Lens question

Discussion in 'Photography Beginners' Forum' started by labpaws, Mar 5, 2009.

  1. labpaws

    labpaws TPF Noob!

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    Im mostly new to all this so sorry if my question is an easy one but want to make sure im thinking the correct way. Im interested in a Sigma 17-70mm lens. Now there is an f2.8 and a f2.8 to 4.5 version. The f2.8 will be a fast lens at all stops but a small DOF. The 2.8 to 4.5 lens will not be as fast on the long end but have a larger DOF. Is this the correct way of thinking or am i all wrong?
    Oh and a word on HSM. If my body has a motor is HSM needed and/or just an added bonus to have HSM
     
  2. Battou

    Battou No longer a newbie, moving up!

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    the 2.8 is a constant aperture wile the 2.8-4.5 varies with the focal leingth. And you have the basics correct but variable apreture is not exactly basic :lol: The 2.8 will be 2.8 at both 17mm and 70mm wile the 2.8-4.5 means that the maximum aperture will be 2.8 at 17mm and 4.5 at 70mm.
     
  3. j-digg

    j-digg TPF Noob!

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    Also when you see or hear the phrase 'fast lens' it means that it's max. aperture is wide enough to allow you to use a fast shutter speed, not that it will be fast at all stops.
     
  4. lockwood81

    lockwood81 TPF Noob!

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    Battou answered the first question quite concisely.

    As for your other question about the DOF. Both of those will be able to have a large DOF. The 2.8 is just the max aperture, they will both have a minimum of about 22, creating a large DOF.
     
  5. AlexColeman

    AlexColeman TPF Noob!

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    It all indicates the starting aperture. You can go up from there.
     

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