f/stop and the aperture ratio

Discussion in 'Beyond the Basics' started by beddingfield, Jan 14, 2020.

  1. beddingfield

    beddingfield TPF Noob!

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    This is supposed to be a fixed ratio, that merely expresses the diameter of the opening allowing light to passé through the diaphragm and then onto the film.

    Its supposed to be a fixed ratio, never changing. a simple "length to diameter".

    Its also supposed to be equal between ALL lenses despite overall diameter.

    For example, common wisdom would tell me that an f/stop number of 8 or 13 will be the same diameter hole regardless of LENS DIAMETER. But why isn't that what actually happens in life?

    For example if I consider these lenses

    100-300 vivitar FD macro zoom, 52 mm thread and tube diameter

    75-210mm vivitar FD zoom with 62mm thread and tube diameter

    630-1300mm Vivitar telescope zoom, at 630mm length its f/8, at 1300 its f/13

    Yet in operation, the 100-300 and the 650-1300 have a very visible drop in visible light when you switch them between these f/stops. YET the humble 75-210 has no change that the eye can detect.


     
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  2. Original katomi

    Original katomi TPF Supporters Supporting Member

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    Whoosh right over the top of my head.
    I have one lens that is one stop brighter for any given light reading and I allow for that.
    I am a fossil and for me it’s about the photography and not proving that I can do math.
    I did that to get my yacht masters
    Even the reading that my light meter give me are treated as advice not set in stone.
    So whilst for some the y times the alpha equals the whatnot is their cuppa and good on those that like that sort of thing I think I will pass
     
  3. Tropicalmemories

    Tropicalmemories No longer a newbie, moving up!

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    I think it's because f numbers are such a simple measure that you the differences?

    In practice, different lens designs and lens glass have variable impacts on the light passing through.

    Cinematographers use the t-stop system, which I think is more accurate?
     
  4. Designer

    Designer Been spending a lot of time on here!

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    I hope I don't confuse you with my answer.

    Yes, the aperture ratio is calculated the same for all lenses, but you'll see that for some zoom lenses the aperture varies with the change in focal length. But remember that the ratio is the opening divided by the focal length, so if that ratio changes with the focal length, then it makes perfect sense.

    Now to address your other question; a larger lens (tube, glass, and diaphram) will have a physically larger opening, but the ratio is still dependent on the size of the opening divided by the focal length. In other words, some physically larger lenses can have a smaller maximum f-stop than some physically smaller lenses. It all depends on the design of the lens.
     
  5. Soocom1

    Soocom1 Been spending a lot of time on here!

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    The "f" in the f stop is a mathematical symbol for the concept of 'function".

    Function (mathematics) - Wikipedia


    This translates to a system where multiple numbers representing different aspects produce a singular result.

    The f stop is a ratio of focal distance to the image plane.

    But it also incorporates other ideas into those numbers resulting in the final product.
    So an f-22 with a focal distance of 50mm is going to be a different diameter in a 100mm lens for the same mount system.

    This is important because of the registration distance of the camera.
    So an f22 for a canon 7D and 50mm lens will be a different diameter than that of a 50mm Mamiya 645 lens mount.
     
  6. beddingfield

    beddingfield TPF Noob!

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    The big vivitar PROVEs that the change is massive, its an issue with those lenses.

    Yet the companies making lenses simply tell us non professional people, "this aperture number will be the same no matter what lens you use".
     
  7. Soocom1

    Soocom1 Been spending a lot of time on here!

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    Take what I just wrote and explain it to someone usi g a iPhone for a Fakebook selfie
     
  8. RVT1K

    RVT1K No longer a newbie, moving up!

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    I understood it to be a ratio of how much light reaches the film/sensor with respect to how much light there would be if nothing were in the way. An example as I understand it...f/4 represents that 1/4 the light reaches the film/sensor, f/9 means 1/9 the light etc.
     
  9. ac12

    ac12 Been spending a lot of time on here!

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    This is NOT a simple subject and you appear to be confused.

    There are TWO things that you are confusing; F-stop and T-stop.
    • F-stop is a computed mathematical number. It is NOT the amount of light actually passing through the lens.
      Because it is a computed number, the actual amount of light that is transmitted through the lens can and does differ between lenses set to the same F-stop.
    • T-stop is based on TRANSMITTED light, not a formula.
      The transmitted light at particular T-stop number will be the SAME between lenses calibrated in T-stops.
    At it's simplest f-stop = FL / aperture diameter. Or spin the equation; aperture diameter = FL / f-stop.
    So a 100mm lens / 50mm aperture = f/2 or 100mm lens / f/2 = 50mm aperture.

    There are also TWO apertures:

    The primary aperture is the diameter of the objective lens. This limits the max amount of light that the lens will gather.
    In the example above, a 100mm lens with a max f/2 aperture will have an objective lens of 50mm in diameter.​
    This is just like an astronomical telescope.

