Passing Signals Within a Large Group of Canonicalized Pages Takes Time
If a large group of pages is included within a canonicalized set, Google will still have to crawl and process all of these pages to find the canonical and pass on any link equity if the canonicalised pages are linked to externally.
Canonicalizing Paginated Pages Back to Main Page Can Cause Crawling & Indexing Issues
Canonicalizing pages in a paginated set back to the first page can be problematic because Google may see that these are different pages and ignore the rel canonical. Alternatively, if Google does follow the rel canonical to the main page, this could mean links and content on other pages might be missed.
Hreflang is a Minor Canonicalization Signal
Google uses hreflang as a small signal when picking the canonical version of a page, but this needs to be backed up by consistent signals provided by rel canonicals, internal linking and sitemap files.
Implement Clear Canonical Version Per Country & Hreflang Between Equivalent Canonical Versions
Don’t specify a canonical between different country versions of a page, because Google will likely only index the preferred version. John recommends having a clear canonical version per country and implementing hreflang between the canonical versions for each country.
Google Doesn’t Support Canonical Tags for Images
Canonical tags don’t work for images, so you can’t fold together different sizes of an image to make sure only the best size is indexed, for example. Google is looking into how best to use srcset for image search, however.
Canonical Selection Doesn’t Affect Rankings For a Group of Pages
Google will simply show one URL out of a group of pages that have been canonicalised. The ranking will be the same for that group of URLs no matter which one Google decides to show.
Different Signals Determine Google’s Canonical Selection
John confirmed that rel canonical, redirects, internal linking, URL parameters and sitemaps are all signals Google uses to decide which page is the canonical from a group of pages that have been folded together.
Canonicalise Duplicate Pages Between Your Sites so They’re Not Seen as Doorway Pages
Use the canonical tag if you are offering the same products on lots of different sites so Google doesn’t suspect that these are doorway pages.