Does a zoom lens distance scale mean the lens is parfocal?
Asked 9/25/2017
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If a zoom lens has a focus distance scale on it, does that imply the lens is parfocal and will stay in focus as you change focal length? Or can a zoom lens have a distance scale even if focus shifts slightly while zooming?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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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.
Originally by user4262. Source · Licensed CC BY-SA 4.0
user4262
8y ago
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No. A distance scale does not mean a zoom lens is parfocal.
Many photographic zooms are varifocal, meaning focus can shift somewhat as you zoom. A parfocal lens is specifically designed to maintain focus at the same subject distance while changing focal length.
The presence of a distance scale only means the lens provides an approximate indication of focus distance. These scales are usually coarse, with limited markings, so they are useful as a guide but not highly precise. That lack of precision is separate from whether the lens is parfocal.
So the two ideas are different:
- Parfocal: focus stays consistent while zooming.
- Distance scale: shows approximate focus distance at the current focus setting.
A zoom lens can have a distance scale and still be varifocal. If you want to know whether a specific lens is parfocal, the practical test is simple: focus on a subject, zoom in or out, and see whether focus remains unchanged.
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