1. A method comprising: processing an image of a present scene in a vehicle's environment to determine a first exposure value for capturing an average amount of light for the present scene to identify objects in the present scene with a camera, the first exposure value including a shutter time value and a gain value; adding, by one or more processors, a fixed offset shutter time value to the shutter time value and a fixed offset gain value to the gain value to generate a second adjusted exposure value; using the second exposure value to capture a first exposure image with the camera, wherein the first exposure image is a dark exposure image; using the first exposure value to capture a second exposure image with the camera; and using the first exposure image and the second exposure image to control the vehicle in an autonomous mode, wherein the first exposure value is determined in order to maintain the average amount of light for the present scene in a predetermine range based on logarithmic control for the shutter time value and linear control for the gain value.