Optical lens basics (III)
The main parameters and evaluation of optical lens.
They are focal length, field of view, object distance, aperture, shutter, etc.
The most complete evaluation of the lens is MTF (Modulation Transfer Function). But due to aberrations (for calibration reasons), each range of the lens has an MTF value. These ranges refer to:
1) the paraxial part,
2) Off-axis part,
3) When there is asymmetric distortion in the optical system, the above two parts are sub-parts in different directions. Each part has its own MTF value for different wavelength ranges of radiant energy. MTF is the most commonly used and optimal indicator for evaluating imaging systems. And it is also the best indicator to guide the integration of machine vision systems.
Aperture common sense
What is Aperture
The lens of the camera has a device that controls the amount of light transmission, called the aperture. The larger the aperture, the greater the amount of light transmitted. The smaller the aperture, the less the amount of light transmitted.
But the aperture alone cannot fully describe the intensity of the light acting on the film. The distance between the lens and the film is also related. So it is related to the focal length of the lens. The smaller the aperture is, the closer the film is, the stronger the effect of light.
How to calculate apeture
Aperture is the value obtained by dividing the focal length of the lens by the diameter of the aperture, expressed as f.
For example, there are A, B, and C lenses. And the focal length of lens A is 50 mm. And the maximum aperture diameter is 25 mm. The aperture factor is 50/25 = 2. So we say that it is a f2 lens. The focal length of lens B is 35 mm. And the maximum aperture diameter is 17 .5mm. So the aperture factor is 35 / 17.5 = 2. And we also say that it is a f2 lens. The focal length of the C lens is 100mm. And the maximum aperture diameter is 25mm. Then the aperture factor is 100/25 = 4. We say it is f4 lens.
The aperture of the B lens is smaller than that of the A lens. But the aperture factor is the same, so the intensity of the light transmitted to the film is the same. The aperture of the C lens is the same, but the aperture coefficient is different, so the intensity of the light transmitted to the film is different.
The apertures of all camera lenses examples f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, etc.
The aperture has two functions:
- Control the amount of light penetration: the intensity of light passing through the lens to the film is inversely proportional to the square of the aperture coefficient, that is, the intensity of the light acting on the film by the two adjacent apertures has a relationship of twice. f2 is twice that of f2.8, and f2.8 is twice that of f4.
- Adjusting the depth of field: The so-called depth of field is the clear range that can be imaged before and after the lens is in focus. Its lens focal length, aperture, and the distance of the subject of the subject are related. The shorter the focal length of the lens, the smaller the aperture, and the distance of the subject. The larger the depth of field, the larger the clear range.
So as far as the aperture is concerned, the small aperture has a large depth of field, which clearly and distantly shows the sharpness of the far and near; the large aperture has a small depth of field, which can make the subject stand out and express the blurring sense of the subject before and after the subject. It is worth mentioning that if the front and back scenes are clear, a small aperture should be used, but it should be small enough to cover the desired depth of field. It does not have to be too small, too small will be affected by diffraction, but will reduce its resolution!