Clients of the biggest ad agencies aren't yet enthusiastic about spending money on Web video ads, and a few things need to change before they embrace it, a panel of digital ad execs told the NewTeeVee conference in San Francisco.

The biggest barrier is time, as ad spenders and buyers are still trying to figure out where they want to spend money.

"Clients are used to working with a certain model. When money shifts to a new model, it takes time to adjust," says Jason Tsai, senior vice president at Universal McCann.

Brand advertisers are used to buying time for video ads on broadcast and cable networks, know how to measure it and are comfortable with spending dollars there.

By contrast, ad buyers are still trying to figure out production costs, units of measurement and what kinds of videos their ads will end up being paired with. 

One of the reasons so many Web advertisements are re-purposed TV ads is because the creative teams are skilled at making them efficiently, which buyers like.

That's not to say that lots of providers of content produced specifically on the Web aren't selling video ads. But the amount is still just a fraction of the ads produced and sold for broadcast and cable networks.

"There's an issue (among ad buyers) about production costs and whether you can get what you want for the money," said Mike Parker, director of digital strategy at Goodby Silverstein. A big part of his job is to help clients understand the digital landscape.

For ten years, there's been an evolution in what advertisers are comfortable with online. First, they had to get used to online banner ads, then search advertising. Now they're starting to explore social media ads and video advertising.

"I tell clients, you have to be comfortable with being uncomfortable. All video is not created equal. Professionally-produced, clients are comfortable putting their image next to. But you also have five-minute spots. Also, standards are key, said Jon Raj, chief digital officer for ad giant OMD.

The other thing holding back spending on video advertising is the lack of standards for length and form of the spots and metrics for measuring them.

"It comes down to measurement, they want to be able to measure. Until you get a standard, advertisers won't come in the way we all want them to," Raj says.