The f word revisited

Our new colleague Olaf Ulrich, in a thread in this department, has persuaded me that my coinage, a few years ago, of the expression "f/number", to be used in lieu of "f number", "F number", "f-number", "F-number", and perhaps other variants, as the name of the familiar expression of the parameter of a lens that approximates the impact of the lens on exposure, was ill-advised.

By way of further background, one way we customarily express that parameter is, for example, "f/3.5". I must admit that I do not even know whether the term "f-number" (to use one common form of the name) actually means:

• "f/3.5", or
• 3.5

We can play with this through the following exercise:

"What was the f-number for that shot?"

Answer A: "F/3.5"

Answer B: "3.5"

In any case, my plan is to revert to the use of one of the more familiar forms of the name in my technical writing. I'm looking for guidance as to which one to adopt. Please fell free to suggest ones not mentioned above.

Thanks.

Best regards,

Doug

"f/xx", always presented as a ratio, which it actually represents (i.e. sometimes called "focal ratio" by optic gnomes), and always with a lower case f. "F-stop", or just "stop" is far more commonly encountered language than "f-number" (which I've rarely heard used in conversation).

Hi, Ken,

Ken Tanaka said:

"f/xx", always presented as a ratio, which it actually represents (i.e. sometimes called "focal ratio" by optic gnomes)

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In fact, "focal ratio" in optical science means the quantity f/D (which is the "denominator" of the symbol, "f/xx").

It is not useful to treat the symbol "f/xx" as a ratio, even though it has the form of one.

If we treat the symbol "f/xx" as a real ratio, it just turns out to always equal D, the diameter of the entrance pupil.

Best regards,

Doug

Reply deleted by author.

Hi, Ken,

I had said:

It is not useful to treat the symbol "f/xx" as a ratio, even though it has the form of one.

If we treat the symbol "f/xx" as a real ratio, it just turns out to always equal D, the diameter of the entrance pupil.

Click to expand...

I have reconsidered that statement (in particular, the first sentence). Now, I say:

One significance of the notation "f/3.5" is that it tells us that the actual diameter of the entrance pupil (D) is f/3.5 (the focal length divided by the number given).

Best regards,

Doug

Hi Doug,

I've followed the discussion in the parent thread with great interest. While my knowledge of optics isn't enough to contribute, it is enough to understand what's being said, and I've learned a great deal. Many thanks to you, Bart and Olaf.

I (and most other people I talk to) use the term f/number to describe a lens - e.g. my macro lens is a Sigma 180mm f/3.5 - and the term f-number to describe its use - e.g. I took the shot at f-16 (or just f16). In the first case f/3.5 is a (simple) formula which measures a physical attribute of the lens; in the second case f-16 is just a number which is used as a convenient means of communicating with other photographers. I don't know if this helps, or whether I've missed the point completely.

Regards,

Stuart

Well of course the convention of saying, "I used f 3.5" has always implied we were using an aperture number (N) of 3.5, but your point is taken: it is not accurate when written as "f N", and can create confusion when writing about optics and optical formulae, so writing it as "f/N" makes perfect sense.

Doug_Kerr said:

... one way we customarily express that parameter is, for example, "f/3.5". I must admit that I do not even know whether the term "f-number" (to use one common form of the name) actually means:

• "f/3.5", or
• 3.5

We can play with this through the following exercise:

"What was the f-number for that shot?"

Answer A: "F/3.5"
Answer B: "3.5"

Click to expand...

First of all, in colloquial language you may expect either answer, as these two usually are considered equivalent. It takes some decent amount of nitpicking to discriminate between the two. But of course, there is a difference.

Basically, f-number is a colloquial term which is layman's talk for "aperture number;" it obviously comes from the fact that it usually is written in close conjunction with the 'f' symbol. Aperture numbers are the numbers written on aperture rings or displayed in camera control displays. As a seasoned photographer you may feel "aperture" and "aperture number" were just the same---but every beginner who fails to understand that bigger numbers don't mean bigger apertures is telling us they're not the same. I guess many consider the 'f/' which is always preceding the f-number just a kind of decoration which is supposed to make it clear to the reader that the number is referring to an aperture and not to, say, a magnification, a focal length, or whatever. Many even think they may just as well use a capital-letter F and/or omit the slash. But as a matter of fact, 'f/' means exactly that: "focal length divided by" followed by the aperture number. After all, the aperture is focal length divided by aperture number.

So in the example above, answer B is correct.

-- Olaf

Hi, Olaf,

Olaf Ulrich said:

First of all, in colloquial language you may expect either answer, as these two usually are considered equivalent. It takes some decent amount of nitpicking to discriminate between the two. But of course, there is a difference.
. . . .
After all, the aperture is focal length divided by aperture number.

Click to expand...

Indeed. f/(f/D)==D [My typography here for "is identically equal to"]

So in the example above, answer B is correct.

Click to expand...

I agree completely.

My current choice of language (when there is a need to be thorough) is: "The f-number [in a certain example] is 3.5; it is presented thus: 'f/3.5' ".

Best regards,

Doug

Doug_Kerr said:

: "The f-number [in a certain example] is 3.5; it is presented thus: 'f/3.5' ".

Best regards,

Doug

Click to expand...

That is as I understand it too. But with the invention of texting and fast responses required everywhere it's like the text message that says ILY....or CUL8R