Why is the eyepiece a large field of view and what will happen to a small field of view?

The field of view is too small to focus and the resolution is not high.

The principle of the telescope is that the convex lens becomes an enlarged inverted image and an enlarged vertical virtual image. The most important thing is to increase the aperture of light. The larger the aperture of light, the higher the resolution of the image. 10MM eyepiece is the most commonly used, and can be used for observation except for some particularly large and dim deep space objects.

The magnification of the system obtained by 20MM eyepiece is relatively low, which can be used to see open clusters, large-field spherical clusters, nebulae and galaxies. It can also be used to find stars, that is, to find the target celestial body and then change it into a short focal length eyepiece for high-magnification observation. 4MM eyepiece can get high magnification, which is mainly used to distinguish binary stars, observe the details of the planet and the moon surface, and check the accuracy of the objective lens.

The eyepiece of the astronomical telescope doesn't seem to have such a short focal length as1.35 mm. The optical elements of the eyepiece focus the light collected by the telescope, so that a clear image of the target or area pointed by the telescope can be observed. When the eyepiece is installed in the telescope focusing tube, it can bring the extremely distant object closer to the focus and study the enlarged image.