Slow-motion footage is the most useful diagnostic a club player has access to, and the most commonly misused. A phone shoots at a high enough frame rate to freeze a contact point, which means anyone can now see what only coaches with specialist equipment could see twenty years ago. What the footage will not do is tell you which of the things you can now see actually matters.
What to film, and from where
Two angles answer most questions. From directly behind the baseline, roughly in line with the player, you can see swing path, contact height relative to the body, and whether the ball is being met in front or beside. From the side, level with the contact zone, you can see the kinetic chain, whether the legs and hips lead or the arm goes first, and how much the trunk rotates.
Film from a tripod or a fence, not from a hand. Camera movement makes frame-by-frame comparison useless, because you cannot tell whether the body moved or the frame did.
Frame rate matters more than resolution
A ball leaving the strings at club pace travels several metres in the time a standard 30fps camera takes one frame, so at that rate contact is simply not captured. Anything shooting at 120fps or above will place a frame close enough to contact to be worth studying, and most phones offer it. Resolution is largely irrelevant for this purpose; a low-resolution frame at the right instant beats a sharp one taken a fifteenth of a second too late.
The mistake almost everyone makes
Comparing a single frame of their own stroke against a single frame of a professional's. Still images show positions clearly and sequence poorly, and it is the sequence that produces the shot. A player whose frame at contact looks wrong may have arrived there by a sound sequence and simply be half a frame late; a player whose contact frame looks correct may have got there by leading with the arm. What is worth comparing is the order in which things happen and the timing between them.
The corollary is that the useful comparison is usually against your own footage from a month ago, not against someone whose body and grip differ from yours.
What footage will not show you
Anything about decision-making. Video shows a stroke in isolation, stripped of the context that made it the right or wrong shot to play. A technically flawed forehand hit at the correct moment beats a beautiful one hit from a position that never should have been reached, and no camera angle reveals that.
It also flatters practice. Strokes filmed on a fed ball look substantially better than the same strokes under pressure, which is why footage taken in a match, even badly framed, from the fence, is more diagnostic than anything shot in a lesson.
A workable routine
Film one stroke per session rather than everything. Take twenty repetitions, pick the three that felt best and the three that felt worst, and look only at what differs between those two sets. That comparison is far more informative than measuring either against an ideal, because it isolates the variable the player can actually feel, which is the one they can change.
Comparing footage over time
The most valuable use of video is longitudinal and almost nobody does it. Filming the same stroke from the same angle once a month, and keeping the files, gives a player the only reliable evidence about whether a change actually took. Memory is unreliable here. A stroke that now feels natural feels as though it always did, and the footage from before the change is the only thing that disagrees.
It also settles the recurring question of whether a plateau is real. A player convinced they have not improved in a year is usually wrong in one direction or the other, and six clips answer it.
Common questions
Which two camera angles answer most questions?
Directly behind the baseline in line with the player, which shows swing path and contact height, and side-on, which shows the contact point relative to the body. Between them they cover most technical questions.
Does frame rate matter more than resolution?
Yes, by a wide margin. A ball leaves the strings fast enough that at 30fps contact falls between frames entirely. A sharp image of the wrong moment tells you nothing.
What is the commonest mistake with video?
Comparing one frame of your own stroke with one frame of a professional's. Stills show position clearly and sequence badly, and almost every technical problem is a sequencing problem.
What will footage never show you?
Decision-making. Video shows the stroke stripped of the situation that made it the right or wrong choice, so a technically clean shot that was tactically wrong looks perfect on camera.