Diopter for my 17-40, do I really need one?

Hi all,

Been using a 17-40 on my 5D in Aquatica housing. Been hearing great things about using a B+W +4 Diopter on the lens for increased sharpness. First of all, is it really unsharp on the corners? Never thought so, but on the other side I always thought my 15mm pics looked sharper.

Second, does it loose depth of field and reduce the angle with the lens wide open (at 17mm)?

Thank´s, Nuno.

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