I have straight edges when using some brushes

[stevec5501051]

I am trying to create a smoke portrait but when using the smoke brushes I get a straight edge on two sides which spoils the effect. Can I get rid of this? 

[c.pfaffenbichler]

Please post a meaningful screenshot including all pertinent Panels (Options, Brush Settings, Brushes, …). 

Wjat does the actual Brush Tip look like? 

[manal shanableh]

when you define a brush, do not define it as square, select using magic wand tool for example, to select the smoke, then select> similer, you can repeat this to have more details from the smoke, then apply feather to selection, the define the brush.

please not the selection in my example.

there is many ways to do.

[stevec5501051]

Many thanks for your help