f number details

sklnner

TPF Noob!
Joined
Feb 19, 2012
Messages
2
Reaction score
0
hi all,
im studying about photography on the internet and i have some theoretic questions while i know how it works on application

F-number - Wikipedia, the free encyclopedia
article:
the f-number is the focal length divided by the "effective" aperture diameter......
.....The f-number is given by "f/D" where f is the focal length, and D is the diameter of the entrance pupil
according to the previous quote, i understand (im not british though so my english are ><) that f-number=f/D
and that when we see "f/8", that means that the f-number is "f/8", that is, not just "f" or just "8"

an img at the top right reads as following: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8
and the note below reads the following:
Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one.
questions:

1) but on the image, "f/2" is a number higher than "f/4" for example since the denominator is lower, so how are these "increasing f-numbers" since the second number is lower?

2) if the f-number is the focal length divided by the "effective" aperture diameter (i know it's not (always) the hole of the pupil and not even the hole of the blades but the aperture "bottleneck" point of the final idol - lets not get into that), how come the "f/1.4" represents the higher aperture? the aperture ('s diameter and thus the aperture) is the denominator right (remember? "f/D") ? so the denominators are 1.4, 2 ..., 8 and 8 is the higher one, not 1.4
as i said im not british and im kind of sleepy, but i can't go to sleep until i found out what's going on

my next question is pretty predictable
3) am i misunderstanding something, or is the f-number actually not an officially defined term and is actually the number the photographers use when they are referring to their cameras' setting for the aperture and thus in "f/8", is the "8" part? cause that would explain all the above

thank you for your time
 
1/8 is smaller than 1/2 right?
there you go:) why double? 1.4 is a rough number. It's really the Square Root of 2...or about 1.41421356
 
F numbers are fractions. f/8 = 1/8, f/2 = 1/2, f/5.6 = 1/5.6 etc. 1/2 IS larger than 1/4.
 
First, your question 3 is spot on: f-numbers are just numbers, for example '4'. The aperture is f/4, the f-number is 4. The larger the f-number, the smaller the aperture. I have the impression that a lot of photographers are confused about the f-number, calling it f/4 or 1/4, for example, rather than '4' - and they pass this confusion on.

The f-number is a defined term, and for photographic use it is defined by ISO, the International Organisation for Standardisation.
It is defined as the focal length divided by the diameter of the entrance pupil. The entrance pupil is the image of the physical diaphragm (iris) as seen from the front of the lens. The smaller the entrance pupil, the larger the f-number.

If you wish, I'll be happy to explain why it is the entrance pupil diameter and not the physical diameter of the iris (except in special circumstances, such as when there are no lens elements in front of the iris). The entrance pupil is also the correct point of rotation for stitched panoramas, by the way.
 
Last edited:
F numbers are fractions. f/8 = 1/8, f/2 = 1/2, f/5.6 = 1/5.6 etc. 1/2 IS larger than 1/4.

I had "Aperture dyslexia" for a while. When someone said bump up your A....I bumped it up from f/8 to f/16.....whew, thank goodness I got that one figured out now. It is easy at first to get it wrong when you are new. But that is a great way of explaining it, if you can cook and use measuring cups, you can understand A :)
 
thank you all very much for your time, i figured it out and i identified the source of my silly confusion which came even though i had it all clear in my mind at first

reply #1 was not very helpful but thank you anyway

reply #2 was wrong (as it turns out) and not very helpful either but thank you anyway
as i now understand and the reply #4 confirmed, the f-numbers are not fractions and the numbers you wrote are aperture values if you see them strictly as mathematical values and not as camera setting values
it's the same camera setting value on the camera though so many would be referring to an f-number or aperture the same way
btw "1/2 IS larger than 1/4"? i didn't say otherwise :)

reply #3
figured out the problem before i need to check out your link, but i will and thank you

reply #4
thank you too, i located the wiki part that confused me:
for example, if the focal length is 16 times the pupil diameter, the f-number is f/16"
f/16 as an arithmetic value would be the aperture right? but the author was probably referring to the camera setting not imagining that someone would actually stupididly interprete the phrase "the f-number is f/16" as |f-number|=|f/16|
edit: well the previous sentence in the article clarifies this but i didn't pay attention, anyway
btw no need to explain about the 'effective aperture', i read some article about that and hopefully i haven't confused anything yet :)
 
Last edited:
f/16 as an arithmetic value would be the aperture right? but the author was probably referring to the camera setting not imagining that someone would actually stupididly interprete the phrase "the f-number is f/16" as |f-number|=|f/16|
btw no need to explain about the 'effective aperture', i read some article about that and hopefully i haven't confused anything yet

Or it could just be that the author was sloppy, ignorant or confused, or wished to confuse other people. The ISO definition is quite clear and unambiguous.

Be careful when reading that article linked to - I just skimmed through it, and it doesn't give me that nice warm fuzzy feeling that the author has a particularly good understanding of the subject. Life is too short to correct every piece of bad information on the internet.

It might be better to call it the 'entrance pupil', because 'effective aperture' has another technical meaning that is definitely not the same as entrance pupil.
 

Most reactions

New Topics

Back
Top