Due to the objective conditions, any optical system can not generate ideal image in theory, and the existence of various phase differences affects the imaging quality. The following is a brief introduction of the differences.
1, chromatic aberration
Chromatic aberration is a serious defect of lens imaging. When polychromatic light is the light source, monochromatic light does not produce chromatic aberration. White light is composed of seven kinds of red, orange, yellow, green, blue, blue and violet. The wavelengths of various kinds of light are different, so the refractive index is also different when passing through the lens. In this way, a spot on the object side may form a color spot on the image side.
Color difference generally has position color difference and magnification color difference. The position chromatic aberration makes the image observe at any position, with color spots or halos, making the image blurred. And the magnification color difference makes the image have color edge.
2. Spherical aberration
Spherical aberration is the monochromatic aberration of the point on the axis, which is caused by the spherical surface of the lens. The result of spherical aberration is that after a point is imaged, it is not a bright spot, but a bright spot in the middle and the edge is gradually blurred. Thus, the imaging quality is affected.
The correction of spherical aberration often uses lens combination to eliminate. Because the spherical aberration of convex and concave lenses is opposite, different materials of convex and concave lenses can be selected to glue together to eliminate. In the old model of microscope, the spherical aberration of the objective lens is not completely corrected, so it should be matched with the corresponding compensating eyepiece to achieve the correction effect. Generally, the spherical aberration of the new microscope is completely eliminated by the objective lens.
1. Wisdom difference
Hui difference belongs to the monochromatic difference of the point outside the axis. When an off-axis object point is imaged by a large aperture beam, the emitted beam will pass through the lens and no longer intersect at a point, then the image of a light point will get a ticking point shape, like a comet, so it is called "coma".
Astigmatism is also the monochromatic difference of off-axis points that affects the definition. When the field of view is very large, the object point on the edge is far away from the optical axis, and the light beam has a large inclination, which causes astigmatism after passing through the lens. Astigmatism makes the original object points become two separate and mutually perpendicular short lines after imaging, and after integration on the ideal image plane, an elliptical spot is formed. Astigmatism is eliminated by a complex combination of lenses.
3, field music
Field curve is also called "image field bending". When there is a field curvature in the lens, the intersection of the whole beam does not coincide with the ideal image point. Although clear image points can be obtained at each specific point, the whole image plane is a curved surface. In this way, the whole phase plane can not be seen at the same time during the microscopy, which makes it difficult to observe and take photos. Therefore, the objective of research microscope is generally flat field objective, which has corrected the field curvature.
In addition to the field curve, all the above mentioned differences affect the clarity of the image. Distortion is a difference of another property, and the concentricity of the beam is not destroyed. Therefore, it does not affect the clarity of the image, but causes distortion in the shape of the image compared with the original object.
（1） When the object is located outside the two times focal length of the object side of the lens, a reduced inverted real image is formed within the two times focal length of the image side and outside the focal point;
(2) When the object is on the double focal length of the lens, the same size inverted real image is formed on the double focal length of the image;
(3) When the object is within the double focal length of the lens object and out of the focal point, an enlarged inverted real image is formed outside the double focal length of the image;
(4) When the object is on the focal point of the lens, the image cannot be imaged;
(5) When the object is located within the focus of the lens object, there is no image formation on the image side, and an enlarged vertical virtual image is formed on the same side of the lens object which is far away from the object.
The imaging principle of the microscope is to use the above-mentioned (3) and (5) laws to enlarge the object. When the object is between f-2f in front of the objective lens (F is the object focal length), an enlarged inverted real image is formed outside the double focal length of the objective lens image. In the design of microscope, the secondary image (middle image) magnified by the objective lens is magnified again by the eyepiece, and the magnified vertical virtual image (relative to the middle image) is formed at the object side (the same side of the middle image) of the eyepiece and the open sight distance (250mm) of the human eye. Therefore, when we are looking at the mirror, we can see the image through the eyepiece (without additional conversion prism) and the image of the original object in the opposite direction.