How to Set Alt Text Focus Point for Mastodon Image Posts

When you upload an image to Mastodon, the platform automatically generates a thumbnail preview. The default crop often cuts off important parts of the picture, such as a face or a key object. Mastodon lets you set a focus point within the alt text editor to control which area of the image remains visible in previews. This article explains how to find and use the focus point feature in Mastodon’s web interface to improve your post thumbnails.

What the Focus Point Does and Why It Matters

Mastodon generates square or rectangular previews of attached images in timelines and profile galleries. Without a focus point, the platform centers the crop on the image’s geometric center. This can cut off faces, text, or logos that are positioned near the edges. The focus point tells Mastodon which pixel coordinates to treat as the center of the crop. The rest of the image is cropped outward from that point.

The feature is built into Mastodon’s alt text editor on the web interface. It does not require any third-party tools or browser extensions. The focus point is saved as metadata alongside the image in the post. Once set, it affects how the image appears in all Mastodon clients that support the focus point property, including official apps for iOS and Android. Third-party apps may ignore the focus point if they do not implement the Mastodon API for image cropping.

Prerequisites

You need a Mastodon account on any instance that runs Mastodon version 4.0 or later. The focus point editor appears only on the web version of Mastodon. Mobile apps do not provide a way to set the focus point during post composition. You must use a desktop browser such as Chrome, Firefox, Edge, or Safari. The image file must be in a supported format: JPEG, PNG, GIF, or WebP. The maximum file size depends on your instance’s limit, typically 8 MB or 16 MB.

Steps to Set the Focus Point for a New Image Post

Follow these steps on the Mastodon web interface to attach an image and set its focus point before publishing your post.

1. Open the compose box
   Log in to your Mastodon instance in a desktop browser. Click inside the compose text area at the top of the home timeline.
2. Upload the image
   Click the image icon in the compose box toolbar. It looks like a small picture frame. Select a file from your computer. The image appears as a thumbnail below the compose box.
3. Open the alt text editor
   Click the alt text button on the image thumbnail. The button label says “Edit” or shows a pencil icon. The alt text editor dialog opens.
4. Type the alt text description
   In the text field, enter a meaningful description of the image. This text is read by screen readers. The focus point feature does not require alt text to be present, but accessibility best practice is to always include it.
5. Locate the focus point circle
   Look at the small preview of the image inside the alt text editor. A translucent circle with a crosshair appears on the image. By default, the circle is positioned at the center.
6. Drag the focus point to the key area
   Click and hold the circle. Drag it to the most important part of the image. For example, drag it to a person’s face, a product logo, or a text headline. Release the mouse button. The circle stays at the new position.
7. Confirm the focus point
   Click the “Apply” or “Save” button in the alt text editor. The dialog closes. The thumbnail in the compose box now shows a preview based on the new focus point. If the preview still looks wrong, repeat steps 3 through 6.
8. Post the message
   Add any text you want to your post. Click the “Publish” button. The image appears in the timeline with the crop centered on the focus point you set.

What to Avoid and Common Mistakes

Several pitfalls can prevent the focus point from working as expected. Understanding these issues helps you avoid them.

Focus Point Not Saved After Closing the Editor

If you close the alt text editor without clicking “Apply” or “Save,” Mastodon discards the focus point. Always confirm the change before closing the dialog. If the thumbnail in the compose box returns to the default center crop, reopen the editor and drag the circle again.

Focus Point Has No Effect in Some Clients

The focus point is part of the Mastodon API. Not every third-party app reads this metadata. If you use an app like Tusky, Fedilab, or Metatext, the crop may fall back to the default center. This is not a bug. It is a limitation of the app’s implementation. To ensure the best appearance for all viewers, compose posts with the focus point on the web interface and test the result in the official Mastodon app.

Image Too Small to Benefit From a Focus Point

Images smaller than the timeline thumbnail dimensions, typically 400×400 pixels, are not cropped at all. Mastodon displays them at their original size. Setting a focus point on a small image has no visible effect. Use images with a minimum width and height of 600 pixels to see the crop behavior.

Mastodon Alt Text Focus Point vs Default Crop

Item	Focus Point Set	Default Center Crop
Thumbnail center	User-defined point	Geometric center of the image
Visibility of edge content	Preserves the area around the focus point	May cut off faces, text, or logos near edges
Supported clients	Mastodon web, official mobile apps, and API-compliant third-party apps	All clients
Setup effort	Requires dragging the circle in the alt text editor	Automatic, no user action needed

You can now control how Mastodon crops your image previews by setting a focus point in the alt text editor. This feature works best for images with a clear subject that is not centered. Next time you upload a photo with a face or a product logo, drag the focus point to that area before publishing. For advanced control, consider editing the image outside Mastodon to place the subject in the center of the frame, which removes the need for a focus point entirely.