For excellent cooking results always remember to pre-heat the oven.
Pre-heat the oven to about 35° F (20° C) above the desired cooking temperature. When opening the oven door to insert the food, some heat will escape, and in addition the food and the roasting pan/baking sheet will be cold, resulting some degrees of heat loss.
The higher pre-heat temperature will have several benefits: tastier and crisper food, shorter cooking times and some energy savings! Don’t forget however to re-set the temperature selector to the original recipe temperature setting after inserting the food.