[Brightcolours]

Brightcolours, I've no idea how you calculate a f/2.8 - it has f/1.8 and you can't just multiply the aperture by the crop factor - that would only be fair, if the lens would have to bring enough light to a double as large sensor.

That is just the nature of things. To get an equivalent lens, you multiply the focal length to get similar field of view. You multiply the f-value to get a similar aperture size and similar DOF. 

 

You can just multiply the focal length by crop factor, and similarly, you can just multiply the f-value by crop factor.

 

Simple example:

Suppose we have a 85mm lens on an 1.6x crop camera (compared to 135 format "full frame), with an focal length/aperture ratio of f/1.8.

To get a similar FOV we multiply by crop factor: 85 x 1.6 = 136mm.

To get a similar aperture we multiply the f-value by crop factor: f1.8 x 1.6 = f2.88, so lets say about f2.8.

Lets check if that is correct. 

Size of the aperture of the 85mm f1.8 lens:

85mm / 1.8 = 47mm aperture.

Size of the aperture of a 135mm f2.8 lens: 

135mm / 2.8 = 48mm aperture.

 

The 85mm f1.8 lens on 1.6x crop and the 135mm f2.8 lens are equivalent. They offer each respective sensor the same FOV angle, they offer a similar aperture size for the light to pass through, they will offer a similar DOF.

 

So this Sigma lens offers for APS-C what the f2.8 standard zooms offer on FF.

 

It has an aperture which is hard (if ever) to get for a range between 18 and 24mm - except some pricey 24/1.4 FF lenses. I can see that as a standard zoom for D7100. Given what the 35/1.4 performs, Sigma is really reaching for a high place in lens heaven.