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# Thread: Aperture Value and Zoom Lenses

1. ## Aperture Value and Zoom Lenses

According to the Canon website, and just about every other source on the web, the f/ #of a lens is calculated as [focal length] / [physical aperture diameter]. This fraction is simplified to 1/[f/ #]. This is why most lens barrels have the f/# written as 1:__.

So, if the maximum physical aperture remains constant, as the focal length changes as a lens zooms, the aperture value (f/ #) must change. However, many high end "constant aperture" zoom lenses are available. If the f/ # does not change as the lens zooms, the lenses physical aperture must increase. Right?

Ex.- Canon 70-200mm f/2.8 L IS.

At 70mm- 70/a = 1/2.8, a = 196

At 200mm- 200/a = 1/2.8, a = 560

2. ## Re: Aperture Value and Zoom Lenses

Originally Posted by Flammel
If the f/ # does not change as the lens zooms, the lenses physical aperture must increase. Right?
Yes - kinda. "Apparent" aperture anyway.

3. ## Re: Aperture Value and Zoom Lenses

That is one of the factors that makes lenses with constant apertures expensive. It is also the reason that makes them so valuable to the shooter. As the focal length increases, a photographer needs a higher speed to attain sharp images (1/focal length for full frame cameras or 1/focal length x crop factor for crop cameras). If a lens stops down as the focal length increases, the shutter speed may not be sufficient to attain a sharp image. If the aperture remains the same as the focal length is increased than the shutter speed can also remain constant.

4. ## Re: Aperture Value and Zoom Lenses

Only if the shutter speed meets your formula requirements.

Lens 70-200 f2.8 assuming no VR or IS, 70mm f2.8 @ 1/120 is great, but 200mm f2.8 @1/120 in not so great.

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