so basic of a question - fast lens?

Discussion in 'Photography Beginners' Forum' started by a_spaceman, Oct 30, 2008.

  1. a_spaceman

    a_spaceman TPF Noob!

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    very silly and i guess basic question.
    i read here and there od people talking about fast and slow lenses. what does that mean?
    thank you in advance for enlighting a beginner!
     
  2. hankejp

    hankejp TPF Noob!

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    Just shooting from the hip but they may be either talking about the aperature for the shutter speed.
     
  3. tenlientl

    tenlientl TPF Noob!

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    when a lens has a wider aperture, it means you 'can' use a faster shutter speed without having to sacrifice the amount of light that gets through.

    maybe the 2nd part isnt really the reason, but yes, wider aperture
     
  4. Dao

    Dao No longer a newbie, moving up!

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    Yes, usually it is reference to the max aperture a lens is capable of.

    For example, "EF 70-200mm F/2.8L IS" is faster than "EF 70-200mm F/4L IS" lens.

    In other words, EF 70-200mm F/2.8L lens has a wider maximum aperture than the EF 70-200mm F/4L version.

    With the wider aperture, the fast lens can use a faster shutter speed to obtain the same exposure with the slower lens with slower shutter speed (narrower aperture).
     
  5. Pugs

    Pugs TPF Noob!

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    If you think of the lens like a hose, light as water, and the "perfect" exposure as a bucket. The question is how long does it take to fill the bucket to get that "perfect" exposure. If the water (light) is flowing at a constant speed, the you can fill the bucket "faster" by getting a bigger hose.

    SO, the lens maximum f/stop number is how "big" of a hose the lens is. Things are a little reversed so stay with me here. The smaller the f/stop number is, the LARGER the aperture opening is. So this means an f/1.2 is a LARGER aperture than an f/22.

    With this in mind, if a lens is called a 50mm f/1.8, then it will be a wider "hose" (aperture) than a 50mm f/3.5. Lenses are typically identified by their focal length (50mm in this example) and their maximum aperture (f/1.8 and f/3.5 in this example).

    Therefore, the 50mm f/1.8 is "faster" lens than the 50mm f/3.5 because it lets more light through meaning that you can leave the "water" running for less time. Basically, you can use a faster shutter speed to get the same amount of light if the lens is set at its maximum aperture.

    Does this analogy make sense or did I completely throw a handful of dirt in the bucket making the water muddier?
     
  6. a_spaceman

    a_spaceman TPF Noob!

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    thank you all for the help! that was useful
    just one more question for concreteness' sake ;)
    what is the difference if i take the same photo first with a slower shutter speed and less aperture and then with faster shutter and higher aperture?
    as the bigger aperture allows faster shutter it will be easier to get sharp images without a tripod or of moving objects. that's where faster lenses' benefit lies, yeh?

    god tonight i can't put two words in a row. sleeping deprivation. i apologize for that.
     
  7. Pugs

    Pugs TPF Noob!

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    Smaller f/stop number (wider aperture) means shallower depth of field. Larger f/stop number (smaller aperture) means deeper depth of field.

    And yes, faster shutter speeds do mitigate camera shake quite a bit.
     
  8. a_spaceman

    a_spaceman TPF Noob!

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    so, and this will hopefully be the very last question, if i want to adjust the depth of field i have to consider the shutter speed as well while taking the photo?
    i mean, does the different aperture make a big difference?
    thanks a lot!
     
  9. Pugs

    Pugs TPF Noob!

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    If you want to adjust the depth of field and get a good exposure, you need to balance the amount of light coming in by adjusting the aperture and the shutter-speed.

    Wider aperture balanced with faster shutter speeds = shallower depth of field.

    Narrower aperture balanced with slower shutter speeds = deeper depth of field.

    And YES, depth of field makes a HUGE difference. Just poke around at some of the pics posted in the galleries here and notice the difference between a portrait with nice bokeh (blurred out background) and a portrait with the subject AND the background in perfect focus.

    Or pay attention to the difference between landscapes in which the focus is utterly soft for most of it but has one element that isn't the subject is in focus.

    Play with it in your camera and you'll see a tremendous difference.
     
  10. Turnerea

    Turnerea TPF Noob!

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    If the situation calls for manipulating the DOF, then you can shoot in aperture prioroty mode. This allows you to pick an aperture (thereby "picking" a DOF), and the camera meter will choose the correct shutter speed.
     
  11. djacobox372

    djacobox372 No longer a newbie, moving up!

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    Also: most lenses have poor sharpness at there maximum aperture--even the expensive ones!

    Generally f8 and above produces ideal image quality, below that you are sacrificing quality for speed--this is true with any lens, just less noticable with higher quality/faster lenses.
     
  12. tsaraleksi

    tsaraleksi TPF Noob!

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    The best lenses (with regards to sharpness) have "good to great" sharpness at their maximum aperture and excellent sharpness stopped down. This is true in only a select few lenses, however-- in my experience a couple of examples include the Canon 35 and 85L primes, 70-200 2.8L zoom, and the Nikon 300 2.8. Super-fast lenses and some telephoto primes are at their sharpest between 2.8 and 4, rather than all the way down to 8.
     

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