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  1. #11
    Join Date
    Dec 2007

  2. #12
    Join Date
    Feb 2006
    Des Plaines, IL

    Cool A distorted point of view ...

    Okay ... discussing this subject is always a test of understanding.

    Thje "super lens 18-250" is a terrific quick answer to most daily shooting. In fact, it solves a lot of vacation issues, when getting AN IMAGE is more important than technical accuracy.

    That being said, most lenses wider than 14mm tend to distort the frame. In fact, the 14mm f/2.8 Rectilinear has special elements in it that are used to correct the path of the light and provide less distortion. That usually costs big bucks.

    So, when you widen to 18mm on the 18-250mm lens ... on a lower end (A700 or less), you are really seeing "effectively" a 27mm shot, because the APS-C sensor 'crops' the image. This is also known as the dcf or digital cropping factor (1.5x). At the focal length, you are literally warping the light beyond its normal path, then the sensor crops off the outer edges and you have your image. Obviously, they did a pretty good job with the super lens, when you consider this ... but, if you really want a Full Frame 18mm shot ... then the lens you will wind up with is a 12mm on an APS-C. Believe me, now that is some serious light warping.

    I know it sounds a bit confusing and the best thing you can do to get a perspective on this is to get your hands on a Ultra Wide-Angle (UWA) SIGMA 10-20mm f/4-5.6 DC EX zoom, TAMRON SP AF 10-24mm f/4-5.6 Di-II LD or a TAMRON SP AF 11-18mm f/4.5-5.6 Di-II LD zoom (the SONY DT 11-18mm f/4.5-5.6 is effectively the same lens). Either will provide the best zoom available for constant wide shots. The extreme focal ends of the 18-250mm present challenges. The long focal end being challenged by the actual focal distance to the subject.

    Oh what a trangled web we weave, when we struggle to achieve.

    Yes, there will still be distortion even with these other UWA zooms, but it will not be as evident as with the super-zoom (18-250).

    Attachment 48616
    A700 w/ SIGMA AF 10-20mm f/4-5.6 DC EX lens
    @ 10mm (effectively a 15mm-shot on a Full Frame)
    Last edited by DonSchap; 08-31-2009 at 06:46 PM.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickr & Sdi

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