As an undergrad, I took a wide variety of courses, but focused on one or two fields and took advanced courses only in those, and those fields are my strengths. I intend to do a PhD, but now I'm applying for Master's programs, and I actually want to focus on the fields that I received little to none education in my undergrad on, and the subjects that I wasn't very good at. What I want to do for my PhD is more directly related to the subjects I'm stronger at, but a knowledge in those I'm not very strong in would be significantly useful too.
To me, that sounds totally acceptable, since I want to fill in the gaps and learn something new, rather than repeating what I already have a good knowledge on and possibly going a little deeper in it, especially as I took quite a few graduate-level courses in the fields I focused on.
I am wondering if that's acceptable and sounds reasonable to admission committee and graduate schools too. Given that they probably have applicants who are actually applying for what they have a strong background in, would it make sense for me to compete with them with this justification that I want to improve the weaknesses in my background by throwing myself at those fields and "learn to love them", or should I just apply for what I'm stronger in and try to improve my knowledge in other fields through self studying?
(Please note that by "field", I'm actually referring to subfields within a major field. So it's not that I've done Management and I want to study Dance Performance, it's more like PDEs vs Algebra, which would mean applying to Applied Math vs Pure Math.)