Question about variable zoom lenses and aperture

[stevet1]

As I understand it, with a variable zoom lens, as you zoom in, your available aperture gets smaller..

As you zoom in, the amount of light hitting your sensor is reduced. If your aperture gets smaller, that reduces the amount of light hitting your sensor gets reduced even more.

That seems to be counter intuitive to me.

What am I missing, or don't know?

Steve Thomas

[SignifDigits]

Now I'm curious about this too. 

There are (I own two) fixed aperture zoom lenses, and I'm wondering how they achieve that, as well.  I'm guessing that the (typically less expensive and lighter) zoom lenses really have a fixed aperture and the smaller field of view delivers less illuminance/lux/foot candles to the sensor, and that the f-numbers are really just the loss of illuminance (effectively smaller aperture) by that design. If so, then a fixed aperture lens would have to somehow have an expanding physical aperture relative to the narrowing field of view as you zoom in.  That would explain the heavier and more expensive nature of them.  I'm sure someone here will know and tell us.

[TomRamsey]

Think about if you are standing in a tunnel, if you are close to the end you are hit by a lot of light, go deeper in the tunnel and there is less.  In a lens it is about getting light to the sensor and it should fill the entire sensor.  That gets more difficult as the lens get physically longer, which is why fixed aperture zooms are bigger around.  This is the very simple version, the lens elements are also involved in this.  

[normadel]

What do you mean by "As you zoom in, the amount of light hitting your sensor is reduced"? This is what happens when the maximum aperture is reduced  at long focal length. This is what "f/4.5-5.6" in a lens name means. This is what a variable aperture lens means. In order for a lens to keep its maximum aperture (f/number) throughout its zoom range, it becomes a heavier, more expensive lens to design and build.

So YES, the amount of light hitting the sensor at max aperture  gets reduced by zooming in....because the maximum aperture gets smaller. Your autoexposure system takes care of this, by slowing the shutter, increasing ISO, etc.

[stevet1]

What do you mean by "As you zoom in, the amount of light hitting your sensor is reduced"?

As you zoom in, your field of view gets smaller. Ergo, less light is coming in.

It's got nothing to do with your aperture per se.

Steve Thomas