Lens Zoom
Posted on Tuesday, December 29, 2009 in Photography
What lens parameters change when I zoom and focus on a zoom lens?
Suppose I have a 70-200mm zoom lens. I zoom it to 130mm, and half-press the shutter button to auto-focus, something moves inside the lens and adjusted its focus. What lens parameters have changed in the previous two steps respectively? For those lens with non-changeable focus length, there is still a focus ring which you just adjust, what does that change?
A true zoom should stay focused on the same point over the range of focal lengths.
A variable focal length lens without constant focus is called a vari-focal lens such as the Vivitar Series 1 35-85mm Vari-Focal f2.8
Many zooms will change the effective aperture over the zoom range, that is why you see two f stop parameters quoted, unlike very good zooms that tend to have a constant aperture over the whole zoom range.
http://photography.suite101.com/article.cfm/vivitar_cult_classic_camera_lenses
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Pentax Long range zoom lens C61244ST 12-1680mm with COHU 8800 camera
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