Why aperture changes when the lens is zoomed in/out?

[r.k.Logeshwaran]

Why aperture changes automatically in some beginner photographic lens when it is zoomed in/out?

Example: Canon EF-S18-135mm f/3.5-5.6

Why is it designed in such a way?

What is the use of this design?

Topic

[Stip]

Why is it designed in such a way?

Costs.

Both manufacturing and retail price.

[dmullenasc]

From the internet: “The F-stop is also known as the f-number or f-ratio. In photography, the f-number of a camera lens is the ratio of the system’s focal length or capacity to bend light to the diameter of the entrance pupil of the lens. It is also known as the focal ratio.”

In other words, the f-stop is not simply the size of the aperture, it’s a ratio between the focal length and the diameter of the entrance pupil (which is not necessarily the aperture.)  So as the focal length changes in a zoom, the ratio will vary — it takes some engineering to design a zoom lens to compensate for this to achieve a consistent f-stop throughout a zoom (and some big zooms still get darker at the most telephoto end even if in general they keep the f-stop consistent.). So you pay more for a constant f-stop zoom, and often the zoom lens might be bigger as well.

[Author]

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