TBR - I don't understand the M=i/o equation, and when o is negative?

[Padfoot]

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It says that object distance is positive if left of the lens, and negative if right of the lens. But does it matter what type of lens it is or if it's a mirror?

I'm confused because Ex. 10.8a has a diverging lens with a focal length of 15cm and an object at 20cm. According to the i=0*f/o-f equation, i = -8 about. (I had thought o was supposed to be negative because it's on the virtual side, but the explanation showed that it was positive 20).

So M=i/o= -8/20 then? But negative M means that image is inverted, and diverging lens always create virtual upright images.
I understood this the first time i studied it, but now I'm somehow confused again. If anyone could help I would really appreciate!

 

[pepocho]

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