[Rover]

Finally, there's some common ground: everybody is different. Besides, I'm not sure if I'm in a minority or not, but I don't mix different systems in my workflow - the most I had at one given time were 1.3x and 1.6x cameras. Therefore, I did juggle lenses around to get "a lil' wider" or "a lil' longer" than I would otherwise have, but I was never making a big deal of DOF.

 

I had and have fast lenses (f/1.4 primes and f/2.8 zooms - all right, I'm down to one of each now) but most of the time I was using them for working in low light, not for any special defocus effects. I have a 85/1.4 in my sights that may be more in line with what you say, but I'll use it for low light again, because I wouldn't know what to do with it otherwise. 

 

 

Now suppose your girlfriend one day decides to give you a present, a Olympus OM-D EM-5, because in her idea you should have a small camera for when you can't take along your big DSLR.

Ah. If only.  <_<

 

 

And you do not want to seem ungrateful and really want to use it. But you want a pretty wide angle lens to go with it. What do you do? You get anEQUIVALENT focal length lens. So you have to calculate in order to get an equivalent lens And it turns out that on that Olympus a 10mm lens is equivalent to that 16mm on your APS-H camera. So you know what to get.

 

Since I can't use my Canon lenses on it, what's the point? I'll need to get a dedicated set of lenses anyway, and they have their own nomenclature. Luckily I know the multiplier in the case of Âµ4/3, but if you had been speaking of medium format (it'd take a rrrrreally rrrrrrich girlfriend, I agree), I would've been totally at sea. Therefore, I feel little need for calculations - it'd be easier to just memorize the (approximate) multipliers and just act by feel.

Besides, juggling the aperture numbers is opening a can of worms because there are a few quite odd concepts evolving - especially if you start bringing noise into that equation as some people do...