4

If a zoom lens has a focal distance scale on it, does that mean that the lens is parfocal? Or does it just mean that the distance scale is inaccurate?

2

If a zoom lens has a focal distance scale on it, does that mean that the lens is parfocal?

No, many zoom lenses are varifocal. As it's used in photography, the term parfocal means that the lens should remain focused at the same distance as the focal length changes. Every zoom lens that I can think of has a distance scale, but most change focus slightly as the lens is zoomed.

Or does it just mean that the distance scale is inaccurate?

I wouldn't say that they're inaccurate; it's more that they're just not that precise. In other words, they don't tell you very precisely where the lens is focused even if you haven't changed the zoom setting. The distance scales usually have relatively few markings; for example, the scale on a Canon EF 70-200 f/4 L ranges from 1.2m to infinity over a distance of a little more than an inch and with only about 5 intermediate markings.

-2

I have never heard the term “parfocal” applied to a zoom lens, however I understand your question. Parfocal in the jargon of microscopes and telescopes pertains to an interchangeable objective and/or eyepiece. Microscopes often sport a turret that holds an array of objective lenses of various powers. One switches objectives by rotating the turret causing a different objective lens to become optically aligned. The same arrangement is available for telescope eyepieces. Various eyepieces are organized on a rotating turret. Parfocal objectives and eyepieces are adjusted as to their barrel lengths. Such an arrangement maintains focus as the adjustments are made.

If a zoom lens has a distance scale, it likely is accurate throughout the zoom range. It will be easy for you to check. Identify several objects of known distance, now zoom; note the scale readings and the degree of sharpness observed.

  • 1
    Searching for parfocal here in Photo.SE generates enough results to give the impression that the term is used pretty widely. – Caleb Sep 25 '17 at 20:17
  • The word parfocal just means that a lens (or microscope) doesn't change focus when the zoom setting (focal length) changes. In the case of microscopes, the zoom changes by swapping objective lens elements, whereas for a zoom lens, it happens by changing the distance between elements, but the same basic concept applies equally in both cases. – dgatwood Sep 25 '17 at 21:20
  • Plenty of high dollar Cine zoom lenses are parfocal. That's one reason, along with no focus breathing that they often cost $15K or more. – Michael C Sep 26 '17 at 0:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.