I am just totally confused about the rule sign of convex and concave lenses. The general formula: $$1/v-1/u=1/f.$$
Is okay but when solving problem sums sometimes $v$ becomes negative sometimes $u$ and many times $f$. Sometimes both $v$ and $f$ becomes negative. I just can't mug up why this is happening.
Can anyone please explain this to me.
$v$ = image distance $u$ = object distance $f$ = focal length of lens.