What is a parfocal lens and how beneficial is it to photography?

Question

I heard someone say that after using a parfocal lens for the first time, they would never switch back. What is a parfocal lens, how is it different from any other lens, and what are the advantages and disadvantages for photography? Would you pay extra for this or prefer it over varifocal?

Answer

Parfocal lens is a lens which remains in focus when you change the focal length. The non-parfocal lens is called varifocal.

It is very convenient to focus at the maximum focal length and change the zoom afterwards. It is more important for manual-focus lens because a well functioning auto-focus can quickly adjust the lens to keep it in focus.

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