The whole picture is flat, not one corner.

Then taps are the wrong instrument and no number of them will get there. A flat frame is a tone problem across every pixel at once, and one pass over the whole thing does more than four placed lifts ever will — [enhancepic.com](https://enhancepic.com/) is the page for that, and it needs one press.

Half the frame is fine and half is heavy.

That is the awkward middle. Try one lift in the middle of the heavy half and drag the seam. If the seam shows a soft circle rather than a fixed half, the fault is bigger than a patch and belongs to a whole-frame pass.

I can see the problem but I cannot point at it.

That is the test, and it is a reliable one. Anything you cannot put a fingertip on is not local, whatever it looks like.

Reading

One spot, or the whole picture

The head of this subject is a question, not an instruction. Here is a thirty-second way to answer it before you spend six taps finding out.

The test

Open the file. Do not tap anything. Drag the seam to the far right so the untouched original fills the stage, and then ask one question: can you put a fingertip on the problem?

If the answer is yes, and the thing under your finger is smaller than the fingertip, this page is the right one. If the answer is no — if the problem is somewhere between everywhere and nowhere — it is a global fault and placed fixes will chase it around the frame without ever catching it.

What local looks like

A dark speck on a pale sky that stays in the same coordinates across several frames.
A pole tip, a cable end or somebody's finger touching one border of the frame.
One corner in shade because the photographer stood between the subject and the light.
A raindrop on the glass in front of the lens.

What global looks like

The whole frame reads grey and no part of it is blacker or whiter than the rest.
The colour is off in the same direction everywhere, so white things are not white anywhere.
The dark parts hold nothing, in every quadrant, and the bright parts hold nothing either.
Everything is soft, including the things you focused on.

The tool makes this call itself when it can. When the median brightness of the working copy is low and no single quadrant accounts for it, a line appears above the stage saying so, and naming the kind of page that will do better.

The awkward cases

Two situations sit on the line. A photograph with one bright window and one dark room is not global — it is two local areas with a hard edge between them, and a soft placed lift cannot follow that edge. A photograph with several small marks scattered across it is local six times over, and the ceiling here is six, which is a deliberate way of saying that a seventh mark means the problem has changed character.

Answers

The whole picture is flat, not one corner.: Then taps are the wrong instrument and no number of them will get there. A flat frame is a tone problem across every pixel at once, and one pass over the whole thing does more than four placed lifts ever will — enhancepic.com is the page for that, and it needs one press.
Half the frame is fine and half is heavy.: That is the awkward middle. Try one lift in the middle of the heavy half and drag the seam. If the seam shows a soft circle rather than a fixed half, the fault is bigger than a patch and belongs to a whole-frame pass.
I can see the problem but I cannot point at it.: That is the test, and it is a reliable one. Anything you cannot put a fingertip on is not local, whatever it looks like.