What exactly is the advantage of constant aperture in a zoom lens?

Started Jan 20, 2013 | Discussions thread
viking79
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Re: What exactly is the advantage of constant aperture in a zoom lens?
In reply to Simon Devlin, Jan 21, 2013

Simon Devlin wrote:

Hello,

I bought another Pentax-A 70-210 F4 zoom lens as the last one was a mushroom factory.

Anyhow besides the pretty good speed of f4@ 210mm I was wondering how to answer the question of the advantage of constant aperture throughout the zoom range, especially on an automatic exposure lens where my K-30 will quickly adjust the shutter speed or ISO to compensate. Have I answered my own question?

I suppose F4 throughout is also useful for blurring out more of the background.

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Simon
Legacy lens lover

The only reason to have constant aperture is convenience of exposure. For auto exposure, auto flash exposure, etc, a variable aperture zoom is fine.

There are two types of variable focal length lenses. Parfocal, or true zoom lens, and varifocal, which is what most lenses are today. Parfocal lenses keep their focus point at different zoom levels, so focus at 200mm is the same as at 70mm, etc, where varifocal lenses change focus point with focal length. Most autofocus lenses are varifocal, as the camera can easily correct focus. We shouldn't call most lenses today zoom lenses, as they are not, we should call them varifocal lenses, but this won't change anytime soon.

A Cinema shooter would probably want a constant aperture parfocal (true zoom) lens. You could zoom without changing focus point and exposure remains constant. For a photography camera I don't think either are important most of the time.

Eric

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