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Sigma 18-250mm

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    mickrick
    mickrick  10 United Kingdom2 Constructive Critique Points
    9 Mar 2012 - 4:08 PM

    After the short zoom for my wife's Nikon D70S got a sticky zoom ring, we decided to replace it with a superzoom and image-stabilisation, and the Sigma 18-250mm seemed a perfect choice. I was away when she got it. She like the lens, but said "It didn't seem very long". I was sure she was imagining it - focal length isn't a moveable feast, and Sigma are a well respected manufacturer. But, when I got home, bringing some equivalent lenses with me, we tested it. The max zoom gave a wider field of view than three 200mm lens I tested it against! I understand the idea of coverage. I've used a Bronica in the past, and see that by changing from a 120 back to a 35mm back, the magnification changes. But surely the same rated focal length, should give the same field of view, when tested on one camera, at the same distance?

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    ams99
    ams99 Junior Member 365 forum posts England9 Constructive Critique Points
    9 Mar 2012 - 4:24 PM

    I'd be interested in an answer to this puzzle.

    Your conclusion seems logical.........

    HouseMartin
    9 Mar 2012 - 5:09 PM

    Never really noticed this issue so I have just done the same check with a Tamron 70-200 vs the Sigma 18-250. I've got to agree with you guys - the Sigma shot IS a slightly WIDER field of view with them both fully zoomed.

    Now I'm confused and I really do not have an answer, so I shall be following this thread with interest as well in the hope an enlightened one can solve it for us.

    Paul

    discreetphoton
    discreetphoton Site Moderator 93453 forum postsdiscreetphoton vcard United Kingdom20 Constructive Critique Points
    9 Mar 2012 - 6:19 PM

    Read this.
    Different lens, but same rules apply.

    mickrick
    mickrick  10 United Kingdom2 Constructive Critique Points
    9 Mar 2012 - 6:40 PM

    I have the lens boxed, and ready to send back to B&H (who were very helpful, by the way). But I sent a query to Sigma, so I'll let you know if they reply

    discreetphoton
    discreetphoton Site Moderator 93453 forum postsdiscreetphoton vcard United Kingdom20 Constructive Critique Points
    10 Mar 2012 - 8:53 AM

    You may find this with every superzoom lens. The quoted focal lengths are only accurate for subjects at infinity. It makes sense really, because the emphasis on the end use is convenience. It's not convenient to have to move father away from your subject as you zoom in on it, so the focal length reduces slightly inside the lens. Otherwise, the close focus distance of these lenses would be about four feet, rather than the more typical fifty centimetres.
    They may mention this in the manual. I know tamron do.
    If your lens employs this approach, you can see it by using manual focus at the long end. You'll see the angle of view change as you turn the focus ring.

    mickrick
    mickrick  10 United Kingdom2 Constructive Critique Points
    12 Mar 2012 - 8:39 PM

    Thanks for this info'. I tested it at infinity, and it does show (about) 250mm equivalent when compared to my 70-300mm. It still seems a bit of a cheat. I'm a firm believer in the principle of compose, and then step closer, but it's not always an option. If a rattlesnake is only 6ft in front of the lens, I'd still want the maximum zoom range. It would perhaps be helpful if the manufacturers made this plain in their advertising. It's not my first Sigma lens, but I won't be buying another in a hurry!

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