Focal Length Calculator

Focal length is the optical distance from the point where light rays converge in a lens to the digital sensor or film in a camera, measured in millimeters. It determines the angle of view (how much of the scene will be captured) and the magnification (how large individual elements will be). Lenses with shorter focal lengths capture wider angles of view, while longer focal lengths capture narrower angles with higher magnification.

Crop factor refers to the ratio of a full-frame sensor's size (36mm × 24mm) to another sensor's size. It's important because it affects how a lens's focal length translates to field of view on different cameras. For example, an APS-C camera with a 1.5× crop factor using a 50mm lens will produce an image with the same field of view as a 75mm lens on a full-frame camera. Understanding crop factor helps photographers maintain consistent composition when switching between different camera systems.

Lens manufacturers list the actual, physical focal length of a lens rather than any equivalent because:
1. The physical focal length is an optical property of the lens itself, independent of the camera it's mounted on
2. The same lens can be used on different camera systems with different sensor sizes
3. It's a consistent measurement that doesn't change based on application
Listing equivalent focal lengths would create confusion as the same lens would have different stated focal lengths depending on which camera system it's used with.

On a full-frame camera, a focal length of approximately 50mm most closely resembles the field of view and perspective of human vision. This is why 50mm lenses are often called "normal" or "standard" lenses. However, human vision is more complex than a camera lens—we have peripheral vision and our brain processes what we see differently. If you're using a camera with a smaller sensor, you would need a shorter focal length to achieve this same "normal" perspective—about 35mm on an APS-C sensor or 25mm on a Micro Four Thirds sensor.

Focal length itself doesn't change perspective—your physical position relative to the subject does. However, different focal lengths encourage different shooting distances to achieve the same framing, which does affect perspective. Wider lenses (shorter focal lengths) typically require getting closer to the subject, creating more apparent perspective distortion where closer objects appear larger relative to distant ones. Telephoto lenses (longer focal lengths) let you shoot from farther away, which compresses perspective, making objects at different distances appear closer together. This compression effect is why telephoto lenses are often preferred for portraits—they create more flattering facial proportions.

Optical zoom changes the focal length of the lens by physically moving lens elements, genuinely magnifying the image without quality loss. For example, a 24-70mm zoom lens can change its focal length within that range.

Digital zoom, on the other hand, simply crops and enlarges the center portion of the image, effectively reducing resolution and image quality. It's similar to cropping an image in post-processing. When possible, optical zoom is always preferred for maintaining image quality.

Telephoto lenses create more pronounced background blur (bokeh) for several reasons:
1. They magnify the background more, which makes background defocus more noticeable
2. They compress perspective, making background elements appear larger relative to their actual size
3. They typically have a narrower angle of view, showing less of the background

While aperture is the primary factor in creating background blur (wider apertures like f/2.8 create more blur than narrow ones like f/11), a telephoto lens will produce more background blur than a wide angle lens at the same aperture and subject framing.

Yes, you can use full-frame lenses on crop sensor cameras, and in fact, they often perform exceptionally well. The crop sensor will only use the center portion of the image projected by the lens, which is typically the sharpest part with fewer optical aberrations. The only downside is that the lens will effectively have a narrower angle of view due to the crop factor. For example, a 50mm full-frame lens on an APS-C camera (1.5× crop factor) will have a field of view equivalent to a 75mm lens on a full-frame camera. This is actually beneficial for telephoto photography but can be limiting for wide-angle work.