What Is a Variable Aperture Lens? Why Does the f-Number Change While Zooming?
What is a variable aperture lens?
A variable aperture zoom lens has a maximum aperture that changes across its focal-length range. A common example is a lens marked something like f/3.5-5.6.
The first number represents the maximum aperture at one end of the zoom range and the second represents the maximum at the other end.
Why does the maximum aperture change?
The optical design and physical aperture opening determine how much light the lens can transmit relative to its focal length. In a variable-aperture zoom, the maximum available f-number becomes smaller in physical light-gathering terms as you move through the range.
The camera communicates the changing maximum aperture so exposure calculations and settings can be adjusted correctly.
Does the aperture suddenly close while zooming?
The important point is the maximum available aperture. If you select a smaller aperture than the lens's maximum, the camera can maintain that selected aperture across more of the zoom range where the lens supports it.
The exact behaviour depends on the camera and lens, but the marking on the lens tells you the marking on the lens tells you the maximum aperture limits.
What does this mean for low light?
A variable aperture zoom may become less useful for low-light work at the longer end because the maximum aperture is smaller. The camera may need a slower shutter speed or higher ISO if you want to maintain exposure.
This does not make the lens bad. Kit zooms often balance cost, size, focal range and portability.
What about video?
A changing maximum aperture can matter for video if you zoom during a shot and want consistent exposure. A lens with a constant maximum aperture can be easier to manage in that situation.
For normal photography where you change focal length between shots, the difference may be much less important.
Final takeaway
Variable aperture is a normal design choice for many zoom lenses. Check the aperture range and decide whether the longer-end maximum aperture fits your shooting needs rather than treating a variable aperture as a defect.
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