The physics in one paragraph

Two things at impact decide where the ball goes. The face angle sets most of the start direction. The difference between the face and the path sets the curve. Face open to the path, the ball curves right. The bigger the gap, the harder it curves. A slice is a big gap. A fade is a small one. A ball that starts right and flies straight right isn't a slice at all, it's a push, and the fix for that is different. Club face to path has the diagram and the full table of shot shapes.

The driver slices worst because it has the least loft. Less loft means more of the face's sideways tilt turns into sidespin instead of backspin. The same swing with a 9-iron produces a gentle fade. With a driver it produces a banana.

The usual causes

A weak grip

Weak here means the hands are turned toward the target, so that looking down you see one knuckle or none on your lead hand. From there the face wants to return open, and no amount of swing work fixes it. This is the most common cause and the easiest to see on video from face-on: just pause at address and count knuckles.

An open setup

Shoulders pointing left of the target at address, often because the ball is too far forward in the stance, which drags the lead shoulder open. An open shoulder line sends the club across the ball from outside to in on the way down. Down-the-line video shows this immediately. A line across your shoulders at address should point roughly at the target, not twenty yards left of it.

Aiming left to allow for it

Everyone does this and it makes things worse. Aim left, and your body swings even more to the left, which opens the face even more relative to the path, which curves the ball even more. The slice gets bigger and you aim further left. On video, you'll see your feet and the ball flight pointing in opposite directions.

Coming over the top

The downswing starts with the shoulders and arms instead of the lower body, so the club gets thrown out and away from you and then cuts back across the ball. From behind, the club head comes down above the plane it went back on and swings out to the left through impact. This is the classic slicer's move and it's usually a response to the first three causes rather than the root of them. People come over the top because they're trying to get an open face back to the ball.

Hanging back

Weight stays on the back foot through impact, the club bottoms out early, and the face is still open when it gets there. Face-on video shows the head and hips well behind the ball at impact.

What it looks like on video

You can diagnose most of this with a phone on a tripod and ten swings. Put the phone behind you on the line to the target for the first five and in front of you, square to your chest, for the second five. How to film your golf swing with an iPhone covers the distance, height and camera settings, and down the line vs face-on covers which angle shows what. Then:

• From behind, at address, draw a line up the shaft. Watch the downswing in slow motion. A club head that comes back down above that line and swings left through impact is coming over the top. One that comes down on or under the line and out to the right isn't your problem.
• From behind, at address, look at a line across your toes and another across your shoulders. Both should point roughly parallel to the target line. Shoulders pointing left of the feet is the open setup.
• From the front, at address, pause and count the knuckles on your lead hand. One or zero is weak. Two to three is neutral to strong.
• From the front, at impact, compare where your head is to where it was at address. A head that's drifted back toward the trail foot is hanging back.
• The ball flight itself, traced across ten swings. Starting left and curving right is the full slice: face open to a path going left. Starting right and curving right is mostly a face problem.

Look at ten swings, not one. Any single swing can be a fluke. Ten swings with the club coming down above the shaft line every time is a pattern, and a pattern is something you can work on.

Things to try at the range

I'm not a teaching pro and this isn't a lesson. These are the things pros tend to try first, and they're the things you can check on video afterward to see whether they took.

• Strengthen the grip one knuckle. Turn your lead hand to the right on the grip until you see two or three knuckles, and set the trail hand so its palm faces the target. It'll feel strange for about two sessions.
• Aim square and let it slice. Set up parallel to the target for a whole bucket and watch where the ball goes. It'll go right. That's the point. You're removing the compensation so you can see the real problem, and often the slice shrinks on its own because the body stops swinging left.
• Move the ball back an inch or two with the irons and check the shoulders on video. A lot of open shoulder lines close up when the ball moves back.
• Feel the club head pass your hands through impact, as if you're trying to hook the ball into the next bay. You won't. You'll probably hit it straight. Slicers almost never release the club too much.
• Start the downswing from the ground. Feel the lead hip and the lead foot move first, before the arms. It's a feel, so check it on video. What it looks like is the club dropping onto or below the shaft line instead of over it.

One change per session. Film it. Compare the clips with last week's. If the club head is coming down closer to the line and the ball is starting closer to the target, keep going. If nothing changed in three sessions, book a lesson and bring the clips. A pro who can see ten of your swings in slow motion before you've hit a ball in front of them is a pro who's going to help you faster.

Questions

Is a fade the same as a slice?

Same shape, different size. A fade curves a few yards and finishes where you aimed. A slice curves a lot and finishes somewhere else. Plenty of very good players hit a fade on purpose and would never call it a slice.

Why do I only slice my driver?

Because the driver has the least loft and the longest shaft. Less loft means more sidespin from the same face angle, and the longer shaft makes it harder to square the face in time. Your irons are probably fading too, just not enough for you to notice.

Will a draw-biased driver fix it?

It'll help a bit. Drivers with the weight toward the heel close the face a little faster and take maybe ten yards of curve off. They don't fix a weak grip or an open setup, and those are usually the actual problem.

If I strengthen my grip will I start hooking?

Possibly, for a while, and that's progress. A hook means the face is finally closing. From there you back the grip off a touch. Going from a slice straight to a straight shot almost never happens.

I'm left-handed. Does any of this apply?

All of it, mirrored. Your slice curves left, your weak grip shows too few knuckles on your right hand, and over the top swings right through impact. Swap every left and right above.

Can an app measure my face angle?

Not from a phone video. That takes a launch monitor. What video shows is where the club came down, how you were set up, and where the ball started and curved, which is enough to tell you what kind of slice you have. Impact Reel cuts a range video into one clip per swing with lines you draw and a tracer you mark by hand, so the ten-swing comparison is quick. It doesn't measure anything and doesn't claim to. It shows you what happened, in slow motion, swing after swing.