F-Stop


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koayst

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Dec 29, 2006
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#1
Just thinking about this question myself.

How is a lens get its f-stop or speed rated? I mean how does a lens manufacturer rated its lens to say f2.8 or slower? What does the lens manufacturer do to rate it at f2.8 or faster?
 

koayst

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Dec 29, 2006
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#3
I did. But Wiki is trying to explain what F-stop is all about.
 

ExplorerZ

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Jan 9, 2006
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hkchew03.deviantart.com
#4
its writen right at the top and you say they didn't... :nono:
Notation

The f-number f/#, often notated as N, is given by

f/# = N = f/D
where f is the focal length, and D is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or N = 16. The greater the f-number, the less light per unit area reaches the image plane of the system.

The literal interpretation of the f/N notation for f-number N is as an arithmetic expression for the effective aperture diameter (input pupil diameter), the focal length divided by the f-number: D = f / N.

The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image). Note that the common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for all types of camera lens. A focal ratio of f/16 does not always mean that the physical aperture inside the camera lens has diameter equal to one sixteenth the focal length.
 

grantyale

Senior Member
Oct 4, 2004
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#5
It's not about rating something as F/2.8, but designing a lens with a target open aperture of F/2.8 - and apparently you do need to know what F-stop is all about to answer your own question.
 

creampuff

Senior Member
Jul 11, 2006
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#6
Take a 50mm lens as a simple example. The focal length is 50mm. If the lens had a max aperture of f/2, by the formula, the aperture size at the max aperture would be 50/2 = 25mm in diameter. This diameter will progressively get smaller with smaller apertures. It will also vary according to the focal length of the lens.
 

koayst

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Dec 29, 2006
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#7
Hmmm .... if everything is as easy as you guy described, then:

1. Why not make all lens as fast as possible ?
 

ExplorerZ

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Jan 9, 2006
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hkchew03.deviantart.com
#8
Hmmm .... if everything is as easy as you guy described, then:

1. Why not make all lens as fast as possible ?
$$$, lens element have to be bigger = more $, the design is also made harder = $$...
and also the size, imagine a 200mm f1 lens, the front element will be at least 20cm huge (without the barrel)...
even if they make it, i doubt you will pay it...
 

drakon09

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Aug 12, 2005
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#10
And if you can afford a few strong men to carry all that glass for you.
 

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