Alpha on already transparent images..

[dognose]

I'm trying make an image with transparency more see though.
I'm using
convert image.png -alpha on -channel a -evaluate set 50% out.png

However, it removes the areas that were completely transparent and makes them 50%..
So, I'm losing the mask..

How can I keep the mask and bump up the rest of the alpha?

[snibgo]

Instead of setting the alpha, you should multiply it, eg by 0.5.

[dognose]

-evaluate multiply 0.5

works great, THANKS!