F factor and speed of a lens

Discussion in 'Beyond the Basics' started by Vuorilla, Dec 6, 2009.

  1. Vuorilla

    Vuorilla TPF Noob!

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    Hi there,

    I wonder if there is a direct relation between the "f number" and the speed of a lens. Is there a way to quantify how faster a f number is as compared to another f?

    For example, how many times faster is F1.2 compared to F3.5?
    And F1.2 to F1.4?

    I don't know if the term "faster" is the correct one here. Maybe I should say "how much more light does F1.2 let in as compared to F3.5?"

    I enjoy night photography, but my lens, at F3.5, sometimes is too slow to capture a crystal-clear photograph.
     
  2. jnm

    jnm TPF Noob!

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    yes. when someone says a lens is 'fast' or 'slow' they are referring to the max aperture, i.e. f number.

    the standard f-stops are 1.4 > 2 > 2.8 > 4 > 5.6 > 8 > 11.2 > 16 > etc.

    each jump is one full stop, or a doubling of light. there's probably a way to tell how much faster 1.2 is in relation to 3.5 but i dont know it. what i do know is 1.2 is a hell of a lot more light than 3.5. ex. if you had asked how much faster 1.4 is than 4 i could tell you that 4 > 2.8 is twice the light, 2.8 > 2 is twice the light, and 2 > 1.4 is another twice the light.

    twice * twice * twice = 2 * 2 * 2 = 8x more light, i.e. f 1.4 allows 8x more light than f/4 does. i suspect the jump from 1.2 to 3.5 is in a similar range.

    Aperture - Wikipedia, the free encyclopedia
    [​IMG]
     
  3. rufus5150

    rufus5150 TPF Noob!

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    That's precisely why it's expressed how it is -- the f-number is a pure number. All of the units have been factored out, which allows us to compare the amount of light let in by lenses with radically different physical sizes.
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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  5. Vuorilla

    Vuorilla TPF Noob!

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    Oh, I see. Could I say, for example, that if in a certain situation, with certain settings (ISO speed, etc.) the slowest shutter speed that would enable me to get a clear picture would be 1/10 at F4, then at F1.4, I could use 1/80 and I would get the exact same picture?
     
  6. Shockey

    Shockey TPF Noob!

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    tripod.....
    F2.8 lets in light fast and large opening, short depth of field great for action....handhold

    F14 lets in light slow and small opening, large depth of field great for landscapes...tripod

    Shutter speed is a function of f-stop combined with ISO.
     
  7. jnm

    jnm TPF Noob!

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    exposure wise yes. depth of field wise no. you could also do something in between, such as upping the iso one stop and using 1/40 or upping iso 2 stops and using 1/20. exposure is a math problem really. each one of the variables (shutter speed, iso, aperture) does something different to the picture though.
     
  8. raptorman

    raptorman TPF Noob!

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    (1/(2*largest_aperture))²/(1/(2*smallest_aperture))²

    So if you compare 1.4 with 4 you get this:

    (1/(2*1.4))²/(1/(2*4))² = 8.1633 -> F/1.4 allows 8.1633x more light then F/4

    If you compare 1.2 with 3.5 you get this:

    (1/(2*1.2))²/(1/(2*3.5))² = 8.507

    So jnm's suspection was pretty much correct.
     
  9. Dao

    Dao No longer a newbie, moving up!

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    Exposure value - Wikipedia, the free encyclopedia

    Take a look at the EV table from the link. (Table 1)

    On the far left column, that is the EV value. For example, EV=8, every settings (shutter speed and aperture) on that row are equal to EV 8. So you can say (F/4, 1/15) settings will yield the same exposure as (F/11, 1/2)


    Let take another example.
    Shooting in "Areas in open shade, clear sunlight", according to wikipedia, you can take a photo with EV 12 settings (based on ISO100). So if you want to have more objects in focus in the photo, you may take the photo with (F/11, 1/30)
    , but if you are taking a photo of a kid running around, you may need a faster shutter speed such as (F/4, 1/250). Both should give you the same exposure.
     
  10. Stosh

    Stosh TPF Noob!

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    As I understand it, f/1.4 is exactly 8x more light than f/4. Your math is likely correct, but the beginning number is wrong. It would technically be f/1.4142135........ or the easier way is the square root of 2. Of course this would be cumbersome to print on a camera lens.
     
  11. Josh66

    Josh66 Been spending a lot of time on here!

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    It could just say "50mm f/√2" instead. ;)
     
  12. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Yep, there's a bit of rounding for the f-numbers.
     

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