What causes images to have flat depth?

Long focal lengths (ie, high zoom factors) tend to remove depth because as the camera is far away, everything is shot from the same angle and this removes perspective. But is is really the distance that does it, cropping a picture taken with a shorter lens (but from the same place) would yield the same result.

• Flat light. That is, light that is either from behind the camera and very close to parallel to the lens' optical axis (such as an on-camera flash) or extremely well diffused such as on an overcast day.
• Smaller apertures (larger f-numbers) combined with longer focal lengths that force shooting from further away to get the same framing will tend to "flatten" the perspective.

It is typically called "lens compression," where things farther away appear nearer to the subject/closer together. And the opposite of this is typically called "lens distortion," where something closer to the camera appears larger than it should... like someone's nose in a portrait.

Neither effect actually has anything to do with the lens, and it is not compression nor distortion; it is simply perspective. And it is caused by differences in relative distances.

For example; start with a 50mm lens, a subject at 10ft, and the background at 40ft (30ft behind the subject). And then you switch to a 200mm lens. To keep the subject the same size in the frame w/ the 200mm lens you have to move back 4x the distance to 40ft (+30ft); which negates the increased magnification at the subject distance. But you did not move back 4x the distance to the background; you are only 70ft from it, when 160ft (4x40) is required to negate the increased magnification of the 200mm lens at the background distance... more than 50% of the increased magnification remains, so the background details appear larger and closer to the subject.

At any given distance/camera position you can change the lens/magnification and the perspective will not change; because the relative distances do not change... the change in magnification affects foreground/subject/background equally.

Neither effect actually has anything to do with taking a picture... it is the difference between trying to judge the space between two parked cars a block away, as compared to judging the space between the cars when you are much closer to them.

Optically, a long lens is said to compress perspective. When a longer focal-length is used, the relative distance between scene elements is shorter. You can visually play with the concept using the Nikon Lens Simulator. In it, choose a lens which a high zoom ration, say a 28-300mm and play with the zoom slider. You will see the scene looking flatter as you advance to 300mm.

Most images that look flat are probably not produced by such long focal-length but by the scene itself and how it is lit. Remember, an image is always 2D. It is flat by definition and when we perceive depth, it is because there are cues in the image which our brain interprets as depth.

• Relative size of objects is an important cue, if elements look much smaller in the background compared to the foreground, it gives a better sense of depth. Image that the street photo has a person near the front that appears to take 1/2 or the image-height and one in the back that is only 1/10th of the image-height.
• Shadows have a huge impact on how we perceive depth. When elements cast shadows on each other, then your brain figures out which is in front of which and that cause the impression of depth. When new photography students complain their images look flat, it is nearly always because of poor lighting which results in a scene with no shadows, such as an overcast sky or photos taken with the sun behind the photographer, causing all shadows to be cast behind objects and therefore not captured in the image.