What's the difference between a folding lens and a normal lens? Under what circumstances?

Refractive lens, also called reflective lens and reflective telephoto lens, is a special form of super telephoto lens. General photographic lens, when the light passes through the lens, it enters from the first group of lenses and directly reaches the film. Telefocal lenses are divided into ordinary telephoto lenses and ultra-telephoto lenses. The focal length of ordinary telephoto lens is close to that of standard lens, while the focal length of ultra telephoto lens is much larger than that of standard lens.

2, aperture is different:

A catadioptric lens has only one aperture, and it is a smaller aperture. Using only a small aperture will reduce the size of the lens, but there is only a small aperture, so the illumination of the framing system is low, which is not conducive to exposure control. Only exposure time or neutral grey mirror can be used to control exposure, but camera programmed exposure and shutter priority automatic exposure cannot be used, so the depth of field cannot be effectively controlled.

3, the picture effect is different:

The depth of field of catadioptric lens is small, so focus carefully. Try to hold the camera with a tripod to avoid camera shaking. Refractive lenses have a small depth of field, which can produce a hazy effect beyond the unique depth of field.

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