That's because of the 1.5x crop factor of DX-format Nikon cameras, which have APS-C (Advanced Photo System - Classic) sensors. They're smaller than the full-frame sensors fitted to the D700 or D3x, which are the same size as a frame of 35mm film.
On DX Nikons, an equivalent focal length of 27mm gives reasonably good wide-angle coverage but can be a little limiting.
At the telephoto end, using FX (full-frame) lenses on DX cameras can be a big bonus. The crop factor plays into your hands, turning a 300mm lens into one with an effective focal length of 450mm.
The tables are turned at the wide-angle end, where FX lenses with even the shortest focal lengths lose more than a little something in translation. For example, an 18mm FX lens on a full-frame camera will give an extremely wide angle of view.
But on a DX camera it will only be about 65 degrees from side to side, or 75 degrees from corner to corner.
DX-format ultra-wide lenses generally need to be designed specifically for APS-C cameras, and aren't compatible with full-frame DSLRs. The rear element extends further into the camera, enabling ultra-wide DX zoom lenses to give a typical range of about 10-20mm or 10-24mm.
At 10mm, the effective focal length is 15mm and the angle of view is much more impressive, at 105 degrees on the diagonal.
Zoom lenses are much more popular for ultra-wide shooting on DX cameras, to the extent that ultra-wide prime lenses are almost impossible to find.
However, we've included the Samyang 14mm prime lens. It's the only lens in the group that's designed for full-frame cameras, but still gives an impressively short effective focal length of 21mm on DX cameras. The diagonal angle of view is 114 degrees (FX) or 92 degrees (DX).
Zoom lenses are convenient, and reduce the frequency with which you need to change lenses on your camera. In the ultra-wide DX class, most lenses have a 2x zoom range of 10-20mm, whereas the Nikon and Tamron lead the field with a 2.4x range of 10-24mm.
Most photographers tend to use ultra-wide lenses at or near their widest zoom settings most of the time. However, there's a lot to be said for shooting at 18mm with one of these lenses, in preference to a standard kit zoom lens, because barrel distortion is generally less pronounced.
The majority of ultra-wide lenses on the market have rectilinear optics. This means that if you were to shoot a sheet of paper from head on, its sides should be as straight as possible in the resulting image.
An alternative is a fisheye lens. These have curvilinear optics that produce a pronounced distortion effect. In the same paper test, the sides of the sheet would look as though they were bowing outwards.
There are two types of fisheye lens, circular and diagonal. The former produces a circular image that only fills the central part of the image sensor, whereas diagonal fisheyes project a larger image circle that covers the whole frame.
We've included the Tokina 10-17mm fisheye zoom in this group, which is a diagonal fisheye and gives full coverage on an APS-C sensor throughout its zoom range.
Whether it's rectilinear or curvilinear, any ultra-wide lens is brilliant for exaggerating perspective. Shoot an object or person from close up and the background will appear to fall away at a much greater rate than when you shoot with a standard lens.
You can use this phenomenon to creative effect. For example, you could artistically distort the shape of three-dimensional subjects, or make tall buildings on a skyline appear to lean in towards each other dramatically.
Depth of field
Yet another bonus is that when you're shooting at really short focal lengths, you can get an enormous depth of field, enabling you to keep both close foreground subjects and distant background objects sharp within an image.
For example, if you shoot at 10mm with an aperture of f/11 and manually set the focus distance to its hyperfocal setting of 45cm, everything in the frame will be sharp from just 22.5cm away, all the way to infinity.
When it comes to autofocus, the Nikon lens and all the Sigmas in this group feature ring-type ultrasonic systems. These deliver super-fast, practically silent autofocusing complete with full-time manual override.
By contrast, the Tamron and Tokina lenses use conventional electric motors, which are slower and noisier. The Samyang has no autofocus at all, so focusing is a strictly manual affair.