Ozzel's Final Mistake.

To recap what we know:

General Veers reports to Vader that the Imperial Fleet moved out of light speed, and they detected an energy shield protecting a portion of the sixth planet of the Hoth system. It's strong enough to stop a bombardment.

Vader criticizes Ozzel for bringing the fleet out of light speed to close to the system and making the Rebels alert to their presence.

They are still able to land ground troops and heavy artillery.

So what was the problem??? The energy shield would be there regardless of where they came out of light speed. And wouldn't it be wiser to come out as close to the planet as possible? If you were too far away the Rebels would see you coming and have more time to evacuate before the blockade was completely in place.