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Thread: f-stop vs. f-number

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    f-stop vs. f-number

    I use the term "f-stop" to refer to the traditional sequence of stops that double or reduce by one-half the amount of light reaching the film or sensor. I use the term "f-number" to refer to the infinate possible aperture sizes between standard f-stops. Is this right?

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    xpatUSA's Avatar
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    Re: f-stop vs. f-number

    Quote Originally Posted by Abitconfused View Post
    I use the term "f-stop" to refer to the traditional sequence of stops that double or reduce by one-half the amount of light reaching the film or sensor. I use the term "f-number" to refer to the infinate possible aperture sizes between standard f-stops. Is this right?
    Interesting post, Ed.

    It's OK for your own understanding.

    However, to be pedantic (as I so rarely am) the term "f-number" does include all possible f-stop values, whereas you've excluded the stopped values by saying "between standard f-stops". In other words, many people would say "an f-number of 5.6" and be understood quite clearly. "f-number" is really slang, although widely used and abused to point of respectability ;-)

    Some people just use "aperture", as in: " an effective aperture of f/4.12 " - for example.

    Or, one could say that "f-number" is referring more to the format f/N than it is to an actual value of N.
    Last edited by xpatUSA; 5th October 2012 at 03:52 PM.

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    Re: f-stop vs. f-number

    Ed: if you said to me the "F-number" is 22, or the "f" is 11, I would believe that you are talking about the f-stop value, which would be f-22 or f-11. As to you defination of "f-number" as "the infinate possible apertures sizes bewteen standard f-stops", is wrong in my opinion, as the aperture size is not varible inbetween standard stops. You can set your camera to increase or decrease 1/2 stops between full stops, or even 1/3 of a stop but you will never get a f-value of (or number) of say f-8.35. So there are only full stops, 1/2 stops, and 1/3 stops all from a full stop value, thus no infinate possible apertures sizes. Just use the term "f-stop" with the value, it will help as not to confuse others and yourself when others talk about f-stops.

    Cheers:

    Allan

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    Administrator Manfred M's Avatar
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    Re: f-stop vs. f-number

    Quote Originally Posted by Polar01 View Post
    As to you defination of "f-number" as "the infinate possible apertures sizes bewteen standard f-stops", is wrong in my opinion, as the aperture size is not varible inbetween standard stops.
    A small bit of trivia for you Allan - if you are using a lens designed for video, the statement is true. Those lenses do not have click stops and do have infinitely variable iris settings. As the distinction between still and video cameras is blurring, you can certainly get video lenses for Canon and Nikon mounts.

    The video lenses don't call them f-stops, but rather t-stops. The "t" stand for transmission and unlike camera lenses, where the f-stop is purely based on the geometry of the lens aperture, the t-stop also accounts for light loss within the lens. Samyang has come out with lenses for both still and video. Optically, they are idential but the features plus gearing for follow focus are included. I've seen some Canon lenses as well, for APS-C and full-frame video camera, but don't know if they are purpose designed video lenses or repurposed still lenses.

    http://www.usa.canon.com/cusa/profes...e24mm_t1_5_l_f

    http://www.syopt.co.kr/eng/product/VDSLR_f24mm.asp

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    Re: f-stop vs. f-number

    Well, my D7000 comes up with, for example, f/7.1 in P Mode. Where does this come from?

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    Re: f-stop vs. f-number

    Whatever the number it is it is the result of a mathematical calculation of dividing the the focal length of the lens by the aperture so it is possible to have any number and decimal places. Some may think in 'pure' whole numbers but that ignores the potential for f/2.8 or f/4.5 or f/6.3 to list just three common examples. I also suggest one should use / and not -.
    However "words mean what I mean them to mean" as one famous wordsmith put it.
    It may be common practice to call video lens by t-stops but in the history of film and video this is a recent practice and all my lenses were f/stop labled.

    Once upon a time people used round holes of various sizes on a strip or disc to "STOP" the amount of light reaching the emulsion and that is the origin of the word I believe .... ie Waterhouse Stops ... LOL

    While the current 'standard' is for the f/2 series numbers awhile back many european cameras came with their own system with stop numbers like 6.3,9,13,18 .... both systems result from the basic division f/a.
    Last edited by jcuknz; 5th October 2012 at 09:05 PM.

