What Is Parfocal vs Varifocal? Why Does Focus Change When You Zoom?
What does parfocal mean?
A parfocal lens is designed so that focus remains substantially maintained as you change focal length. This can be useful when zooming during video because the subject does not require constant refocusing.
The real-world performance depends on the lens, camera and focus method, so the term should not be interpreted as a guarantee of perfect focus in every situation.
What is a varifocal lens?
With a varifocal design, changing focal length changes the focus relationship, so the lens may need to refocus after zooming.
Many still-photography zoom lenses work perfectly well this way because photographers often choose the focal length first and then focus for the final composition.
Why does it matter more for video?
A visible focus shift during a zoom can be distracting in video. If you intend to perform smooth zooms while recording, lens behaviour during zooming becomes more important.
For photography, the difference may have little practical impact because each frame can be focused independently.
Can autofocus hide the difference?
Modern autofocus can refocus quickly enough that the difference may be less obvious in some workflows. However, autofocus behaviour can vary with subject movement, lighting and camera settings.
Manual-focus video workflows make the lens's optical focusing behaviour more apparent.
What should buyers check?
If video zooming is important, test the exact lens at different focal lengths and observe whether the focus position changes. Do not assume that every zoom lens is parfocal.
Also check focus breathing, autofocus behaviour, maximum aperture and stabilization if video is a major part of your work.
Final takeaway
Parfocal and varifocal describe how focus behaves as a zoom lens changes focal length. For still photography the distinction may be minor, but for video zooming it can become an important part of lens selection.
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