Focal Length
Calculate the required focal length, magnification, and angle of view for any camera lens and sensor size. Free online photography calculator with interactive charts.
About This Calculator
The Focal Length Calculator helps photographers, cinematographers, and optical engineers determine the correct lens focal length needed to capture a subject at a given distance with a specific camera sensor. By entering your subject's real-world size, your camera sensor diagonal, and the shooting distance, this tool instantly computes the required focal length along with the magnification ratio and diagonal angle of view.
The calculation uses the thin lens equation: Magnification = Image Size / Object Size, and Focal Length = (Distance x Magnification) / (Magnification + 1). The angle of view is derived from the sensor size and focal length using the formula: Angle of View = 2 x arctan(Sensor Size / (2 x Focal Length x (Magnification + 1))). These formulas are fundamental to geometric optics and are used by lens designers worldwide.
Common Sensor Sizes
Full-frame (35mm): 43.3 mm diagonal — APS-C (Nikon/Sony): 28.2 mm — APS-C (Canon): 26.7 mm — Micro Four Thirds: 21.6 mm — 1-inch: 15.9 mm — Medium format (44x33): 55 mm. Enter your specific sensor diagonal for the most accurate results.
Regional Notes
Focal length is universally measured in millimeters and does not vary by region. Sensor sizes and lens availability are consistent across global markets (IN, US, UK, EU). The most common lens mounts (Canon EF/RF, Nikon F/Z, Sony E, Fujifilm X, Micro Four Thirds) are sold worldwide with identical specifications.
Frequently Asked Questions
What is focal length and how is it measured?
Focal length is the distance (in mm) between the lens and the camera sensor when the lens is focused at infinity. It determines how much of a scene your camera captures — shorter focal lengths give a wider angle of view, while longer focal lengths magnify distant subjects. Common values range from 14mm (ultra-wide) to 600mm (super-telephoto).
How does sensor size affect the effective focal length?
Sensor size directly affects the angle of view for a given focal length. A smaller sensor (APS-C or Micro Four Thirds) crops the image, making the lens appear more zoomed in. This crop factor is roughly 1.5x for APS-C and 2x for Micro Four Thirds. For example, a 50mm lens on APS-C gives an equivalent angle of view to a 75mm lens on full frame.
What is the difference between optical zoom and focal length?
Optical zoom is the ratio between the longest and shortest focal lengths of a zoom lens (e.g., 24-70mm has 2.9x zoom). Focal length is an absolute measurement in millimeters that describes the lens's angle of view. Prime lenses have a single fixed focal length, while zoom lenses cover a range.
How do I calculate the right focal length for portrait photography?
For portraits, a focal length of 85mm to 135mm (full frame equivalent) is ideal as it provides natural facial proportions and pleasing background compression. On APS-C cameras, a 50mm to 85mm lens achieves similar results. Use this calculator to determine the exact focal length by entering your subject size, sensor size, and shooting distance.
What is the relationship between focal length and angle of view?
Angle of view decreases as focal length increases. A 14mm lens has a roughly 114° diagonal angle of view (ultra-wide), a 50mm lens has about 46° (standard), and a 200mm lens has approximately 12° (telephoto). The exact angle depends on your camera's sensor size.
Does focal length affect depth of field?
Yes, longer focal lengths produce shallower depth of field at the same aperture and subject distance. A 200mm lens at f/2.8 creates much more background blur than a 24mm lens at f/2.8. However, depth of field also depends on aperture, subject distance, and sensor size.
What is the standard focal length for a normal lens?
A normal lens approximates human vision with an angle of view around 40° to 50°. For full-frame cameras, this is typically 50mm. For APS-C it is around 35mm, and for Micro Four Thirds about 25mm. Normal lenses produce natural-looking perspective with minimal distortion.