What constitutes a wide angle lens?

The kit lens my D7100 came with, the 18 to 105, a has a wider angle than some of the wide angle lenses i have seen sold for Nikon, such as the 35mm for example

yet mine is not listed as a wide angle lens

so, is it all about the lowest mm, or is it more complicated than that as i assume it is?

The 18-105 is not listed as a wide angle lens because it is a ZOOM lens. Yes, it has a wide angle on one end, and telephoto on the other, but the term "wide angle lens" refers to a non zoom lens, or more commonly called a PRIME lens. It has ONE focal length. A prime lens can be wide angle, or "normal", or telephoto, such as a 28mm lens, or a 50mm lens, or a 135mm lens.

Every camera type has a focal length that is classified as "normal". A normal lens delivers an angle of view that is considered the human perspective. If we mount a lens that is 70% of the "normal" focal length or shorter it delivers a wide-angle view. The shorter the lens, the wider the angle of view. Conversely, a lens with a focal length 200% of "normal" or longer delivers a narrow angle of view we classify as telephoto.

First we need to figure out what is "normal". The Nikon D7100 is classified as an APS-C format. This is alphabet soup for Advanced Photo System - Classic. First we need the specifications of the D7100 as to format dimensions. Checking the specifications it is - 15.6mm height by 23.5mm length. By custom, the "normal" focal length for this camera is approximately equal to the diagonal measure of the format. We figure this out mathematically, it is 28.2mm. This value is the focal length that delivers a "normal" angle of view. This works out to approximately with the camera held in the landscape (horizontal) position.

Now wide-angle being 70% of "normal" works out to 20mm or shorter. A 20mm will deliver an angle of view of A 15mm will deliver an angle of view of

As to telephoto: We will use 30mm as "normal"; thus if you mount a 150mm the view will be 150 30 = 5 meaning a 5X magnification. In other words, the 150mm delivers the same as a view as a 5X binoculars.

At the 18mm end of the zoom, that lens is a wide angle lens.

It goes from 18mm (wide angle) to 105mm (telephoto).

Any lens which has a focal length of less than 35mm is considered wide angle on your camera. The shorter the focal length, the more wide angle it is.

On a 35mm film camera or full-frame DSLR a 50mm lens is considered a "normal" lens since its angle of view approximates that of the human eye. On a crop-sensor DSLR like your D7100 with its 1.5 crop factor a 35mm lens has the equivalent angle of view of a 52.5mm lens on a 35mm film camera or full-frame DSLR which makes it a "normal" lens on your D7100.

The 18mm end of your 18-105mm zoom has the equivalent angle of view of a 27mm lens on a 35mm film camera or full-frame DSLR. This makes it a modest wide angle lens on your D7100.

Suppose you have a friend who owns a full-frame Nikon DSLR and a 17mm wide angle lens. If you wanted to come close to having as wide a view as her then divide 17 by 1.5 which will tell you you'd need an 11mm lens on your D7100.

Nikon offers a 10-24mm and 12-24mm.

Sigma offers a 10-20mm, 12-24mm and 8-16mm