If you're using a 50mm, F1.4/1.8 lens, I suggest u drop the f.stop to
F2.8. The pictures will turn out sharper (Even at the focus point). I think 50mm sharpest f.stop is
F4-F8. For most of the lens, the widest aperture is not the sharpest. Depending on the lens, it gets shaper as f.stop goes lower and looses sharpness nearer to the largest f.stops, ie. f22-f32. -This does not apply to expensive lens and Macro lens!-
For filling in of the models.
During daytime, It is not advisable to use a flash because it cannot help much. What i suggest for you when you're shooting models is to carry a
reflector with you. It is the best way to fill in by using the available light and reflect back onto the model. However, you'll need a helper for that case.
For reflectors, you do not have to get the round reflectors most photographers uses. For my case, I go cheapo by using a huge white cardboard bought from Popular. $2 and effective

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Thats my 2-cents~ Sorry if I am wrong.
Zee,
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I draw with light. Photography