Sensor size and aspect ratio

Bit over my head at first but after some research it became clear.

Back in the film days, the rectangle that captured the image on a standard SLR was one size: 24mm x 36mm .

People usually refer to a sensor’s size by its “crop factor.” That’s the number you use to find the 35mm equivalent of a given lens. It’s just like taking the middle of an image and throwing away the outside edges. If a sensor is 24mm x 36mm, then there is no crop factor, since it covers the same area as 35mm film

The Nikon D300 that i will be using during this course uses a 12.3 megapixel , 23.6 x 15.8 mm CMOS DX sensor in the 3:2 aspect ratio.

Focusing on Nikon they use two sensor sizes full frame (FX) and 1.5x (DX).

Nikon DX sensors, , have a crop factor of 1.5x. What this means, is that relative to 35mm film, the image will appear enlarged by approximately 50%. So shooting with a 24-70mm lens is "equivalent" of shooting with a 36-105mm lens on a FX full frame camera.