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  1. #1
    Faugh a' ballagh Sean Dempsey's Avatar
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    Lens speed and relative light at different shutter speeds... my bumbling question

    I have a question about fast lenses, I hope I can make it clear.

    Okay, here's my example, i don't know how else to get my question out:

    2 identical cameras with identical tripods, settings, subjects, everything.

    One has a 70-200 f/4, the other a 70-200 f/2.8. Both expose for the same subject.

    If the Aperture Priority is set, say at an f/11, with both lenses register the same shutter speed? Or will the faster lense register a faster shutter, since more light gets in at the same aperture?

    Or the reverse, if the Shutter Priority on both cameras is set to say a 1/200, will the faster lense have a f/11 while the slower lense an f/5.6?

    I hope I am being clear. Basically I am asking, if the speed of a lense is faster at the wide open end, does that mean that its faster at each respective, comparative shutter or aperture value? Or does the faster lense just go faster on the bottom end? I hope this makes some sense.
    A good craftsman never blames his tools.

  2. #2
    Toon Army Foot Soldier straightarm's Avatar
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    It's independent of lens

    The particular lens being used doesn't matter. f11 is f11 whether you use a 50mm f1 lens or a 600mm f4.

    For example if you used a separate hand held meter, it would know which lens you were using; it would simply give you a shutter speed and aperture combination eg 1/500 and f8.

    Simon
    Simon, bombadier 1st class

  3. #3
    It's hurricane season... again...
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    I could be wrong, but I think that the term "faster" is misleading. I think that lenses with wider apertures are considered faster because when they are wide open, they require less shutter time to develop the photo... in other words, in the same lighting, a meter reading of 1/500 at f/2.8 would equate to 1/250 at f/4.0, hence the term "slower". I don't think it has anything to do with the speed of the mechanics in the lens itself.

    Please feel free to correct me if I'm wrong... that's just what I've deduced from my limited experience.
    Kristin
    Canon Bird Nerd #2


  4. #4
    has-been... another view's Avatar
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    Yes, all correct. F11 is f11 is f11 - doesn't matter what lens you're using. The advantage to fast lenses are that you have that extra stop or two if you need them so you can use a faster shutter speed. Faster lenses give you more options to work with.

  5. #5
    Sleep is optional Sebastian's Avatar
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    Everyone's correct here, at identical ISOs, if each lens is set on f/11, they will have identical shutter speeds. The point of a faster lens is that you can open it up to f/2.8 if you need to and get faster shutter speeds than the guy with the lens that maxes out at f/4. Plus you get shallower DOF.
    -Seb

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    How to tell the most experienced shooter in a group? They have the least amount of toys on them.

  6. #6
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    Question Value of Fast Lenses

    Hello All:

    I've been following this thread and was hoping someone could provide a little advice about lens purchases. I have pretty good experience with cameras but have been largely a point and shoot digital person for the last 3 or 4 years. I currently own:

    Canon EOS Rebel 2000 with a Sigma 28-105 (f2.8-4.0) zoom and a Sigma 70-300 (f4-5.6) zoom.

    I have pretty much moved over to digital with a couple of "point and shoot" cameras including a G2, but now want to move to a Canon EOD 10D. Because of the 1.6X magnification factor of the CCD, I feel that I may need a more wide angle lens selection and have been looking on B & H for Canon or Sigma lenses that I think might be appropriate.

    What I'd like to know is whether it is worth spending several hundred dollars more for a lens that is f2.8 at all focal lengths than one that is f2.8-4.0 or how much light you lose by going to one that is f3.5- 5.6, or similar. The reason I ask is that recently I borrowed a Nikon D100 and lenses to take photographs indoors without a flash. Because of the environment, we could not use flash and needed to turn the sensitivity up, which made the photos a little noisy and I still had pretty slow shutter speeds (sub 1/40th). This lens was a 24-85 Nikon Zoom with about f3.5-5.6. If I purchase a lens that is f2.8-4 or even f2.8 at all focal lengths, how much faster shutter speeds could I expect?

    It does seem that ultra wide zooms with fast optics are much more expensive than wide zooms with fast optics, so any thoughts people have on whether to keep the 28-105 (f2.8-4.0) zoom and invest more in a 16-35 (f2.8) zoom or to get a 24-135 (f2.8-4) zoom is appreciated.

    Part of my dilema is that I want to have a maximum of three good zoom lenses to cover my requirements. One for wide, one for daily, and one for telephoto and the choices on B & H are enormous.

    Many thanks,

    Paul

  7. #7
    Junior Member
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    I think that is a valid question because full throttle on a 4 liter car is probably faster than full throttle on a 2 liter car.

    If f11 means something then it should allow the same amount of light through, regardless of the type of lens. I think it would be meaningless to have f-stops if f11 on a fast lens allowed more light through than f11 on a slow lens.

    I think the advantage of a faster (f2.8) lens compared to a slower (f5.6) lens is that you can get more light through by choosing the large aperture on the fast lens and so have a faster shutter speed. As I just posted, that does come at a cost, which in some cases can be over $1000 difference from an f4 to an f2.8. Some of the ultra fast sports telephoto lenses cost $7000 because of the engineering required to make an 800mm (f2.8) lens.

  8. #8
    Sleep is optional Sebastian's Avatar
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    Charles,

    The answer is rather simple. An f/2.8 lens will let in two more stops of light than an f/5.6 lens. That means that if you're shooting at 1/40 with the 5.6, you will be shooting at 1/160 with the 2.8 at the same ISO.

    Is that worthe the extra grand? Absotutely, if you find yourself needing sharp images in such conditions.
    -Seb

    My website

    (Please don't edit and repost my images without my permission. Thank you)

    How to tell the most experienced shooter in a group? They have the least amount of toys on them.

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