Why does nikon....

Started May 28, 2013 | Discussions thread
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Fred Mueller Senior Member • Posts: 2,478
maximum f stop is also a set physical characteristic ...

David - for equivalent depth of field, as crop factor varies, you need to use a larger or smaller aperture, so lens equivalency across sensor size is doubly more complex than just focal length - you've walked into the tar pit of lens designation and are just showing a novice lack of familiarity - no worries.  The easy rule of thumb is to just add 50% to the focal length of a DX lens and subtract one stop to get the FX equivalent.  But after a while you will just know without the mental gymnastics and Nikons correct and only possible labels will make perfect sense.

So: 18mm @ f/2.0 on DX =  27mm @ f/2.8 FX

and if you are really perceptive you may suddenly realize that for equivalent depth of field, FX gives up one stop of light gather "power" - oops, there goes most of the vaunted FX iso noise advantage !

Also many lens makers DO actually (parenthetically) explicate the 35mm full frame equivalencies of their products, especially for point and shoot cameras, which have wide angle values that start in the single digits ...

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