Why hasn't Nikon released a f/2.8 DX zoom?

Lance B wrote:

That's because it needs to be big.

You can't get away from the laws of physics and an f2.8 lens is generally large. Let's take a 70-200 f2.8 zoom. The minimum size of the front element is always going to be the 200mm focal length divided by the max aperture of the lens of f2.8, ie 200/2.8 = 72mm and that is regardless of format, so this lens will be the same dor FX as it is for DX. The only real advantage of DX is that you have the crop factor meaning that your 200mm lens is "equivalent" to 300mm in FX. When I say "equivalent" it sort of is, but that is a whole new discussion.

Things get a little more difficult when you go wide angle, however. The problem is that due to the lens mount distance to the sensor, which was designed for 35mm film not APS C or DX digital, wide angle lenses for DX need to be designed differently. You will find that DX wide anlge lenses which have the same Field of View and the same maximum aperture as their FX counterparts, will be at least as large and larger in many cases, as their FX counterparts, negating the benefit of the smaller DX sensored bodies.

Some of these design considerations of wide angle lenses have to do with angles of light falling on the smaller sensor of the DX camera. Part of the problem being that due to the lens mount distance being designed for FX not DX, that the light from wide angle lenses on DX does not fall perpendicular to the sensor and can cause light fall off and other issues. Therefore, these lenses need different design conmsiderations which generally make these lenses a little larger than the FX counterparts.

If you really want to get technical, there are other factors as well, like the fact that f2.8 on FX really needs to be f2 on DX to get the saem narrow DOF when wide open, which means that the DX lens wuld even be larger than their FX counterparts! Imagine how big the 17-55 f2.8 lens would be if it were f2 and imagine the cost!

So, looking at your Nikon 17-55 f2.8 lens, it is wide angle up to about 30mm in FOV terms and this is part of the reason it needs to be large and at least comparable with their FX counterparts.

--
Lance B
http://www.pbase.com/lance_b
http://i.pbase.com/o4/21/489821/1/53232844.SydneyPanoVertSmall.jpg