Zoom and aperture

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ak3888

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Hi All,

I am rather new to all these so this might be a dumb question to most here. :bonk:

Why would zooming in reduces the aperture?

AK
 
ak3888 said:
Hi All,

I am rather new to all these so this might be a dumb question to most here. :bonk:

Why would zooming in reduces the aperture?

AK
Click to expand...

The lower priced kit zoom lenses often have a variable aperture f3.5 - f5.6 say, usually the larger aperture (f3.5) is at the wide end of the zoom (18mm).

This is a way of keeping the size of the lens smaller, and cost down, it is inherent with the design.


Looking at my example, at the telephoto end 105mm the aperture will be f5.6 wide open. If you wanted f2.8 at 105mm you would need a larger front element than you do for f5.6.

Fixed aperture zooms f2.8 70-200 for example are large, heavy and expensive.





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Thanks Martyn.

Point taken that its a cost and design issue. But I still don't get why zooming would change the aperture for these lenses.

Do you (or anyone else) have a link to a diagram that illustrate how it works?
 
ak3888 said:
Thanks Martyn.

Point taken that its a cost and design issue. But I still don't get why zooming would change the aperture for these lenses.

Do you (or anyone else) have a link to a diagram that illustrate how it works?
Click to expand...

Aperture is a ratio of the physical size of the opening to the focal length of the lens, when the focal length changes during zooming the ratio changes.

So 50mm f4 the dia of the aperture will be 12.5mm (50/4), if we zoom that to 100mm the dia stays the same so we have 100/12.5 = 8 or f8.

The diameter of the aperture blades does not change, however the focal length has, thus altering the ratio or f number.

In practice it is not that simple, many other factors are involved, however the principal of my basic example would hold true.
 
Not sure exactly what aspect you're getting at, but either way it is the inverse square law that drives everything relating to the magnification of an image and its brightness. That is, twice the diameter, four times the area.

As Martyn points out, with f/numbers this ends up with the f/ratio between the focal length and the physical diameter of the entrance pupil.
 
Thanks Martyn! Yes, that makes more sense now. But... (yes, I am thick :bonk:)

I now understand the F number would change with the focal length, but if diameter of the aperture blades doesn't change, why would longer focal length capture less light? (I am assuming the F number also indicate how much light it lets through)

And how does the fixed aperture zoom lens work then?
 
Fixed maximum aperture lenses work by increasing the diameter of the aperture opening as you zoom in, thus maintaining the focal length/aperture ratio.
 
ak3888 said:
Thanks Martyn! Yes, that makes more sense now. But... (yes, I am thick :bonk:)

I now understand the F number would change with the focal length, but if diameter of the aperture blades doesn't change, why would longer focal length capture less light? (I am assuming the F number also indicate how much light it lets through)

And how does the fixed aperture zoom lens work then?
Click to expand...

The aperture blades sometimes do change, but the effective size of the aperture, as seen through the lens, changes as the elements move back and forth and also in relation to eachother.
 
ak3888 said:
Thanks Martyn! Yes, that makes more sense now. But... (yes, I am thick :bonk:)

I now understand the F number would change with the focal length, but if diameter of the aperture blades doesn't change, why would longer focal length capture less light? (I am assuming the F number also indicate how much light it lets through)

And how does the fixed aperture zoom lens work then?
Click to expand...

Longer focal lengths gather light from a smaller area (they have a smaller Angle of View), therefore they let in less light even if the size of the aperture does not change.

Fixed (or Constant) Aperture Zooms.
My simplified explanation :)
The f stop is not actually given by:
the focal length/the diameter of the aperture
but, as Hoppy mentioned earlier, by:
the focal length/the diameter of the entrance pupil.

The entrance pupil is the apparent size of the aperture as viewed from the front of the lens and differs from the real aperture size depending on the magnification due to the front group of elements (lenses).
So when you look at the aperture blades through the front of the lens you are actually seeing a magnified image.

In a constant aperture zoom lens the position of the front elements changes as the lens is zoomed and the magnification therefore also changes. The lens is designed so that the magnification keeps the diameter of the entrance pupil at exactly the right size so that the f stop remains constant throughout the zoom range.

This is more complicated than just zooming the lens without worrying about the relationship between focal length and entrance pupil diameter so constant aperture zoom lenses are dearer.
 
Thank you everyone!

I think I kind of get now. A bit more complicated than I imagine, thats why I was missing the plot.

Obviously a lot more to learn. Thanks again.
 
My thinking is that when you zoom in and out the lenses change position and in doing so change which element is the limiting factor - THE aperture stop (as my lecturer at uni called it). I doodled this in paint for how THE aperture stop is different at different object points, but it works just the same as if you moved the different lenses about inside the lens (as far as I understand it)
18756837.png


Hopefully that makes some sense!
 
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