aberration

aberrations are generally divided into two broad categories:chromatic aberrationand monochromatic aberrations. chromatic aberration for shortcolor difference, is due to the refractive index of the lens material is a function of wavelength, the resulting aberration. It can be divided into two kinds of position chromatic aberration and chromatic aberration of magnification. monochromatic aberrations are referred to even in highlymonochromatic lightare divided into two categories: blurring the image and deforming the image according to the effects produced. The former category hasspherical aberration, coma and astigmatism. The latter category has field bending and distortion.

1. color difference (Chromatic aberration)

chromatic aberration is a serious defect of lens imaging, which occurs when polychromatic light is the light source, and monochromatic light does not produce chromatic aberration. white light by red Orange yellow green Qing Blue purple seven kinds of composition, all kinds of light wavelength is different , the refractive index is also different when passing through the lens, so that a point on the object side may form a color spot on the image side.

chromatic aberration generally has position chromatic aberration, chromatic aberration of magnification. Position chromatic aberration makes the image observed in any position with color spots or halos, making the image blurred. And the chromatic aberration of magnification makes the image have colored edges.

2. spherical aberration(Spherical aberration)

spherical aberration is the monochromatic phase difference of a point on the axis, which is caused by the spherical surface of the lens. The result of spherical aberration is that after imaging, a point is not a bright spot, but a middle bright. bright spots with gradually blurred edges. This affects the imaging quality.

spherical aberration is often eliminated by the combination of lenses. Since the spherical aberration of convex and concave lenses is opposite, convex and concave lenses of different materials can be selected to be cemented to eliminate. In the old type of microscope, the spherical aberration of the objective lens is not completely corrected, and it should be matched with the corresponding compensation eyepiece to achieve the correction effect. The spherical aberration of the general new type microscope is completely eliminated by the objective lens.

3. Hui Cang(Coma)

coma is a monochromatic phase difference at the point outside the axis. When imaging an off-axis object point with a large aperture beam, the emitted beam passes through the lens and no longer intersects a point, then the image of a light point will get a comma strong, like a comet, so it is called“Hui Cang“.

4. astigmatism (Astigmatism)

astigmatism is also an off-axis point monochromatic phase difference that affects sharpness. When the field of view is large, the object point on the edge is far away from the optical axis, and the beam tilt is large, causing astigmatism after passing through the lens. The astigmatism causes the original object point to become two separate and perpendicular short lines after imaging, which form an oval spot after synthesis on the plane of imagination. The astigmatism is eliminated by a complex combination of lenses.

5. Field Song (Curvature of field)

field song is also called“image field bending“. When there is a field curvature in the lens, the intersection of the entire beam does not coincide with the ideal image point. Although a clear image point can be obtained at each specific point, the entire image plane is a curved surface. In this way, the entire phase cannot be seen clearly at the same time during microscopic examination, causing difficulties in observation and photography. Therefore, the objective lens of the research microscope is generally a flat field objective lens, which has corrected the field curvature;.

6. distortion (Distortion)

all the differences mentioned earlier affect the clarity of the image, except for the field curvature. Distortion is a phase difference of another nature, and the concentricity of the beam is not destroyed. Therefore, the sharpness of the image is not affected, but the image is distorted in shape compared with the original object.

(1) When the object is outside the double focal length of the lens, a reduced inverted real image is formed within the double focal length of the image and outside the focal point;

(2) When the object is located on the lens side of the two-fold focal length, an inverted real image of the same size is formed on the image side of the two-fold focal length;

(3) When the object is located within the two-fold focal length of the lens, outside the focal point, an enlarged inverted real image is formed outside the two-fold focal length of the image;

(4) When the object is at the object-side focus of the lens, the image cannot be imaged;

(5) , when the object is located within the focal point of the lens object, the image side is not formed, and a magnified upright virtual image is formed on the same side of the lens object side farther than the object.