These fatties are best to really help open out the eye and amplify lashes—the densely-packed bristles deposit product at the base, which thickens the bottom of the lashes and makes the lash line look fuller.
Pro tip: Position the brush at the base and really wiggle it through the lashes to max out their potential.
Big love: Bobbi Brown Eye Opening Mascara and M.A.C False Lashes

The teeth-like combs in these wands deposit less product on the lashes, so instead of bulking them out at the roots, it pulls the hair though the brush, thereby lengthening the lash. The wider spaced bristles also help define individual lashes.
Pro tip: Instead of pulling the wand straight up and through the lashes, use a diagonal movement. Wiggle the brush at the base of the lash line and when you pull the wand along the lashes, pull towards the outer corner of each eye. So, if you’re working on your right eye, drag the brush out to the right instead of straight up. That helps the lashes set in a cat-eye shape.
Big love: Ciate London Triple Shot Mascara, Clinique High Impact Volume Mascara and Estée Lauder Sumptuous Infinite Daring Length + Volume Mascara

The best thing you can do for curved lashes is to invest in an eyelash curler (try M.A.C Full Lash Curler). The second best is to get a mascara with a curved wand to give your lashes a bit of a bend.
Pro tip: run the curved wand through the lashes a couple of times to coat them with mascara. Then, draw the wand halfway through the lashes again, and instead of brushing it all the way out, hold the wand with the lashes engaged in it for a few seconds to set the curl.
Big love: Clinique High Impact Curling Mascara

Slimmer brushes let you to get closer to the eye, allowing the bristles to reach out and grab the shortest of hairs, defining and thickening them. On those with longer lashes, these wands give a more natural look and help separate each lash.
Pro tip: Avoid oversized brushes, as those will not be able to target shorter hairs.
Big love: M.A.C Extended Play Lash and Bobbi Brown Intensifying Long-Wear Mascara