Comparing lens sizes on Digital and Film cameras?

Does anyone know how to compare lens sizes on digital SLR cameras compared to conventional film SLRs?

I'm am used to having an idea for the field of view through a conventional 28mm wide angle lens. I understand a digital equivelent is about 18mm?

Is there a formula that can be used to compare?

We're likely to buy a Nikon.

they all have conversion factors ranging from 1.3x, 1.5x, 2x

depends what brand you are talking about, with a 1.5x a 100mm lens on a film body becomes 150mm on a DSLR (with 1.5x)

tell us the brand and can give you the factor

The conversion for Nikon is 1.5x. So an 18mm lens is 27mm, the closest you'd get to 28mm. It's very simple, just multiple the numbers by 1.5 to get the 35mm equivalent.