Is consent required by law to take a picture or to record audio / video of a conference talk?
Assume that the conference does not explicitly prohibit unauthorized audio and visual recordings of the presentations, and ignoring ethical/political/any other non-legal issues.
I am mostly interested in the following locations:
Yes
The presentation of an artistic work such as a conference presentation is covered by copyright. The copying by means of audio or visual recording is subject to the normal rules of copyright, either you need the presenter's/organiser's permission (copyright probably vests in multiple people) or the usage must be fair use (USA) or fair dealing (other jurisdictions).
Still photography would be OK provided you did not capture any Powerpoint slides etc. which are also copyright.
Notwithstanding, see other questions on this site regarding photography/video/audio recording. I won't list them since you can do your own search.
