F-stop enquiry

As far as I have ever heard, the f stands for Focal length. Since the effective aperture (the diameter of the lens opening) is equal to the focal length of the lens divided by the aperture number, these values were written as F/16, F/11 . . .

If anybody can do better, I would like a better story.

Not a better story, but I think F-stop may be more slang than a legitimate term. I've never seen this term in the more serious optics books; they invariably refer only to f-number. All else as Ed says.

This is from Ansel Adams, A Biography by Mary Street Alinder. I highly recommend this book as it is that rare thing: informative and a good read:

"...Willard (Van Dyke) remembered that he proposed "U.S.256," the old system name for f/64. He said that Ansel responded, "U.S. 256 is not good, it sounds like a highwat." Willard continued, "He then took a pencil and made a a curving 'f' followed by the dot and 64. The graphics were beautiful and that was that." At first, it was written "Group f.64" in the style of the old aperture notation, but that was soon updated to the new notation with its slash, "Group f/64." To those familiar with Ansel's handwriting, the f in the Group f/64 exhibition invitation appears nearly identical to his own typical, very musical looking f."

So it appears the notation f goes back to around 1932. i suspect the "f" itself is derived from a mathmatical notation but happens to work well in a photographic context.

Any math majors here to confirm or disprove my theory?

The curvy f without the slash could be a sign for integration, as part of a math formula. It would normally be written rather large too, enough to cover two lines at least, with a number as an upper limit near the top of the f, and a lower limit at the bottom of the f.

I don't know how you would apply that to photography, light, and ratio of sizes of openings though.

Good to be a pioneer, that way you can make stuff up.

Maybe it was Ansel's way of saying "f*** you" to the pictorialist school of photography?

Another question:

Does anyone know the story behind the T-stop expression?

From Ellis post if it was written "Group f.64" then F. would be F stop as in telegram speak

T-stop merely stands for Transmission stop. It factors in loss of light in the optical elements. It is used most frequently in movie making and scientific work where extreme accuracy is necessary.

it could be named after the inventor of the lens cap, E.F. Stop.

It stands for 'Fenestra', latin term for 'window'. Get it?

Submitted for your consideration (from the Lens FAQ in the "Optics" section of this web site):

Q3. What is meant by f-stop?

A. The focal length of the lens divided by the diameter of the aperture (as seen from the front). It is also called an f-number, and is written like f/8, which means the aperture diameter is 1/8th the focal length.

The term is used both in regard to the maximum aperture of a lens and in regard to the aperture selected in a specific situation.

The brightness of the image on the film is inversely proportional to the f-number squared. The depth of field increases but diffraction is worsened when using a large f-number. The effective f-number for all 3 effects changes if the lens is focused extremely close. See Q7.

The term "stops" purportedly comes from old technology in which the aperture was selected by turning a wheel with various sized holes in it, each one of which let in twice the light of the preceding one. Thus the phrase "open up a N stops" means to change to an aperture allowing in 2^N times as much light, and conversely with "stop down N stops".

Sounds a little more plausible than "fenestra", doesn't it?

Well, Stan, if I wanted to make it up, I would probably have come up with something more interresting, but the truth is, that IS where F came from. We all know the meaning of F ratios, but the poster wanted to know where the F came from. Look it up.

It could also mean "fenetre", which is French for "le window".

After searching on this for hours, I have to go with it being a math equation for Focal Length ("F") over ("/") diameter. A lens with 14 inch "focus" (focal length) which is four inches in diameter he is talking about an f /3.5 lens -- (14 / 4) = 3.5

It's origins appear to go back to the mid 1800's and was used in a letter from Irving Pobboravsky to John William Draper. In it, he gave the above formula with an accompanying detailed explanation of what the F/ on the new lens stood for ("focal length over diamter").

This is what I read in a magazine recently. The f stands for fraction of the focus length of the lens. In the old days when a 50mm lens really was 50mm physically, an f/2 is an aperture 1/2 the size of the focal length (not sure its diameter or what).

So for the same light gathering power, ie same f stop: f/2 of a 50mm lens, the aperture has to be 25mm large f/2 of a 200mm lens, the aperture has to be 100mm large

Of course modern optics meant focal lengths no longer need to have a direct relation to the physical size. The f number is now only a indication of light gathering power. However, a longer lens still requires a larger aperture than a shorter lens, which is why it fast telephoto lenses costs so much.

That's what I understand.

Should it then be writen as f/ or as f.

Which is more accureate

It's meaningless on it's own because it is a ratio, thus you should write f/number.

The measurement of the focal length and amount of light is also what I read. But there's more. I also read that the f actually came from the guy who came up with this measurement of light. I want to say he was from Germany and his name was very long. I can't think of it to save me. It began with an F. As the measurement caught on it was tagged with his name. Over many many years, it became simply f stop. I got this from text in an actual photography class.