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Message: <blockquote data-quote="Bob Blaylock" data-source="post: 210906" data-attributes="member: 16749">It seems to me that the key point doesn't stand out prominently enough here.  Just the way the text fell.  Let me break it out so that it hopefully stands out better…  On those Nikon DSLRs that do not have a built-in focus motor in the body (which is anything in the D3?00 or D5?00 series, the D40, or D60), autofocus is only available when using a lens that has a focus motor in the lens.  Nikon designates such lenses as “AF-S” or “AF-I”.  Any autofocus-capable lens which is not “AF-S” or “AF-i” will only autofocus on the higher-model bodies that have a focus motor in the body.  You can use them on a D3100, but you'll have to focus manually.  Again, a point made here, that may not be clear enough.  I assume that the OP knows that 50mm is the “standard” focal length for a lens for a 35mm camera, and is supposing that it is also so with his D3100.  The D3100, like most of the more basic DSLRs, has a smaller image sensor than a full 35mm frame, and so the lens focal lengths should be scaled down accordingly, by a factor of 1.55.  A lens that would be most exactly equivalent on this camera to a 50mm on full-frame 35mm camera would have a focal length of 32.25mm; 35mm is close enough.  A 50mm lens on this camera would be equivalent to a 77.5mm lens on a 35mm camera.  Nikon uses the designation “DX” to refer to this smaller frame size, and “FX” to refer to those that use a frame size similar to that of a full-frame 35mm camera.  If you want a “standard” lens for a DX-format camera, that is equivalent to a 50mm lens on an FX camera or a 35mm film camera, then what you want is a 35mm lens, not a 50mm lens.</blockquote>

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