In the Auto Exposure Mode your camera selects an appropriate aperture - the opening that permits light to pass into the camera -  and shutter speed - how long the shutter is allowed to remain open - for the scene you are photographing

Introduction

Using your camera’s Auto Exposure Mode will allow you to take acceptable pictures without having to rely on charts and tables. In Auto Exposure Mode, the camera uses its built-in light meter to choose an aperture that will expose your subject at a medium tone. It then selects a shutter speed to match that aperture. The light meter measures the amount of light reflected from the subject at which the camera is pointed, assuming that all subjects reflect 18% of the available light. In many instances, and in hurried circumstances, this is the easiest way to take a good picture.

The fern and the tree in this photograph are both medium in tone, reflecting about 18% of light. In average light conditions and when shooting from a comfortable distance, using the Auto Exposure Mode will give you an excellent picture.

This sage desert scene was shot in early morning when the light was even. Although the picture contains both white and dark brown horses, the overall tone of the composition is medium, with the subjects on average reflecting 18% of light. Again, in this situation, Auto Exposure Mode will give you a properly exposed image.

This costal image is composed of many different tones: the water is light, the grass is medium and the rock is dark. However, once again the overall tone of the landscape is medium and the Auto Exposure Mode can be used.

This image is taken in late afternoon when the light is even. Although the rock formation has different tones, however,  the overall tone of the rock is medium reflecting 18% of light. In this situation, Auto Exposure Mode will give you a good picture.

Conclusion

When photographing medium tone subjects you can usually leave your camera in the Auto Exposure Mode allowing it to choose an appropriate combination of aperture and shutter speed. However, learning to use your camera’s aperture-priority and shutter speed-priority options will give you more opportunities for creativity and allow you to take great pictures.