So what happens? When the mail is opened, the redirection service of Adobe Campaign registers the opening, then pushes back the empty pixel resource path. It is done for statistics metrics of course.
Then, how can be managed the time spent to read (or not) the mail ? On web sites, the usual tagging/tracking tools, such as Adobe Analytics, Google Analytics, AT Internet/Xiti, etc, try to manage the duration by either capturing the instant of the user changing URL through his browser, the ideal situation. It is not read time per se, but an estimation. And most often, they manage that over a specific duration without activity, the user has left the page. But it is a convention. An approximation. There are no ways to do an exact measurement in all the cases, except when the user browses to another page.
For email clients, it is more difficult, because emails clients don't push back information to the original source. If heavy client, or mobile app, and even for most of webmail clients, you have absolutely no information, for instance whenever people change of email records in their client.
Hence it is not a problem of Adobe Campaign, but a general concern.
However, Litmus email analytics tool try to manage a clever analytics measurement with pixels opening analysis algorithm (not public):
Tom, I don't know about pixel tracking being available in Campaign. Still, I'll ask if there are plans to enhance this. If that happens, that would probably be in AC Standard (not sure which version you use).