You pick up your phone to check whether a message came in. A while later you are still holding it, and you never read the message.
Wallpaper is not the cause of that, and no wallpaper fixes it on its own. But the lock screen is the first thing in the chain, and one of the few parts of a phone you fully control. A picture that gives your eye nowhere to stop stays out of the way.
This is about what makes an image easy to look past: low contrast, slow movement, and nothing in it that starts a thought.
What "distracting" actually means here
A distracting wallpaper is not simply a loud one. It is one that competes with whatever you unlocked the phone for, and two properties do most of that competing.
The first is contrast. Your eye is drawn to edges, and an edge is anywhere light meets dark. A photo with hundreds of small edges offers hundreds of small invitations, and reading white text over it takes a fraction more effort each time.
The second is meaning. A photo of a person, a street you know, or a trip you took is a prompt. You look and something starts: a memory, a plan, a message you meant to answer. An anonymous stretch of water has nothing to say, which is the point.
Contrast is what your eye chases
Colour matters less here than it might seem. A deep blue and a deep teal side by side are a large colour difference and a small brightness difference, so the boundary is soft and your eye does not lock on to it. Black next to white is a small colour difference and an enormous brightness difference, and that edge cannot be ignored.
This is why a dark underwater scene can be full of colour and still read as calm, while a black and white city photo can be tiring. To judge a wallpaper, squint at it. That removes fine detail and leaves the brightness structure. If the squinted version is a soft gradient, it will sit quietly under your clock. If it is a scatter of bright dots, it will not.
Slow motion is ignorable, sudden motion is not
Movement at the edge of your vision is handled differently from movement you look straight at. Sudden appearance, and sharp changes of speed or direction, get flagged for attention. Useful for noticing something approaching you, unhelpful when you are reading a bank alert.
Movement that is slow, continuous and roughly predictable does not get flagged the same way. A jellyfish pulsing, a school drifting through a shallow arc, light shifting across sand: these register as ambient. A fish darting the full width of the screen does not. So the question is not whether a wallpaper moves, but how fast, how far and how sharply. In Aquarium 3D: Live Wallpapers, the difference between the busy reef scenes and the deep sea ones is largely that: same idea, different speed.
Faces and places carry a story; water does not
There is a reason a quieter phone usually means an abstract or natural background. Photographs of your own life are meaningful, and meaning is what you do not want while checking a time.
Water is unusually good at this. It is interesting enough that you do not resent looking at it, and it carries no obligation. It does not remind you of anyone, and there is nothing in it to plan around.
Building a lock screen you can read in half a second
Keep the top third simple, because the clock and any widgets sit there and neither gets a shaded backing plate. Keep the middle band calm too, since notifications stack there through the day. Put whatever you want to look at in the lower half.
Avoid a bright object near a screen edge, since that is where peripheral vision is most sensitive. And judge the result with your notification stack full rather than empty, because a picker preview shows the best case.
Give the calm wallpaper a Focus, not your whole day
The change that sticks is the automatic one. Build a plain, dark lock screen and attach it to a Work Focus, then keep your busier picture on a second lock screen for the evening. Touch and hold the lock screen while the phone is unlocked, swipe to the screen you want, and tap the Focus button below it. Turning that Focus on then switches the wallpaper for you, and the change of picture becomes a signal that work has started.
Where a calm wallpaper does not help
A wallpaper does not reduce the number of times your phone asks for you. If you unlock and open something immediately, the background you passed over was never the problem.
The controls that change that are elsewhere: Focus modes with an allowed-notification list, the scheduled summary that batches non-urgent alerts, turning off badges, and moving the worst offender off your first home screen page so opening it takes a search rather than a thumb reflex. A quiet wallpaper improves every glance a little. It is not a replacement for changing what interrupts you.
Quick answers
Does a dark wallpaper save battery?
On an OLED iPhone, a little. Black pixels are not lit, so a mostly dark image draws less power than a bright one at the same brightness. On LCD models such as iPhone XR, iPhone 11 and the older SE, the backlight is on regardless.
Is an animated wallpaper worse for focus than a still one?
It depends on the speed. Slow, continuous motion tends to be ignorable, while anything that darts or appears suddenly is not. If unsure, use a still frame during working hours and keep the motion on a second lock screen.
Should I just use a plain colour?
You can, and it is the least distracting option there is. It also tends to feel unfinished, and a wallpaper you dislike gets changed quickly. A dark underwater scene sits in between: quiet enough to ignore, interesting enough to keep.
What about the home screen?
The home screen does not animate on iPhone, so choose it purely for how icons read against it. Blurring the lock screen image, an option offered when you customise the pair, is usually safest: it keeps the colour and removes the detail that fights with icon labels.
Where to get it
If you want a quieter lock screen without spending an evening cropping footage, Aquarium 3D: Live Wallpapers has slow underwater scenes framed for a phone, including deep sea and jellyfish views that stay dark behind the clock, with a preview so you can watch the motion first.
Choose the slowest scene you still find interesting. That is usually the one you keep.