Parfocal lens

Parfocally called zoom lenses (photography) or ocular ( microscopy, astronomy), their focus when changing the focal length and the magnification remains constant. This makes it possible, during the manual focus at the telephoto position (longest focal length, highest magnification ) to focus and only then to be determined by the change in the focal length of the desired image. For a more precise manual adjustment of focus is possible because one can judge the focus based on the enlarged in telephoto position viewfinder better.

• Photography