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Relation Between Focal Length and Perspective

None actually…

A very common myth you’ll read around forums especially when discussing lenses of different focal lengths are the statements “just zoom with your feet”, “just take a couple of steps forward/back”, the idea was, moving with your feet gives you a similar image – it doesn’t.

Perspective of a scene changes when the subjects or the camera position moves towards or away the subject in relation to the background. However, merely zooming in and out from the same camera position will not alter perspective.

Take a look at this series of photographs:

24mm

24mm

50mm

70mm

70mm

105mm

105mm

The subject’s (scooter) relative size to the frame are about the same in all four image because I moved the camera’s position. As I moved from wide to tele, I progressively moved the camera backwards in a straight line. The same focus point in my camera was used (left center), focusing on the black rectangle on the vertical bar of the scooter.

Keep an eye on the following areas:

  1. The grass line. See how in the 24mm photo, the line runs along a 30-degree angle from lower-left to upper-right. As I increase my distance from the subject, the angle is reduced (leaning more towards the base of the frame.
  2. The angle of the scooter’s step-plate/rear tire. Like the grass line, it gradually straightens out in relation to the base of my frame. As a result, the perceived length of the step-plate in the 105mm shot is longer than the step-plate in the 24mm image, because the camera was farther from the subject than the 24mm image. We all know the scooter’s size never changes.
  3. Lastly, the relative size of the monkey bar’s shadow in the upper right wall.

When you shoot a scene where the background is an integral part of the composition, knowing where to position yourself in relation to the subject and before choosing a focal length is important. The ability to crop out backgrounds or amplify the characteristic of the background without significantly altering your main subject’s position and appearance is what zooming does, but you need to physically move the camera position or your subject in order to change the perspective.

105mm

105mm

The two shots are taken with the same lens (Canon EF 24-70mm f/2.8L USM), same background, same subject. Only the focal length was altered and the shift in camera position. The shift in camera position played a big part of distorting the background’s perspective. Notice how subtle manipulation of perspective altered the impact of the images.

Before I finish, I must point out one very frequent term that’s misused by many magazine writers and forum advisers as well – compression.

Specifically, perspective compression. Changing focal length alone DOES NOT compress a scene. I repeat, going from 50mm to 200mm does NOT compress the scene. You are magnifying the farther subjects, but you’re not compressing the scene. You need to physically alter the position of the lens to do that.

Compressing a scene means that the scene remain the same. You get the same subject elements in a scene, but ordered in a different manner. Lenses cannot do that unless you move the position of the camera itself. When you zoom from 50mm to 200mm, you magnify the far elements only to the extent of what the frame can accommodate!!! You’ll lose elements on all four sides of the frame as you zoom in and out with different focal lengths, that’s not perspective compression, that magnification and cropping.

105mm

24mm (Cropped) - Reference Image - 6 blocks in scene.

105mm

105mm ZOOMED - This is NOT compressed as only three blocks remain in the frame.

105mm

105mm - Compressed scene as all 6 blocks are in the scene as the camera was moved backwards.

You can compress a scene either you compress the subjects themselves (move them closer together), or move the camera’s position to include compensate the cropping of the longer focal length.

You can compress a scene either you compress the subjects themselves (move them closer together), or move the camera’s position to include compensate the cropping of the longer focal length.

Shooting from wide angle to a zoomed image doesn’t change perspective, just the amount of image cropped and magnified.

The first set of photos are taken with 28mm, 128mm, and 250mm equivalent, actual images.

The second set compares the 250mm original from the 128 and 28mm crops. As long as the camera position doesn’t change, the images offer the same visual perspective.

If you want to work with equations and stuff (I don’t hehe), download this PDF from ScubaGeek.com and submerge yourself with equations (pun intended).

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