Accessibility is a hot topic these days, and alt text is one of its most significant building blocks. There are many comprehensive resources and tutorials out there, so I won’t get into what alt text is or how to write it (if you need an intro, start here: OSU Digital Accessibility – Alternative Text for Images). In this post, I’ll address a few issues where guidance is less clear-cut and that have come up in my conversations with instructors.
Does alt text have a character limit?
You’ve done the work and written a detailed alt text that you’re proud of. You hit “done” and, much to your frustration, the Canvas editor is flagging your image and saying: “Alt attribute text should not contain more than 120 characters.” What’s going on here? Is there really a limit, and why is it so?
Well, this is one of those things where you’ll find lots of conflicting information. Some people say that assistive devices only read the first 140 characters; others, the first 150; yet others argue there are no such limits with modern tech. See this article: 100, 150, or 200? Debunking the Alt text character limit, which has more info and references, including a nod to NASA’s famous alt text for the James Webb telescope images.
One thing is clear though: alt text should be short and sweet, to make it easy on the users. Keep the purpose in mind and address it as succinctly as you can. However, if your carefully written alt text still exceeds Canvas’s limit of 120 characters, don’t fret – that constraint is probably too restrictive anyway. But if the image is complex and needs a much longer description, use a different method (see more options below).
How should I use a long description?
When you have an image that contains a lot of information, such as a graph or a map, you need both alt text and a long description. The alt text is short (e.g., “Graph of employment trends 2025”), while the long description is detailed (e.g., it would describe the axes and bars, numbers etc.). The W3C Web Accessibility Initiative (WAI) – Complex Images Tutorial explains a few ways you can add a long description. The most common ones (and that I would recommend) are:
- Put the long description on a separate page or in a file and add a link to it next to the image.
- Put the long description on the same page in the text (under a special heading or simply in the main content) and include its location in the alt text (e.g., “Graph of employment trends 2025. Described under the heading Employment Trends”.)
The advantage of these methods is that everyone, not just people using assistive technologies, can access them. The description can benefit people with other disabilities or those who simply need more help understanding complex graphics.
But wait, what about image captions? Do they duplicate alt text?
Image captions can be used in various ways: as a short title for the picture, as related commentary, or as a full explanation (see an example of alt text vs. caption). In any case, avoid duplicating content between the caption and the alt text. If the caption doesn’t include a sufficient description, make sure you have that in the alt text. Alternatively, you can keep the alt text very short and use the caption for a longer description that everyone can read (I wouldn’t recommend very long ones, though – those may be better placed elsewhere, as described above).
For web pages, it’s best to add the caption using the <figcaption> element. This ensures that your caption is semantically linked to its image. If you like editing the HTML in your LMS, check out the W3Schools tutorial on the HTML <figcaption> Tag.
Should the alt text describe people’s gender, race, age etc.?
It really depends on what you are trying to convey and how much you know about the individuals in the image. Are those details significant? If yes, you should include them. Are you making any assumptions? Make sure not to project your own ideas about who the person is. This guide from University of Colorado Boulder: Identity and Inclusion in Alt Text is a great resource to refer to when faced with these decisions.
It’s 2026! Can’t I just get AI to write the alt text?
You’re right that AI tools can be a great help in writing alt text or long descriptions! We often recommend ASU’s Image Accessibility Creator. But, as you’re aware, LLMs are not always correct. Moreover, they don’t know what exactly you want your students to get from that image (well, you could tell them, but that may be as much effort as writing the alt text yourself…). Make sure you always check the output for accuracy and revise it to fit your purpose and context.



