This may be a dumb question but..

I could figure out the ATM / STM (straight or angle) but what was the difference between the STM and STS?

Thanks

I believe the difference is magnesium frame (M) vs. steel (S).

The STM has a magnesium frame, the STS has aluminum.
Not sure about the optics, whether they changed anything or not.

There are few ounces in weight savings with the M being lighter. Glass is the same.

Lee

Thanks! I always wondered more about the glass than the weight but this helps alot!