Focal length and perspective

Zooming from the same position

A longer focal length shows a narrower angle of view on the same sensor size. Individual objects appear larger in the frame, and less surrounding space fits inside it. If the viewpoint remains unchanged, their perspective size relationships fundamentally stay the same. A matching crop from the wide-angle photograph therefore shows similar perspective to the longer lens from that position, apart from lens characteristics and image quality.

Test this with two cups: place one nearby and another noticeably farther away on a table. Photograph both at short and long focal lengths without moving the camera. Then crop the wide photograph to match the tighter framing. The distant cup does not suddenly become relatively larger merely because you zoomed. You selected a smaller portion of the same spatial arrangement.

Stepping back for equal framing

Now change the experiment: keep the front cup approximately the same size in the frame. With a longer focal length, you must step backwards. The distance difference between the cups then becomes smaller relative to the overall shooting distance. The rear cup looks relatively larger and the scene more tightly layered. This is often called telephoto compression, but the changed viewpoint creates it.

For a portrait, first choose a distance that gives the facial proportions and interaction you want. Then choose focal length for framing. If a cramped room prevents you from stepping back, a longer lens cannot provide a complete portrait with a different spatial relationship; it simply cuts off more of the subject.

Two tests with a person in front of a gate

Place a person a few steps in front of a gate and photograph from a fixed mark with a wide and then a tighter view. Crop the wide frame afterward so that the person appears equally large. Apart from resolution, depth of field and optical aberrations, the relative sizes of person and gate remain broadly the same. You kept the viewpoint; the longer lens merely recorded a smaller portion of it.

For the second test, move substantially farther back with the longer focal length until the person again occupies the same size in the full frame. The gate now appears larger relative to the person. Your distance to the person has increased substantially, while the additional distance to the gate matters less proportionally. This is often described as telephoto compression, but it results from the changed position.

For a travel portrait, first decide how large a building behind the person should appear. Then find a position that creates that relationship and a focal length that gives the desired framing. If a wall prevents you from stepping farther back, turning the zoom ring cannot force the desired relationship into existence.

Include the background deliberately

A person in front of a distant building can look very different with two approaches. Photographed close up with wide angle, the person dominates and the building appears small. From farther away with a longer lens, the building can become much more prominent behind them even when the person stays similarly sized in the final frame. Check whether that background becomes not only larger but also more distracting.

Moving sideways also changes overlap. A small step can remove a lamp post from behind a head without any zooming. For composition, this shift is often more important than the exact millimetre figure. Look for clear relationships between foreground, main subject and background before settling the framing.

Perspective is not lens distortion

Straight lines appearing curved are a different issue from the size relationship of near and distant things. Barrel or pincushion distortion relates to lens projection and can often be corrected with a profile. That cannot simply transform the spatial proportions of a face photographed very close into the appearance of a more distant viewpoint. Make that decision during capture where possible.