sorry but I"m idiot but what is the x zoom equalivent of a 400mm on a full frame camera?
Not sure if my comments below are necessary, if you are familiar with the following, please disregard.
As mentioned before the term "zoom" is not a good measure to determine the "reach" of a lens or how far it can "zoom in" so to speak. Actually "zoom" may not even refer to a lens' magnification.
A zoom lens is any
lens where the focal length (the one in mm) can vary, i.e. it is adjustable (as opposed to "prime" lenses, where the focal length is fixed, you cannot zoom in or out, e.g. 50mm). So yes, this Sigma 18-250mm is roughly a 13.9x zoom. But for example my ultra-wide Sigma 10-20mm is also a zoom lens, even though it's still a wide angle at the 20mm setting and capable of capturing group pictures indoor even when fully zoomed in. Since it's a 10-20mm lens it is ergo a 2x zoom lens.
At the same time a big, heavy Nikon or Canon (new model) 200-400mm lens is also a zoom lens, and also a 2x zoom lens (400/200=2) even though it is a serious lens for wildlife photography and has a great reach to bring wildlife close (especially used on a crop sensor DSLR, and/or used with a teleconverter.)
That is why in order to determine how far a camera lens can "reach", bringing far objects close to you, it's better to know the focal length of the lens (mm). (And also the sensor size, the smaller the sensor the greater the reach due to the crop factor).
This is fairly often confused by people. When I walk around with my 600mm lens (120-300 2.8 + 2x TC), my 120-300mm zoom lens becomes a 240-600mm zoom lens, which is quite a reach for wildlife, however, it's still only a 2.5x zoom
. Sometimes people are disappointed to hear it's "only" a 2.5x zoom
Last edited by gabebalazs; 06-09-2013 at 02:02 PM..