What is a "fast lens?"

Discussion in 'Photography Beginners' Forum' started by feRRari4756, Dec 26, 2008.

  1. feRRari4756

    feRRari4756 TPF Noob!

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    Hey guys in these thread i keep hearing the term "fast lens."

    What does this mean? How do you determine if a lens is fast?

    and are they better for taking action shots of brids and stuff?

    Thanks
     
  2. Battou

    Battou No longer a newbie, moving up!

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    A lens with a large maximum aperture I.E. 50mm f/1.4 (fast) as opposed to 50mm f/3.5 (slow)

    The fast lens gathers light at a faster rate so to speak allowing for faster shutter speeds with lower lighting at the cost of Depth of Field, it's advantages are innumerable.
     
  3. Ins3rtNam3H3r3

    Ins3rtNam3H3r3 TPF Noob!

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    A fast lens is a lens that has a large aperture which allows it to let in more light and threfore lets you use a faster shutter speed.
     
  4. seth-trenda

    seth-trenda TPF Noob!

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    A fast lens is a lens with a low aperture number. your "fast lenses would be like the canon 2.8 IS (image stabilizer) or your 1.8.. They can be better...it all depends on the amount of light you have when shooting. When you are shooting at 2.8 your DOF is small and it might be harder to get action in focus. Fast lenses are more handy for shooting in low light situations. I hope that this helps.
     
  5. AlexColeman

    AlexColeman TPF Noob!

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    Depends on aperture, wider is faster, because you can use a faster shutter speed.
     
  6. feRRari4756

    feRRari4756 TPF Noob!

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    ohh okay. so it doesnt really affect how clear it autofocuses right?

    because i am trying to get clearer action shots of birds. so a fast lens would really help that right? birds in the daytime)
     
  7. dxqcanada

    dxqcanada Been spending a lot of time on here!

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    Wider apertures on a "fast" will permit you to use higher shutter speeds.
    If you use higher shutter speeds it will reduce blur cause by camera shake, and blur caused by motion of the subject.

    Most camera's will focus easier with a faster lens.
     
  8. Jurence

    Jurence TPF Noob!

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    Have any of you read the 6+ posts before you and realized that the answer was already given?
     
  9. stsinner

    stsinner TPF Noob!

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    That wasn't necessary.


    I like the hose and bucket analogy given by some other member another time someone asked this same question-Think of getting a picture to your sensor is like filling a bucket. Using a garden hose (f/5.6 lens), it will take a while. Using a fire hose with a larger diameter (f/1.8), it will fill much faster. Therefore the fire hose is a "faster" hose.. And an f/1.8 lens is a "faster" lens.

    Also, aperture numbers are inverse of the way most people think-I remember it by thinking that the f-number refers to the amount of the lens opening that is covered by the aperture blades.. At 22, even though the number is bigger, the hole to see the image through is much smaller (more of the lens of covered by the blades, hence the larger number) and it's "slower" because it will take more time to get sufficient light through that hole to fill the sensor (bucket).
     
  10. Jurence

    Jurence TPF Noob!

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    I think it was, it would have been a bit odd seeing how we are almost at page 2 and that would be nothing but the same response to the same quesiton :lol:
     
  11. feRRari4756

    feRRari4756 TPF Noob!

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    Okay thanks guys. The lowest I can get my camera down to is f/4 on my 70-210mm

    and f/1.8 on my macro lens.

    I know the macro is "fast" since its an f/1.8 but im not sure about the f/4 (I know you guys said that f/4 is "normal speed" but is it different on a zoom. In other words, if f/4 "fast" for a zoom lens?

    THanks so much guys
     
  12. dxqcanada

    dxqcanada Been spending a lot of time on here!

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