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Zoomed Callout Circle to Circle Line

Name: SciFig
Author: SciFig

A clean zoomed callout circle with a leader line connecting a detail region to a magnified circle — drop it onto any figure to highlight small structures.

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Core Subject (e.g., Cas9 protein cutting DNA)

Action / Details (e.g., Double strand break, detailed molecular view)

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What is Zoomed Callout Circle to Circle Line?

A zoomed callout circle to circle line is an annotation element that magnifies a small region of a figure: a circle marks the area of interest, a leader line connects it to a larger circle, and the larger circle shows the enlarged detail. It is used in micrographs, schematics, and screenshots to point readers at fine structures without cropping the image. With SciFig you describe the callout in plain language and generate a clean, editable zoomed callout circle, ready to relabel and export.

Why this annotation exists

It resolves the conflict every micrograph has: the reader needs the wide field for context and the fine detail for evidence, and a single magnification cannot supply both. The callout keeps them on one panel.
It is honest in a way that a cropped inset is not — the source circle stays in place on the overview, so the reader can see exactly where the magnified region came from and that it was not cherry-picked from off-frame.
It preserves the figure's spatial argument. Cropping to the detail alone discards the anatomical or structural context that often carries half the claim.
A leader line makes the correspondence unambiguous. An unconnected inset floating in a corner forces the reader to hunt for the source region, and in dense images they frequently guess wrong.
It saves panel real estate: one image plus a callout replaces a two-panel A/B layout and its second caption.

How to construct it

The source circle: drawn on the overview at the true boundary of the magnified region, not larger and not approximate. Its size must be consistent with the stated magnification factor.
The destination circle: the magnified crop, masked to a circle, sized large enough that the feature of interest is actually resolvable at print size.
The connector — either a single leader line between circle centres/edges, or the two tangent lines that form a cone or frustum. The tangent-pair reads as a projection and is the clearer choice when the magnification factor is large.
A ring stroke on both circles, same colour and weight, chosen to contrast with the underlying image (white with a thin dark outline survives on both bright and dark micrographs).
Two scale bars, not one. The overview and the magnified circle have different pixel sizes; carrying the overview's bar into the inset is a real and frequently published error. Alternatively state the fold-change (e.g. 4×) explicitly.
Optional: a fill or slight desaturation of the overview outside the source circle to push the callout forward — used sparingly, since altering image tone in a data panel has integrity implications and must be disclosed if applied non-uniformly.

Where it is used

Microscopy figures: a low-power overview of a tissue section with a high-power circle on a single cell, a synapse, a nucleus, or a labelled structure.
Electron microscopy, where the magnification span between overview and detail is largest and the callout is often the only way to bridge it.
Anatomical and clinical illustration — an organ-level drawing with a magnified view of the tissue, valve leaflet, or lesion in question.
Materials and structural figures: a device schematic with an enlarged view of a junction, an interface, or a surface feature.
Software and UI documentation, screenshots in supplementary methods, and instrument diagrams where one small control or port needs to be identified.
Posters and teaching slides, where the viewing distance is long and small features must be lifted out to be seen at all.