why do they call fstop fast?

Discussion in 'Photography Equipment & Products' started by BLD_007, Mar 4, 2010.

  1. BLD_007

    BLD_007 TPF Noob!

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    Why is low fstop called fast? the lens doesn't move, the shutter does.

    or is there a part in the lens that moves during capture. other than zooming and your normal lens movement
     
  2. PhotoXopher

    PhotoXopher TPF Noob!

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    It lets the light in fast; you can get a faster exposure at f/1.4 than you can at f/11.
     
  3. usayit

    usayit No longer a newbie, moving up!

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    well... technically light doesn't actually move faster... its constant.

    Larger aperture (lower number, Av) requires less time (shutter, Tv) to achieve proper exposure.
     
  4. Dwig

    Dwig TPF Noob!

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    equals larger f/stop.

    The terms "fast" and "slow", when discussing a lens or a lens' maximum f/stop, date from times long ago (140+ years) when most photographs were taken at the lens' maximum aperture. The plates (this is before flexible plastic film) were so slow that exposures were very long (multiple seconds). The only practical exposure control was exposure time; you generally couldn't afford to stop a lens down as wide open still resulted in painfully long exposures. As wider aperture lenses were developed they were thought of as "fast lenses" because they produced a good exposure in lens time.
     
  5. BKMOOD

    BKMOOD TPF Noob!

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    Simply put...

    Let's say your lens is a bucket. This bucket needs to be filled with water (light) before a picture is made. Water comes through the hole in your lens (the aperture).

    If the opening of your bucket is small (say, f22 -- a coke bottle sized opening) it will take longer for the bucket to fill with water than if the whole was larger (say, f1.8 -- a jelly jar sized opening). So an f1.8 aperture is faster than a f22 aperture.

    The wider the aperture, the bigger the hole, the faster water (and light) comes in.
     
    Last edited: Mar 5, 2010
  6. Dao

    Dao No longer a newbie, moving up!

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    When we take photo, we want a correct exposure.

    To have the correct exposure, we need to control the aperture (the amount of light entering the camera at a given time), and the shutter speed (the time allow for the light to enter).

    Let's keep the ISO constant.


    To have a correct exposure of a full moon, we need aperture of F/11 and shutter speed of 1/250. However, 1/250 shutter speed may not be fast enough for take a photo with a long telephoto lens such as 300mm (assuming no IS/VR/VC/OS) without camera shake issue. In order to have a faster shutter speed, I can increase the size of the aperture (by decreasing the number) so => aperture F/8 and shutter speed 1/500

    As you can see, we allow double the amount of light entering the camera (1 stop of light) at a given time. But at the same time, we cut the time that allow the light to enter in half. So technically, we have the same total amount of light hitting the film/sensor at the end.

    So for the same exposure, the larger the aperture, the faster the shutter speed.

    Let compare these 2 lenses.

    Canon EF 50mm F/2.8 macro
    Canon EF 50mm F/1.4

    Example: Typical office area -> EV 8

    EF 50mm F/2.8 macro
    Max aperture is F/2.8, with EV8, I need shutter speed of 1/30 at F/2.8

    EF 50mm F/1.4
    Max aperture is F/1.4, with EV8, I need shutter speed of 1/125 at F/1.4

    So which one has a faster shutter speed? Of course, it is the one with larger aperture (F/1.4) and you can say EF 50mm F/1.4 lens is faster than EF 50mm F/2.8 macro lens.



    Edit: I found that the 50mm macro lens is a F/2.5, not F/2.8 .
     
    Last edited: Mar 5, 2010
  7. BLD_007

    BLD_007 TPF Noob!

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    so will we ever see a lens at like .5 or even .1?
     
  8. Dao

    Dao No longer a newbie, moving up!

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    Even if they can make a lens like that ... but it is not going to be practical because the DoF is so shadow that you cannot really able capture much.
     
  9. djacobox372

    djacobox372 No longer a newbie, moving up!

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    likely not, the fastest lens ever made was a zeiss f0.7. Special production for nasa, and only a few of them were made. Later Stanley Kubrick used them in his movie "Barry Lyndon" to film under candle light. The DOF is so small in these shots that typically only one actors face is in focus at any time.
     
  10. LBPhotog

    LBPhotog TPF Noob!

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    The front piece of glass on that Zeiss f/0.7 had to have been HUGE, any idea what the focal length was?
     
  11. usayit

    usayit No longer a newbie, moving up!

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  12. KmH

    KmH Helping photographers learn to fish Supporting Member

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