What does the "f" in f/stop mean?


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mich_2103

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#1
Hello fellow CSnappers,
I am looking for the answer to this question - what does the "f" in f/stop mean?

I have asked myself the same question before but I never bother to go and find out the answer (my procrastination).

Recently, my friend asked me the same question again and asked for my help. Apparently he refered to many photography books, searched many photography-related websites but failed to give him a definite answer.

The closest answer I found was foot-candles but I decide to post this thread to confirm if my answer is correct.

What do you guys think? :dunno:

Rgds,
-Mich- :p
 

raid

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#2
mich_2103 said:
Hello fellow CSnappers,
I am looking for the answer to this question - what does the "f" in f/stop mean?

I have asked myself the same question before but I never bother to go and find out the answer (my procrastination).

Recently, my friend asked me the same question again and asked for my help. Apparently he refered to many photography books, searched many photography-related websites but failed to give him a definite answer.

The closest answer I found was foot-candles but I decide to post this thread to confirm if my answer is correct.

What do you guys think? :dunno:

Rgds,
-Mich- :p

f-stop or f-number or focal ratio is a term used to express the diameter of the aperture in terms of the focal length of the lens...

f-stop = focal length / diameter of aperture

someone correct me if I'm wrong hor...
 

Darren

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#3
f - Focal length

Example - if a lens has a focal length of 28mm and it has f/2.8, f/4, f/5.6 as its aperture, the actual diameter of the aperture opening is

28/2.8 - 10mm
28/4 - 7mm
28/5.6 - 5mm
etc.

For a more detailed explanation - Aperture definition in DPReview Glossary
 

Stoned

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May 7, 2004
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#5
Darren said:
f - Focal length

Example - if a lens has a focal length of 28mm and it has f/2.8, f/4, f/5.6 as its aperture, the actual diameter of the aperture opening is

28/2.8 - 10mm
28/4 - 7mm
28/5.6 - 5mm
etc.

For a more detailed explanation - Aperture definition in DPReview Glossary
Wouldn't that mean that the 200/1.8 has an aperture opening of 10cm? Isn't this larger than the mount?
 

Michael

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#6
What is F-Stop, anyway? F-stop is the focal length divided by the diameter of the lens. For example, a 200mm f/4 lens will be 50mm wide. Get out your ruler and measure it. 200mm/50mm = f/4. That is why f-stop is typically written as F/4, meaning "focal-length over 4" or "focal-length divided by four".

source: http://www.paragon-press.com/lens/lenchart.htm
 

raid

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#7
Also, the f-stops u see on the lens or camera corresponds to a geometric sequence governed by the powers of sqrt(2)...

so...

sqrt(2)^0 = 1.000
sqrt(2)^1 = 1.414
sqrt(2)^2 = 2.000
sqrt(2)^3 = 2.828
sqrt(2)^4 = 4.000
sqrt(2)^5 = 5.656
sqrt(2)^6 = 8.000
sqrt(2)^7 = 11.310
sqrt(2)^8 = 16.000
sqrt(2)^9 = 22.627
sqrt(2)^10 = 32.000


of course for practical purposes... this becomes...
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32

for every stop increased, the amount of light permitted through the lens reduces by half.

so f/2 allows twice as much light to enter as compared to f/2.8

Hope this helps... :)
 

raid

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Aug 25, 2003
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#8
Stoned said:
Wouldn't that mean that the 200/1.8 has an aperture opening of 10cm? Isn't this larger than the mount?
I think it's valid.. the aperture need not be located at the mount area mah... i think lah.... i never had the chance to handle those super big big big big lenses before.. no $$ and no rich friends...but those super fast tele lenses are not exactly small lei..
 

zaren

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Oct 27, 2003
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#9
Stoned said:
Wouldn't that mean that the 200/1.8 has an aperture opening of 10cm? Isn't this larger than the mount?
the lens diameter is 13cm, which is larger than the mount.
 

nikkie

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Jan 7, 2005
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#12
zaren said:
the lens diameter is 13cm, which is larger than the mount.
:nono:
but the effective 200mm is not achieved thro a single glass element.
it is made up of a complex combination of more than ten elements usually (depending on lens design), and
as a result you don't need an aperture larger than the mount.
 

Jan 23, 2005
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#13
Stoned said:
Wouldn't that mean that the 200/1.8 has an aperture opening of 10cm? Isn't this larger than the mount?
The front lens will have to be more man 10cm in diameter.

An even more interesting question is why e.g. a wideangle lens can be much further away from the film than its focal length, and why some tele lenses are shorter than their focal length ;)
 

Apr 12, 2005
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#14
Focal length in lens is much more complex than the distance from the center of the lens to the focal point is the focal length because it's not a just a single element equi-convex thin lens.


Lens focusing systems : http://www.photozone.de/3Technology/lenstec7.htm

Measuring focal length in camera lenses : http://bobatkins.photo.net/photography/technical/measuring_focal_length.html

In any case, the F/ stops are, as explained by others here, the size of the aperture diameter relative to the focal length used although it may not be an absolute measurement. If you decrease the F/ stops by the square root of 2 (i.e. 1.4142) (e.g. from F/4 to F/2.8), the relative diameter of the aperture is increased by 1.4142x and therefore the relative area of the aperture is increased by 2 because area of a circle (approximate the aperture area) is
p x R x R

When R is inceased by square root of 2, the area is increased by 2......i.e. doubled the amount of light (exposure) and it's called an increase of 1 stop in photography.
 

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