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Circle Of Confusion

Images of point size Objects would ideally be points. For physical reasons (“diffraction”), we only obtain little disks of light.
The so called circle of confusion (also called “Airy disk”) is the smallest disk of light, that an ideal lens can theoretically generate on image side.

The smallest possible diameter of a circular disk that a “perfect” (=diffration limited) lens can generate on image side is given by the Rayleigh Criterion.

SVG of Diffraction at Pixel Level

Diffraction at pixel Level

Contrast

The optical term “contrast” of an image is pretty much what we would expect from our daily use of the word.

contrast1

However, we have to distinguish global contrast …
contrast2
… from local contrast :
local contrast

The global contrast in the two images above is about the same, however the local contrast (the change from pixel to pixel) is less high in the lower image, because of the slight blurring.

The local contrast is described by the MTF-curve.

 

Contrast can be a difference in brightness but also a difference in color
Contrast heavily depends on environmental light. Switch off the light at night and all contrast is gone!
Contrast can be increased or reduced by changing the light situation.
Red writing on a white paper becomes invisible under red light. Both paper and writing look red, the contrast is gone.
Yellow Writing on a white paper is nearly invisible on the image of a monochrome camera. Illumination with blue light increases the contrast nicely.
Formulas used to measure contrast:
  • Weber contrast
    \frac {I-I_b}{I_b}
    (Feature -background) / background
  • Michelson contrast
    \frac{I_{max} - I_{min}}{I_{max} + I_{min}}
    Difference of intensities divided by sum of intensities. Often used in image processing.
  • RMS contrast (Root Mean Square Contrast)
    \sqrt{\frac{1}{M N}\sum_{i=0}^{N-1}\sum_{j=0}^{M - 1}(I_{ij} - \bar{I})^2}
    Standard deviation of the pixel intensities.

CS-Mount

Standardized interface for the mounting of lenses, described in ISO 10935 (1996-12) Optics and optical instruments – Microscopes – Interface Type CS

The diameter of the thread is 1 “(one inch) and there are 32 threads to 1” in length.

The distance between the mechanical stop of the lens and the sensor in cross bolts in air is 12.52 mm. These are about 5mm less than for C-mount lenses.

C-mount lenses can (with an 5mm extension (C-CS-mount adapter))  be used in CS-mount cameras,

However CS mount lenses can not be used in C-mount cameras.
CS-Mount lenses are mostly used in security systems, often these are wide-angle lenses.