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Why no short zoom faster than f2.8 lens?

  • 15-02-2011 4:40pm
    #1
    dinneenp
    Registered Users, Registered Users 2 Posts: 8,154 ✭✭✭


    can a mod please move this to Photogrpahy main forum?
    Hi,
    Just wondering why a short zoom faster than f2.8 lens doesn't exist?
    Would it be too big or what's the reason?
    I'm sure cost isn't the reason.

    I have a 17-50mm f2.8 and am going to get a Sigma 30mm f1.4 as well.(Someone also mentioned a 50mm f1.4 would be good for up close portraits with blurred backgrounds as the 30mm would have some distortion when used like this).

    It would replace the need for having these two/three lenses.

    Cheers,
    Pa.


Comments

  • #2
    Promac
    Registered Users, Registered Users 2 Posts: 1,407 ✭✭✭Promac


    You can get short fast zooms but they're just really fookin expensive:
    http://www.amazon.com/Olympus-Zuiko-14-35mm-Digital-Cameras/dp/B000X1P5R4

    Zooms are slower cause of the extra lenses required to do the zooming and focussing.

    Basically, the size of the aperture (and therefore the amount of light getting in to the sensor) is a function of focal length and, in a zoom lens, that also includes the extra lenses needed for the zoom and focussing mechanism and the spaces in between them.

    See the animation below here:

    Zoom_prinzip.gif

    In a prime lens you don't have the extra distance between the sensor and front lens so you can have a much wider aperture with a lot less glass. Primes also tend to use better quality glass. If you pay extra you can get the same quality in a zoom (Canon L for example).


  • #3
    Borderfox
    Registered Users, Registered Users 2 Posts: 10,263 ✭✭✭✭Borderfox


    Olympus do an f2 70-200 in four thirds


  • #4
    magicbastarder
    Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 50,264 CMod ✭✭✭✭magicbastarder


    fast aperture = more glass.
    zoom lens = more glass.
    more glass = more money.


  • #5
    City-Exile
    Registered Users, Registered Users 2 Posts: 2,547 ✭✭✭City-Exile


    It doesn't really make sense to create such an expensive lens, when f/1.4 is not a practical aperture for very many situations. Even for portraits, you can get such a shallow DoF that only one point of the face is in focus, often the nose, hopefully the eyes.


  • #6
    Borderfox
    Registered Users, Registered Users 2 Posts: 10,263 ✭✭✭✭Borderfox


    City-Exile wrote: »
    It doesn't really make sense to create such an expensive lens, when f/1.4 is not a practical aperture for very many situations. Even for portraits, you can get such a shallow DoF that only one point of the face is in focus, often the nose, hopefully the eyes.

    Dof would be dependant on distance to subject, near mfd that would be true but back off and the dof would be easier to work with


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  • #7
    dinneenp
    Registered Users, Registered Users 2 Posts: 8,154 ✭✭✭dinneenp


    I'm not saying a f1.4 zoom but even an f2 18-50 or so would be great.
    But not cheap.


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