Why does aperture go up with zoom?

Question by -Mersadies-: Why does aperture go up with zoom?
Why does the aperture go “up” (in number) when I zoom? Is there a way to keep it the same no matter what length the lens is at?

Best answer:

Answer by Sakura
The aperture is the opening that determines the amount of light a lens will let through. The amount of light let through is also dependent on the quality of the lens. It turns out that when the focal length gets larger, this opening also needs to get larger in order to let in enough light to expose the image. This is because the amount of light gathered by a telephoto lens is spread over a larger area, thus making the brightness less.

There is no way to keep it the same.

What do you think? Answer below!

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4 Responses to Why does aperture go up with zoom?

  1. MixedMojo says:

    The relative size of the aperture decreases in a zoom lens as focal length is increased. For lens manufacturers, it is harder and more expensive to keep the effective aperture proportional to focal length at long focal lengths. That is why faster, constant aperture zoom lenses like the nikkor AF-S 80-200mm f/2.8 is both more expensive and physically larger than the nikkor AF 28-200mm f/3.5-5.6D, despite the latter having a larger focal zoom range. If your lens has a variable maximum aperture, like for example, f/3.5-5.6 – there is nothing you can do. It won’t effect much beyond one and a third stop in your shutter speed.

  2. John T says:

    This is dependent on the actual physical lens you are using.

    You can buy, for example, a 70-200mm f/2.8 that has a maximum aperture of f/2.8 throughout the zoom range. This lens is quite a bit more expensive and bulky compared to the cheaper consumer zoom lenses.

    Most constant aperture zoom lenses are ‘professional’ lenses and are very expensive and quite a bit heavier/bulkier than their consumer counterparts.

    If you’re willing to spend the extra $ $ , go for it!

  3. Edwin says:

    Manufacturing a variable aperture zoom lens (f3.5/5.6) is much cheaper than manufacturing a constant aperture zoom. For most applications a variable aperture zoom will do fine but whenever possible a constant aperture zoom is a better choice.

  4. Bill P says:

    Do some math.

    Find out how you determine the largest aperture on a lens. It is a simple process. The size of the aperture is totally dependent upon the opening in the camera that accepts the lens. f stop is a ratio of the focal length of the lens and the widest opening that the lens will allow.

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