スマホで撮影したとは思えないほどきれいな写真って英語でなんて言うの？

"The photos are so good you wouldn't think that they were taken on a smartphone."
"the photos" 「写真は」
"are so good that ~" 「〜ほど良い」"are so beautiful that ~「〜ほど綺麗」も言えます。
"you wouldn't think that ~" 「〜とは思えない」
"they were taken on a smartphone" 「スマホで撮影した」

この場合、次のような言い方ができます。

ーI can't believe you took all those pictures with your smartphone.
「あの写真全部スマホで撮影したなんてすごいですね」
can't believe で「信じられない・すごい」
to take pictures with one's smartphone で「スマホで写真を撮る」

ーThose pictures were really beautiful. It's hard to believe they were taken with a smartphone.
「写真、とても綺麗でした。スマホで撮ったなんて信じられないです」
It's hard to believe で「信じがたい・信じられない」

ご参考まで！