Write image captions and alt text for different jobs

A caption and an alt attribute may sit beside the same photograph, but they do different work. Copying one into the other without considering the image's purpose can create repetition or leave important meaning unexplained. Start with what the image contributes to this particular page.

Identify the image's job

W3C's image guidance distinguishes informative images, decorative images and images that perform a function. An informative image needs an alternative that conveys its relevant information; a decorative image normally uses an empty alt attribute. For an image used as a link or control, the alternative should identify the function. The correct choice depends on context, not simply on the file itself.

Consider an illustrative photograph of a notebook beside an article about drafting. If it adds only atmosphere, a long description of the pen, desk and window may not help. If the photograph demonstrates a particular annotation method discussed in the text, those annotations become the important information. The same image can therefore need different treatment in different uses.

Use the caption for context

A caption can identify a person, explain when a photograph was taken, clarify that a scene is illustrative or connect the image to the argument. It can also carry required credit information. Do not use a caption to imply that a stock photograph depicts your staff, office or customer when it does not.

For a hypothetical article comparing two document layouts, a caption might explain which revision is shown and why it matters. The alternative text should convey the relevant visual distinction, or direct attention to a fuller explanation available in the page where appropriate. It should not merely repeat ‘document layout’ because that is the filename.

Review the combination

Read the surrounding paragraph, caption and proposed alternative together. Remove unnecessary repetition while preserving the information needed to understand the page. Complex charts or diagrams may need a fuller textual explanation than a short alt attribute can provide; plan that explanation as part of the content rather than treating it as an afterthought.

Avoid stuffing alternative text with search phrases, promotional language or every visible object. Describe what matters to the image's role. Have someone review the actual implementation as well as the wording, because an accurate description in an editorial document does not guarantee that it appears correctly in the published page.

The editorial question is simple even when the implementation needs care: what would be lost if this image were unavailable, and where should that information be supplied? Answering it produces better captions and alternatives than applying one stock formula to every photograph.