As explained by vtheatre.net, straight make up means that the actor's natural appearance is close enough to the character being played that no alteration needs to be made.
The primary purpose of straight make up is to compensate for the normal conditions of performance: distance and strong lighting.
In the theatre, actor's faces must be read from the back row of the balcony. The natural features of the face tend to become indistinguishable at that distance unless heightened with make up.
While distance is usually not an issue for screen actors, bright lighting is. The face is a three-dimensional object with both peaks and valleys. The nose is the highest peak and the eye sockets the deepest valleys. We can distinguish these features most easily when light falls on them from a single angle above, like the sun or an overhead lamp. This creates a natural interplay of highlight and shadow which reveal the fine contours of the face and its expressions. Stage and studio lighting tend to be much stronger than natural light, and comes from several angles. This tends to erase the natural face shadows. With make up, facial highlights and shadows are re-created with paint so that lighting cannot erase them.
Tip! Learn The 6 Steps of Stage Makeup (The Basic of Theatre Makeup) at wixsite.com.