how to prevent a blown out sky?

[molested_cow]

For close subjects, a flash. If you don't have a flash or don't want to, you can expose it to the sky, then try to recover it in RAW editing (won't always work).

For landscape or still subjects, HDR.

 

[Joel_W]

When I 1st read your post, I thought that HDR imaging would be the best way to handle your blown out sky issues. But after seeing your portraits, and since I don't like HDR portraits, your best options are either fill flash, or a GND filter. Either will work just fine.

You've gotten some great advice as to why you've blown out skies, and several methods to eleviate it. You just need to try a few methods and decide how you prefer to handle it when it pops up again.

The suggestion to checkout YouTube is an excellent one, as there are thousands of photography videos on virtually every topic. Also checkout Adorama's videos at their learning center.

As for sticking with just ISO 400, that's a major mistake. ISO is one part of the 3 leg exposure triangle. Learning how to use all three legs correctly, and to your advantage will make your life so much easier. Insisting to only use one ISO because a Professional Photographer does just doesn't make any sense.