Understanding Focal Length

Focal length is the most important specification on a lens, yet it is also one of the most commonly misunderstood concepts in photography. Understanding what focal length actually does--and does not do--will help you choose the right lens for any situation and compose more intentional photographs.

What Focal Length Means

Focal length, measured in millimeters, is the distance between the lens and the image sensor when the lens is focused at infinity. A shorter focal length means the lens bends light more aggressively, producing a wider field of view. A longer focal length means the lens is more like a telescope, magnifying the scene and narrowing the field of view.

This is why lens categories are described in ranges:

• Ultra-wide (14mm - 24mm): Extreme field of view. Distortion is visible at the edges. Foreground elements appear larger, background elements recede. Used for architecture, interiors, and dramatic landscapes.
• Wide (24mm - 35mm): Broad field of view with less distortion than ultra-wide. Standard for environmental portraits and full-body shots in tight spaces. 35mm is a favorite for street and documentary photography.
• Standard (40mm - 60mm): Field of view that approximates human perception. 50mm is called the "nifty fifty" because it produces natural-looking perspective with minimal distortion. Versatile for portraits, street, and general photography.
• Short telephoto (70mm - 135mm): Compressed perspective that flatters facial features. 85mm is the classic portrait focal length. 135mm works well for headshots and compressed landscapes.
• Telephoto (135mm - 300mm): Long reach for distant subjects. Used for wildlife, sports, and candid portraits where you cannot approach the subject.
• Super-telephoto (300mm+): Extreme reach. Requires steady support. Used for bird photography, field sports, and astrophotography.

Focal Length and Perspective

A common misconception is that focal length changes perspective. It does not. Perspective is determined by your distance from the subject, not by the lens. A 50mm lens and a 200mm lens shot from the same position will produce images with identical perspective. The 200mm image is just a magnified crop of the center of the 50mm image.

This distinction matters in practice. If you want to compress a scene--making foreground and background elements appear closer together--you need to move farther from your subject, not change lenses. A telephoto lens lets you maintain subject size from that distance, but the compression comes from the distance, not the lens.

Focal Length and Sensor Size

A lens's focal length is a physical property and does not change when you mount it on a different camera. However, the sensor size determines how much of the lens's image circle is captured. A smaller sensor crops the image, making the lens appear to have a longer focal length.

This is the crop factor. APS-C sensors have a crop factor of about 1.5x (1.6x for Canon). Micro Four Thirds sensors have a 2x crop factor. A 50mm lens on an APS-C body produces the same field of view as a 75mm lens on a full-frame body (50mm x 1.5 = 75mm).

When comparing lenses across sensor sizes, convert to full-frame equivalent for a consistent reference. The Ricoh GR III has an APS-C sensor and an 18.3mm lens, which works out to approximately 28mm in full-frame terms.

Focal Length and Depth of Field

Focal length also affects depth of field. All else being equal, a longer focal length produces shallower depth of field at the same aperture. This is why 85mm f/1.8 lenses are popular for portraits: they create a thin plane of focus that separates the subject from the background.

This is also why wide-angle lenses appear to have deep depth of field even at wide apertures. A 24mm lens at f/2.8 on full frame keeps most of the scene in focus, making it easier to shoot from the hip in street photography.

Choosing a Focal Length

For a first lens, a 35mm or 50mm prime forces you to work within constraints, which improves your composition skills. Zoom lenses offer flexibility at the cost of aperture speed and image quality at the extremes.

If you own a zoom lens, spend a day shooting at a single focal length. Tape the zoom ring in place if you have to. This exercise teaches you to move your feet to find the right composition, and you will learn which focal lengths work best for your style.

For specific use cases:

• Street photography: 28mm to 35mm. Wide enough to capture context, narrow enough to isolate subjects.
• Portraits: 85mm to 135mm. Flattering compression, comfortable working distance.
• Travel: 24mm to 70mm. Covers wide scenes and detail shots without changing lenses.
• Wildlife: 300mm and above. No substitute for reach.
• Architecture: 16mm to 24mm, with tilt-shift if available. Straight lines matter here.

Summary

Focal length determines field of view. Perspective is determined by distance. Sensor size crops the image and changes the effective field of view. Learn these three facts and you will never be confused about which lens to use for a given situation again.