I was thinking that something I learned long ago was that the definition of a zoom lens included the characteristic of holding its focus as you changed the focal length, that a lens that did not have this trait was a “varifocal lens” as opposed to a true zoom lens.
A quick look at the
Wikipedia reveals that I was correct, but that most consumer-grade “zoom lenses” are really varifocal lenses, which means that you are also correct.
A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes. A lens that loses focus during zooming is more properly called a varifocal lens. Despite being marketed as zoom lenses, virtually all consumer lenses with variable focal lengths use varifocal design.·
·
·
Whereas lenses used in cinematography and video applications are required to maintain focus while the focal length is changed, there is no such requirement for still photography and for zoom lenses used as projection lenses. Since it is harder to construct a lens that does not change focus with the same image quality as one that does, the latter applications often use lenses that require refocusing once the focal length has changed (and thus strictly speaking are varifocal lenses, not zoom lenses). As most modern still cameras are autofocusing, this is not a problem.
That last sentence caught me, given my frequent use of an ancient late-1960s or early 1970s Vivitar 85-205mm ƒ/3.8 zoom lens, which I've always assumed met the definition of a true zoom lens in that it didn't need refocusing at different focal lengths. In fact, I'm pretty sure that it did not, when I shot
this sequence to demonstrate its characteristics. No autofocus there to help me. I am quite sure that I never refocused during the entire course of that sequence, which includes these two pictures taken at 85mm and 205mm with the aperture wide open at ƒ/3.8.
View attachment 258317View attachment 258315
