Can I take pictures from posters and slide presentations in a conference? If I do not ask for permission, is it acceptable to take pictures from posters and slide presentations in a conference by camera? Can I record a presentation?
In the US, audio recording is legal in some states (one-party consent states) and illegal in others (all-party consent states) without permission of everyone involved, so I wouldn't do it without asking. Some conferences or universities may have rules against it (so you could be removed from the premises of a conference or thrown out of a lecture by the professor), or you may find that the speaker finds it rude and confronts you or stops speaking. Neither is particularly professional behavior by you. In other countries, the rules probably vary even more. Also, just because it is legal, that doesn't mean that it will be seen as acceptable by the speaker or the audience.

I see audience members taking photographs of every slide shown at conferences all the time, but I find that speakers and audience members are frequently annoyed. When it happens to me as the speaker, I usually stop and offer to share the slides after the talk with anyone that wants them. That usually stops the photography and settles everyone down a bit.

Also, someone other than you owns the copyright in the slides or poster. I would avoid what is likely a copyright violation by photographing a slide or poster.

Edited to summarize: I wouldn't. People do it all the time. That doesn't make it OK. Get permission if you can or don't do it.