Select your longest focal length and compose a portrait shot fairly tightly within the frame in front of a background with depth. Take one photograph. Then walk towards your subject while zooming out to your shortest focal length. Take care to frame the subject in precisely the same way in the viewfinder and take a second shot. Compare the two images and make notes in your learning log.
As you page between the two shots it can be shocking to see completely new elements crash into the background of the second shot while the subject appears to remain the same. This exercise clearly shows how focal length combined with viewpoint affects perspective distortion.. Perspective distortion is actually a normal effect of viewing an object, for example where parallel train tracks appear to meet at the horizon. A ‘standard lens’ – traditionally a 50mm fixed focal length lens for a full frame camera (about 33mm in a cropped-frame camera) – approximates the perspective distortion of human vision (not the angle of view which is much wider). A standard lens is therefore the lens of choice for ‘straight’ photography, which aims to make an accurate record of the visual world.
The first set were taken at 38mm and 110mm, the second image at 110mm appears cleaner, the shop signage ,doorway, some windows and the lamp have all disappeared yet the subject remains the same.
The second set of images were taken at 18mm and 300mm, notice that at 300mm (Apart from the obvious grain at this length) the sky has vanished and the background has softened, also the line of the path appears to have changed trajectory. Also new information has come into view? Such as the bench and the conical posts which seemed to be behind the subject in the previous shot.
It seems as though the lens zooms in on the backgrounds whilst the subject remains unchanged.