Depth is useful to add visual clarity to your image. It increases the contrast in each and every pixel, making it easier to discern the details. The eye perceives reality in 3-D, and the Depth slider enhances this.
*Tip: When you choose to increase the Depth your photos will usually benefit by increasing the defaults of Exposure. That’s because increasing depth will create a photo that is overall slightly darker.
Definition – Provides more detail in the shadows and highlights and provides a better exposure correction on faces
Contrast – Increases the difference between light and dark areas.
!Note: For extremely dark photos, it is beneficial to turn depth “off” in order to bring out extreme details. Less contrast will reveal the details in the shadows, and thus applying less contrast can be a trick to creating a brighter photo.