    BUT the camera lens designer can do whatever they please and use a larger diameter barrel, if they want to.
    In the example above, the objective lens can be bigger (70mm), but it cannot be less (40mm), than the computed 50mm.
    One reason for a larger barrel is to use common parts from another lens, to reduce manufacturing cost.​
    Optical and mechanical design can create the need to have a larger diameter objective lens than the formula computes.

    Now there is the secondary aperture, that on these 3 lenses is controlled by the aperture ring, which controls the size of the aperture in the iris.

    The iris is in a particular place in the lens based on the optical design. And it's size is not in the formula above. It's purpose is to limit the light even further.
    Note: The visual size of the iris aperture is affected by the optics you are looking through. The actual aperture many be 10mm, but the magnification of looking through the lens may make it seem like 15mm, or the reverse and look smaller at 7mm.

    Now we come to fixed aperture zooms and variable aperture zooms.
    A fixed aperture zoom gives you a fixed f-stop at all zoom FL.
    The diameter of the objective lens can't change, so the iris has to close down, to limit the transmitted light to the same level at all FL.
    The aperture in the iris will be smaller at the shorter FL, and larger at the longer FL
    200mm lens / 50mm objective aperture = f/4
    100mm lens / 50mm objective aperture = f/2
    But this is faster than f/4, so the iris has to close down to limit the light to the same level as f/4.​
    A variable aperture zoom, the f-stop changes based on the FL (it is variable). Larger/faster at shorter FL, and smaller/slower at longer FL.
    The aperture of the iris does NOT change as you zoom, so the computed f-stop has to change.
    200mm lens / 50mm objective aperture = f/4
    100mm lens / 50mm objective aperture = f/2
    Variable aperture lenses are simpler and less expensive to make than fixed aperture lenses, because the mechanism to change the iris diameter as you zoom is eliminated.​

    As for you last question:

    An automatic lens (#1 and 2) is kept wide open (max aperture) until the camera fires, then it stop down for the shot, then opens again.
    So you do not see the light level at the indicated f-stop, unless it is at max. Or you press the Depth of Field preview button on the camera.​
    A manual lens (#3) is at whatever aperture you turn the aperture ring to.

    This is why on the camera, #1 and 2 will not dim/brighten as you turn the aperture ring, and #3 will.
    Note: An auto lens with a Nikon mount that is NOT on the camera, will stop down as you turn the aperture ring.
    Note: Some auto lenses have a manual switch to put them into manual mode.
     
    Last edited: Jan 14, 2020
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  10. ac12

    ac12 Been spending a lot of time on here!

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    No.
    You speak before understanding.

    Unless you are shooting slide film and critically reviewing the slides, I doubt you will see the difference between f/8 on all of the lenses.

    F-stop and T-stop while not the same are close, especially with modern optics and coatings.
    The difference is due to light loss going through the optics. Each element and each air/glass interface has a certain amount of light loss.
    In the past, with zooms with over a dozen lens elements, there was significantly more light loss than a simple 3 element telephoto. And there was no consistency between lenses, enough to be significant, for some people.
    By significant, I mean enough to make an exposure change visible in a movie, when the lens was changed. That is one reason the T-stop was created.

    But even in the 1970s, I do not recall a difference of more than 1 stop, so the difference is NOT massive, as you accuse.

    With modern optics and coatings, this difference is reduced, but not eliminated.

    And if you use a camera with TTL metering, the meter corrects for this.
     
    Last edited: Jan 14, 2020
  11. petrochemist

    petrochemist TPF junkie!

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    Firstly filter thread & tube diameter (presumably at the front of the lens) are both different from the aperture diameter - both will be bigger. Your 'common wisdom' also seems to have completely ignored the focal length.
    It is possibly worth pointing out what the aperture diameter used in the calculation is the apparent size of the opening seen from the lenses entrance pupil. Optical elements between the aperture & the front of the lens, will vary this from the physical size. In wide angle lenses the front element has to be very much bigger than the apparent aperture to prevent vignetting in the image.

    Many lenses remain at their widest aperture no matter the setting on the aperture, until just before the shutter opens. It sounds to me as if your 75-210 is merely doing this.
     
  12. wfooshee

    wfooshee No longer a newbie, moving up!

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    Aperture diameter has nothing to do with lens diameter. The diameter of the aperture actually is irrelevant as a measured number. Its importance is how it relates to the focal length of the lens. 200mm lens at f:4, the iris is 1/4 of 200, or 50mm. A 50mm at f:4, the iris is 1/4 of 50, or 12.5mm. But the actuall= diameter is irrelevant; what matters is that it's 1/4 of the lens's focal length. f:4 mean focal/4. f:16 means focal over 16. The ratio to focal length is how it's measured, not the diameter of anything.

    As for seeing the change when you change the f-stop on the lens, keep in mind that the camera will hold most lenses wide open, at their largest aperture (smallest number) for best seeing in the viewfinder, and setting the actual aperture only at the time of the shutter opening. Unless the lens is in "stopped-down" mode, there will be no visible change when you set the f-stop; it only gets the set f-stop when the shutter is tripped.
     

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