worth buying the sony 50mm e mount 1.8 as alternative lens to 18-55 kit lens

Video only requires about 1080 lines of resolution for 1080p.  Any Sony lens will be plenty good at that (even the worst rated 16mm f/2.8).  For example the NEX 7 for photos requires 4000 lines of resolution (it is a 6000x4000 pixel sensor), or 4 times as many lines.  HD Video is only a couple megapixels per frame vs 24 MP for the NEX 7 sensor.

The reason to buy the Sony 50mm f/1.8 is if you want shallow depth of field.  The kit lens is about f/5.6 at the same field of view, from f/5.6 to f/4 to f/2.8 to f/2 to f/1.8 is about 3.5 f/stops.  This means it lets in a lot more light set at f/1.8 and will also make the background more blurred.

If you are just shooting video at f/5.6 there wouldn't be much difference between the two.

I prefer primes, so I would get the Sigma 19 and 30mm f/2.8 kit for $200 on BH Photo, and then the Sony 50mm f/1.8 for $300 or whatever it is now.  However, the 18-55mm will be fine too if you mostly shoot video and don't need as shallow depth of field.

Eric

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