# Focal Length & Lowest F-Stop



## JohnYoga (Dec 27, 2011)

RE: New D5100 user

Hello Folks,

Why does my lowest F-Stop hinge on the focal length used? On the D5100 + 18-55mm lens, I get F5.6 @ 55mm & F3.5 @ 18mm. I realize that the 3.5 - 5.6 label is shown on the lens, but not sure why F3.5 can't be used at 55mm; it must be optics Physics issue?

Thanks,

John


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## dxqcanada (Dec 27, 2011)

Design of the lens.
There are zoom lenses that are designed with a fixed maximum aperture throughout the entire zoomed focal length.
It costs more to design a lens that way.


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## Josh66 (Dec 27, 2011)

Limitation of the lens.  Designing the lens that way makes it cheaper to produce.

Higher end zooms will have a fixed maximum aperture throughout the zoom range.


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## Kerbouchard (Dec 27, 2011)

You'll notice that as you 'zoom' your lens, the actual lens itself gets longer...At longer focal lengths, the lens is physically different than at shorter focal lengths.  This results in a loss of light, therefore, a difference in maximum aperture.

There are ways to compensate for this and to achieve a constant amount of light throughout the zoom range.  Those lenses will typically cost upwards of a thousand dollars.

In the budget lenses, they make compromises to keep them cheap and lightweight.  One of the big compromises happens to be a changing aperture at different focal lengths.


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## Josh66 (Dec 27, 2011)

And really, the aperture isn't changing - the focal length is.  And because the focal length is changing while the aperture is not - the f-stop changes.  It might sound complicated, but once you have a better understanding on exactly what "f/4" means, it will make sense.


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## JohnYoga (Dec 27, 2011)

Thank you, folks, for answering me so quickly!

John


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## Big Mike (Dec 28, 2011)

I think you've found your answer, but just for the sake of clarity...

The F number is actually a ratio.  When we say F5.6, it's actually 1 over 5.6, sometimes written F/5.6.
It's a ratio between the focal length of the lens and the diameter of the entrance pupil.

So imagine the entrance pupil in your lens.  As you zoom from 18mm out to 55mm, the size of the pupil doesn't change (because of the design of the lens), but obviously the focal length does change...so that changes the ratio, which changes the F number.


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## JohnYoga (Dec 28, 2011)

Excellent Folks!

Thanks for the F-stop clarification, Mike. I now understand it clearly.

John


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## Helen B (Dec 28, 2011)

Big Mike said:


> So imagine the entrance pupil in your lens. As you zoom from 18mm out to 55mm, the size of the pupil doesn't change (because of the design of the lens), but obviously the focal length does change...so that changes the ratio, which changes the F number.



The diameter of the entrance pupil does usually change, because of the changing magnification of the iris by the lens elements in front of the iris as they move to alter the focal length (the iris diameter probably stays the same). For example, 55/5.6 = 9.8 mm and 18/3.5 = 5.1 mm.

Best,
Helen


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## Big Mike (Dec 28, 2011)

Thanks for the clarification Helen.


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## KmH (Dec 28, 2011)

JohnYoga said:


> RE: New D5100 user
> 
> Hello Folks,
> 
> ...


There are zoom lenses that the maximum aperture doesn't change with the focal length. They are known as 'constant' aperture zooms (not fixed aperture). Constant aperture zoom lenses cost more to design and build, and they tend to have wider maximum apertures and better optics, which also adds to the cost.

Here is a constant zoom, wide max aperture Nikon that is about $1500 new - Nikon 17-55mm f/2.8G ED-IF AF-S DX Nikkor Zoom Lens 

Your 18-55 mm is a 'variable aperture' zoom lens.


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## dxqcanada (Dec 28, 2011)

KmH said:


> ... They are known as 'constant' aperture zooms (not fixed aperture) ...



Yeah, I knew it was the wrong word when I typed it.

I blame the cat.


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## JohnYoga (Dec 28, 2011)

Thank you, Keith and Helen!

Regards,

John


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