Is this field brightening normal?

[parallaxerr]

I received my new Helios Apollo 15x70 bins yesterday and first impressions are very nice.

I took a quick look through them this afternoon however, and notice some very prominent brightening in the lower half of the field. I assume it's the sky brightness, before being inverted by the prism.

They're primarily for astro so not a big problem if this goes away at night, but I do enjoy some occasional daytime terrestrial viewing. Is this normal? I've circled it in red in the close up image below.

 

Edited December 6, 2019 by parallaxerr