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    Administrator Manfred M's Avatar
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    Re: f-stop vs. f-number

    Quote Originally Posted by Abitconfused View Post
    Well, my D7000 comes up with, for example, f/7.1 in P Mode. Where does this come from?
    f/7.1 is 2/3 of a stop slower than f/5.6, or 1/3 of a stop faster than f/8. Cameras tend be set in either half or third stops between the full f-stops, and that is what you are seeing here.

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    Re: f-stop vs. f-number

    Quote Originally Posted by Abitconfused View Post
    Well, my D7000 comes up with, for example, f/7.1 in P Mode.Where does this come from?
    It comes from an internal camera calculation. In P mode. the camera "firmware" decides all the settings for the shot, based on what it sees in it's scene metering Department. Because lenses are controlled by an [almost] infinitely variable motor, it can quite happily decide that f/7.1 is perfect for your shot. The same could occur in shutter priority mode . And a similar effect could well occur in aperture priority mode, but with a seemingly non-standard speed number.

    Or, what Manfred sez above!

    Yet another good reason to shoot manual habitually ;-)
    Last edited by xpatUSA; 5th October 2012 at 09:10 PM.

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    Re: f-stop vs. f-number

    That's correct, an f-number can take any value. An f-stop is the distance between two f-numbers where the amount of light differs by a value of two (i.e. double of half).

    The numbers we use are actually naming conventions.

    It's actually has to do with the diameters of the iris (aperture opening) in every f-number (for a fixed focal length). The area (A) doubles when the diameters (D) become larger by a factor of square route of two (about 1.4 since 1.4x1.4=2 approx.) according to the cycle area - diameter formula (A=pi x R x R where R = D/2 is the radius).

    Approximately (all numbers are rounded):
    1 x1.4=1.4
    1.4 x1.4=2
    2 x1.4=2.8
    2.8 x1.4=4
    4 x1.4=5.6
    5.6 x1.4=8
    8 x1.4=11
    11 x1.4=16
    16 x1.4=22

    I also suggest one should use / and not -.
    True
    However "words mean what I mean them to mean" as one famous wordsmith put it.
    Also true

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    Re: f-stop vs. f-number

    Surely cameras do not have an infinitely variable motor controlling things but more likely a stepping motor designed to accommodate the steps required by the designers?

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    Re: f-stop vs. f-number

    Quote Originally Posted by jcuknz View Post
    ... but more likely a stepping motor designed to accommodate the steps required by the designers?
    Yep, also true. The values of the f-number in our cameras are constrained but the (stepper) motor that controls the iris. It only works on carefully predefined steps so we can't change that e.g. with a firmware update.

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    Re: f-stop vs. f-number

    Quote Originally Posted by jcuknz View Post
    Surely cameras do not have an infinitely variable motor controlling things but more likely a stepping motor designed to accommodate the steps required by the designers?
    Quite so. Which is why I said "controlled by an [almost] infinitely variable motor".

    However, it is doubtful that the steps of the motor are made exactly equal to stops or halves or thirds thereof. In most stepper motor applications, gearing is often used for torque multiplication and/or for increased angular resolution (think AF).
    Last edited by xpatUSA; 6th October 2012 at 08:15 PM. Reason: clarified

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    Re: f-stop vs. f-number

    Quote Originally Posted by jcuknz View Post
    However "words mean what I mean them to mean" as one famous wordsmith put it.
    Yes. I think we are getting close to the "how many angels can dance on the head of a pin" issue.

    If I know what you mean, there is no confusion.

    Sometimes the word "thingy" can be appropriate.

    Glenn

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    Administrator Manfred M's Avatar
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    Re: f-stop vs. f-number

    Quote Originally Posted by jcuknz View Post
    Surely cameras do not have an infinitely variable motor controlling things but more likely a stepping motor designed to accommodate the steps required by the designers?
    You are assuming that the control can only be achieved through the use of a stepping motor. If you look at the aperture control on a lens, it is just a lever that connects to an external linkage that can be used to set f-stops. I have a Novoflex Nikon F mount to MFT adaptor, and that is exactly how it works. In the picture below, I have my Tokina f/2.8 11-16mm F mount lens mounted on my AF100 camera using the Novoflex adaptor. The blue ring is how I set the f-stops; 100% infinitely variable; no stepping motor involved whatsoever.

    f-stop vs. f-number

    If I shoot with one of the MFT Panny lenses, then there is an electro-mechanical coupling, and the steps, while small, are discrete, driven by the aperture control system connected to a small thumbwheel seen on the bottom left of the camera body.

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