Addicks boss Lee Bowyer said of Field: "He's a very good player with Premier League experience. He's young, wants to learn and he works hard.
"He knows what is expected coming here, I've told him that we'll improve him. I'm looking forward to working with him. He seems a really nice lad and he wants to learn."
On Leko, Bowyer told Charlton's website: "I'm excited to work with Jonathan because he has a bit of everything. He's definitely something we haven't got here.
"He's going to be exciting to watch. When I've watched him, he got me excited and he had me on the edge of the chair and if he's doing that to me, he'll be doing that to all our fans because every time he gets the ball you think 'go on then, what are you going to do with it this time?'
"He goes past people with ease, he has great pace, he is going to hurt people in behind, so I'm definitely looking forward to working with him."