There is currently not enough evidence to support the effectiveness of face shields for source control. Therefore, CDC does not currently recommend use of face shields as a substitute for masks.
They go on to give examples where face masks aren't suitable, and advice on how to use as face shield in those circumstances.
An ABC7 News article from July 4, 2020 gives similar advice from the CDC:
Officials at the Centers for Disease Control and Prevention were asked if shields are as good as masks.
"I don't think we have data yet that a shield is better or worse," Dr. Anne Schucat with the CDC said. "We are recommending for the consumer, we are saying the cloth face mask is what we are recommending."
They also quoted an expert saying that face shields were as effective:
Some users claim face shields are just as good at keeping you safe as the cloth alternative.
"I think face shields are a great option. One great advantage this time of year is that they aren't as hot as a face mask is," Dr. Michael Edmond, Professor of Infectious Diseases at University of Iowa, said.
A university health clinic looked at the question with an article from July 23, 2020.
They explained that the question of comparing face shields to face masks was complicated due to the differing effectiveness of different masks.
There has not been a study comparing face shields to masks, and doing such study would be challenging since mask materials vary greatly in the community.
However, Veltman says research has found that face shields are beneficial if someone cannot wear a mask.
I feel that the face-shield is just as effective.
Are face shields as effective as face masks to prevent SARS-CoV-2 being spread?