Storing and numerically ordering a values based on lens data - numeric and text values

Hi,

I have a database of lenses and one of their characteristics is 'Focal Length'.

This is measured in mm and the vast majority of values are simply whole numbers. My issue is with zoom lenses which have a variable focal length stored as a text value e.g. 50-100mm.

First question; from a data point of view how should this be stored. If it was single focal lengths I would simply store as whole number, but in what way would I transform my column to make this data most usable? 

Second question; I'm trying to make my tables human readable so ideally if someone wants to produce a table of lenses the zoom's should also be ordered. For example:

If I had the following lenses I would want them ordered as below:

10-50mm
30-70mm
100-200mm

If I order by text however I would get:

10-50mm
100-200mm
30-70mm

And taking this another step up in complexity, if I had a table listing a manufacturer's lenses that produced prime AND zoom lenses I would want them to be show as follows:

- Primes first 
- Zooms second

Example:

Zeiss
   12mm
   16mm 
   24mm

   10-50mm
   30-70mm
   100-200mm

Any help would be very much appreciated.