EDIT

The editor

Everything here is non-destructive. The photograph on screen is the original plus a recipe, recomputed every time; the recipe is what gets saved, copied, pasted and put in a preset.

The Lensmith editor with the Edits tab open: active edits, a snapshot and the history list beside the photograph. — The Edit module with the Edits tab open: what this photograph is currently wearing, the snapshot saved from it, and the history behind it.

The scope and the readout. Histogram, waveform or RGB parade, with the pixel under the pointer read out beside it — R, G, B and L*, in Display P3, the working space.
Tools · Edits. The panels, or this: the list of what is on the photograph rather than the controls that put it there.
Active edits. One row per section, each with its own switch. Turn a section off to see what it was doing; nothing is lost.
Snapshots. A named look you can come back to, with a thumbnail of what it was.
History. Every change as its own entry, newest first. Click one to go back to it; it lasts as long as the app is open.
The foot of the canvas. Rating, zoom, and the before/after switch — or just press and hold anywhere on the photograph.

Working across photographs

Copy the current settings: ⇧⌘C
Paste them onto the selection: ⇧⌘V
Synchronize a chosen set across the selection: ⌥⇧⌘S
Undo · redo: ⌘Z⇧⌘Z

Synchronize is the one that asks: it lets you send the white balance and nothing else, or the whole develop but not the crop. Undo is per photograph and lasts as long as the app is open.

A virtual copy is the other way to keep two readings of one file — a colour version and a black-and-white one — without copying a byte.

Seeing what you are doing

Before / after: Press and hold anywhere on the photograph to peek at the original. \ toggles it and Y cycles the split and side-by-side modes.
Zoom: Z or Space toggles fit and 100%, ⌘+ and ⌘− step it. Zoomed in, only the visible window is re-rendered at the resolution the zoom asks for, so 100% is sharp without the wait — and zooming back out is instant, because nothing was thrown away.
Crop preview: R opens Crop & Straighten from anywhere.

The scopes and the readout

Pinned at the top of the inspector, on every surface that borrows that column — the tools, a layer's settings, the preset browser — because they read the photograph rather than the tool, and losing them on a surface swap would take away the exposure reading exactly while you are changing the exposure.

Histogram: How the light is distributed.
Waveform: Where in the frame it sits. A panorama seam, a graduated filter a few degrees off, a sky a third of a stop brighter on one side — all invisible in a histogram and obvious here.
RGB parade: A cast that changes across the frame, which the global white balance cannot fix and a linear-gradient mask can.
Pixel readout: The value under the pointer, in Display P3, with L* and a neutral indicator beside it — 250 and recoverable, or 255 and gone.

One at a time, because the column has room for one plot and nobody reads four graphs at once.

Edits, history, snapshots

Three lists, in the tab beside the tools.

Edits: What is currently on the photograph, tool by tool. Each row can be switched off or removed, which is the quickest way to ask "what is this one doing?"
History: Every step you took, in order, back to the import.
Snapshots: The states you named yourself — the way to keep a version you can come back to without making a virtual copy.