At the top of many image-editing programs is a histogram. This is a graph showing the brightness of your image on the X axis, with the number of pixels at that brightness on the Y. You can use this to see if your image has highlights that are pure white—known as “clipped”—and from which no detail can be extracted. Affinity has a “Show Clipped Highlights” button, top-right, which shows these up as red areas in your image. The opposite, shadows that are pure black, can be revealed in blue with the “Show Clipped Shadows” button. As you alter the brightness of your image, you can watch the histogram to get an idea of how much latitude you have before areas start to clip.