Similar to として, にして points to a certain standard (A) against which something else (B) is measured. The key distinction is that と often implies B is a logical outcome of A, whereas に suggests B is happening 'in the state of' A. This is reminiscent of the case marking particle に indicating a point in time or space. The addition of the adverbial particle は sets up the entire phrase (A) as a topic for contrast.

にしては is typically translated as 'for (A), (B)', or 'considering (A), (B) is...'. It can follow both nouns and verbs. Although it's composed of multiple grammatical elements, にして is generally treated as a single adverbial particle in modern usage.

• 彼女(かのじょ)は毎日(まいにち)ピアノを練習(れんしゅう)しているにしては、あまり上手(じょうず)じゃない。
  For someone who practices piano every day, she's not very good.
• 習(な)ったばかりにしては、発音(はつおん)がすごくきれいだね。
  Considering you just learned it, your pronunciation is beautiful.
• 冬(ふゆ)にしては今日(きょう)は暖(あたた)かいね。
  For it to be winter, it's pretty warm today.
• あの子(こ)は子供(こども)にしては難(むずか)しい言葉(ことば)をたくさん知(し)っている。
  For a child, he knows a lot of difficult words.

While 'considering (A), (B)' is a common translation, the core nuance of にしては is closer to 'while being (A), (B) is true'. This emphasizes that (A) and (B) exist concurrently, and more importantly, that (B) is not a direct consequence of (A) and may even contradict expectations.

• 安(やす)い値段(ねだん)にしては品質(ひんしつ)はいいけど、高(たか)いものと比(くら)べればやっぱり劣(おと)る。
  For such a low price, the quality is good, but of course it's inferior when compared to expensive items.
• 初(はじ)めてにしてはよくできたけど、プロと比(くら)べるとまだまだだね。
  For a first try she did very well, but compared to a professional, it's still a long way off.