How nomacs Viewer Synchronization Works

nomacs viewer synchronization lets you perform the same action — such as panning or zooming — across multiple open viewers at once, so two images move together instead of separately. It is built for side-by-side comparison: you open the images you want to compare, and any navigation you do in one viewer is mirrored in the other. This is useful whenever you need to inspect differences between two versions of an image at the same position and magnification.

What synchronization actually does

According to the nomacs site, "the same action (like panning, zooming, etc.) can be performed on multiple viewers, making comparing two images easy." The key idea is that the viewers stay in lockstep on navigation, not on content.

• Panning — move one image and the other moves to the same region.
• Zooming — change magnification in one viewer and the other matches it.

Because both viewers track the same view state, you can flip your attention between them without re-navigating each one. That is the whole point: your eyes do the comparing, not your mouse.

Why this matters for comparison work

Comparing two images by hand is error-prone. If you zoom one image to 200% and pan to a corner, then repeat the same steps on the second image, small mismatches creep in — slightly different zoom, slightly different position — and you can mistake a navigation difference for a real difference in the images.

Synchronization removes that class of mistake. Typical situations where it helps:

• Before/after edits — check what cropping, resizing, rotating, or a color adjustment actually changed.
• Two exports of the same source — for example, a RAW file next to its processed version, to judge how the conversion handled detail or color.
• Version comparison — spot what changed between two revisions of an image.

In each case the value comes from holding position and scale constant while only the image content varies.

How to use it

The nomacs site describes the capability but not the exact menu path, so the reliable approach is to work from the feature itself rather than a memorized sequence:

1. Open the two images you want to compare so both are visible as separate viewers.
2. Enable synchronization between the viewers. In nomacs this is the feature the site labels "Viewer synchronization."
3. Navigate in one viewer — pan, zoom, or both.
4. Verify the second viewer followed. If it did not, synchronization is not active for that pair; re-check the setting before continuing.

Expected result: both viewers show the same region at the same magnification, and stay that way as you keep navigating.

Common snags

• Only one viewer moves. Synchronization is a relationship between viewers, so it has to be active for the pair you are comparing. If you open a third image, check whether it joined the synchronized set or is behaving independently.
• Comparing at different zoom levels on purpose. Synchronization is designed to keep views matched. If you deliberately want one image at a different scale, you are working against the feature — turn it off for that comparison.
• Assuming it compares content. Synchronization aligns navigation, not pixels. It will not highlight or diff differences for you; it only guarantees you are looking at the same place in both images.

Where it fits in nomacs overall

Synchronization is one of three capabilities the nomacs site highlights, alongside image viewing and image editing:

So synchronization is the comparison layer on top of viewing and editing: you view or edit images, then use synchronized viewers to judge the result against a reference.

Who should care

If your work involves deciding whether two images differ — checking an edit, validating an export, or reviewing a revision — synchronization is the feature that makes that judgment reliable. If you only ever look at one image at a time, it adds nothing. nomacs itself is described as a free, open-source image viewer supporting multiple platforms, licensed under the GNU General Public License v3.0, with the current version listed as v3.22.0.