structures on the small pixels of the high-resolution camera. The aberrations are particularly evident at the image corners.

High resolution lens

The high-resolution sensor captures even the smallest structures clearly. The edges of the image are also sharp. Detail of 640x480 pixels of a 5-megapixel image (camera with small 3.45µm sensor pixels)

Limiting the resolution: diffraction effects

Light with its wave characteristics is diffracted at the aperture of the lens. The image of an object point is not represented as an ideal point, but as a slightly blurred spot of light surrounded by weak concentric diffraction patterns. The more the aperture of the lens is closed, the greater the effect of the superposition of the diffraction patterns: the resolution of the image is reduced!

Stop down the lens: Limiting of resolution

The diffraction of light at the slit cannot be reduced by any corrective measures, it only depends on the width of the slit (aperture) and the wavelength of the light. Optics of ideal quality do not produce any type of imaging errors, consequently the image sharpness is only defined by the diffraction (diffraction-limited optics)

In practice, however, it becomes apparent that stopping down the lens by approximately 2 f-stops noticeably enhances the imaging quality. Optical errors like coma, astigmatism, spherical longitudinal errors, etc. are reduced by stopping down so that the quality gain of this effect is greater than the reduction of the resolution due to the effect of diffraction at the slit. From stop 8 on, the diffraction effect typically reduces the resolution. Further details are given in chapter "Limiting resolution and MTF".

Important for Machine Vision

Make sure your lenses are of high optical quality. They should be suitable for capturing fine structures on the sensor. The pixel size of the camera sensor is particularly important here. The smaller the sensor pixels are, the higher the resolution of the lens must be!
Use short rather than long wavelength light, as diffraction effects are also wavelength dependent. In the visible light range, this means that you should work with blue or green light rather than red light.

Stop the optics down by approximately 2 f-stops if there is sufficient light. Too little light can possibly be compensated by means of better illumination or using an LED flash controller. If the light is too intense and exposing the sensor too little resulted in image impairment, a neutral grey filter might help to avoid having to stop down the lens too much.

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