Focal Length Calculator

Calculate field of view from focal length and sensor size. Find the equivalent focal length across different camera systems and crop factors.

Focal Length (mm)

Sensor Size

About Focal Length Calculator

Understanding Focal Length

Focal length is the fundamental optical property of a camera lens, measured in millimeters. It describes the distance between the optical center of the lens and the camera's sensor when the lens is focused at infinity. This single number determines two critical aspects of your photography: the angle of view (how much of the scene the lens captures) and the magnification (how large distant objects appear in the frame).

Shorter focal lengths capture wider fields of view — a 14mm lens on a full-frame camera sees approximately 114° horizontally, taking in vast landscapes or entire room interiors. Longer focal lengths narrow the field of view and magnify distant subjects — a 400mm telephoto compresses the scene to just 5.2° horizontally, pulling distant wildlife or athletes close enough to fill the frame.

Lens Categories:

Ultra-Wide: Below 21mm — architecture, interiors, astrophotography
Wide Angle: 21-35mm — landscapes, street photography, group shots
Normal: 40-60mm — general purpose, natural perspective
Short Telephoto: 70-105mm — portraits, close-up work
Telephoto: 105-300mm — sports, wildlife, events
Super Telephoto: Above 300mm — distant wildlife, aviation, astronomy

Crop Factor and Equivalent Focal Length

Not all camera sensors are the same size. A full-frame sensor measures 36×24mm, but many cameras use smaller sensors such as APS-C (approximately 23.5×15.6mm) or Micro Four Thirds (17.3×13mm). The crop factor describes how much smaller a sensor is compared to full frame, calculated by dividing the full-frame diagonal by the sensor's diagonal.

When you mount a lens on a camera with a smaller sensor, the sensor captures a narrower portion of the lens's image circle, effectively "cropping" the view. A 50mm lens on a 1.5× crop sensor produces the same field of view as a 75mm lens on full frame. This is the "equivalent focal length" or "35mm equivalent" — a common reference point for comparing lenses across different camera systems.

Common Sensor Sizes

Sensor	Crop Factor
Full Frame	1.0×
APS-C (Nikon/Sony)	1.5×
APS-C (Canon)	1.6×
Micro Four Thirds	2.0×
1" Sensor	2.7×

50mm Lens Equivalents

System	Equivalent
Full Frame	50mm
APS-C 1.5×	75mm
APS-C 1.6×	80mm
MFT 2.0×	100mm
1" 2.7×	135mm

Field of View Calculation

Field of view (FOV) is the angular extent of the scene captured by the lens, measured in degrees. It can be calculated horizontally, vertically, or diagonally. The formula is: FOV = 2 × arctan(sensor dimension ÷ (2 × focal length)), where the sensor dimension is the width for horizontal FOV, height for vertical FOV, or diagonal for diagonal FOV.

Understanding field of view is practical for many scenarios. Landscape photographers use it to determine how much of a panorama a single frame captures. Surveillance system designers use it to calculate how many cameras are needed to cover an area. Real estate photographers choose wide-angle lenses based on the FOV needed to capture an entire room in a single shot.

The relationship between focal length and FOV is not linear — it follows an inverse tangent curve. Doubling the focal length does not halve the field of view. The change is more dramatic at shorter focal lengths: going from 14mm to 28mm reduces the horizontal FOV from 104° to 65°, while going from 200mm to 400mm only reduces it from 10° to 5°. This is why wide-angle lens choices are more critical for architectural and interior photography, where even small changes in focal length produce significant differences in the captured scene.

Perspective and Compression

A common misconception is that focal length changes perspective. In reality, perspective is determined solely by camera position — the distance and angle relative to the subject. What telephoto lenses do is compress the apparent distance between objects in the scene, making foreground and background elements appear closer together. Wide-angle lenses exaggerate this distance, making near objects look larger and far objects look smaller.

This compression effect is why telephoto lenses are preferred for portraits (typically 85-135mm on full frame). At these focal lengths, the photographer stands farther from the subject, which produces flattering proportions — noses appear less prominent, faces look more natural, and the background is compressed into a pleasing, simplified backdrop. Wide-angle lenses shot up close distort facial features, making noses and foreheads appear disproportionately large.

Understanding these characteristics helps you choose the right focal length for your creative intent. Want to show the vastness of a landscape with dramatic foreground elements? Use a wide-angle lens positioned low and close to an interesting foreground subject. Want to isolate a subject with a compressed, creamy background? Use a telephoto lens from a greater distance. The combination of focal length choice and shooting position is one of the most powerful compositional tools available to photographers.

Frequently Asked Questions

What is focal length in photography?

Focal length, measured in millimeters, describes the distance between the optical center of a lens and the camera sensor when focused at infinity. It determines the angle of view (how much of the scene is captured) and the magnification of the image. Shorter focal lengths provide wider angles of view, while longer focal lengths provide narrower, more magnified views.

What is crop factor and how does it affect focal length?

Crop factor is the ratio of a camera sensor's diagonal to a full-frame sensor's diagonal (43.27mm). An APS-C sensor with a 1.5× crop factor makes a 50mm lens behave like a 75mm lens on a full-frame camera in terms of field of view. The actual focal length doesn't change, but the smaller sensor captures a narrower portion of the image circle.

How do I calculate field of view from focal length?

Field of view is calculated using the formula: FOV = 2 × arctan(sensor dimension ÷ (2 × focal length)). For horizontal FOV, use the sensor width; for vertical, use the sensor height; for diagonal, use the sensor diagonal. A 50mm lens on a full-frame sensor (36mm width) gives a horizontal FOV of about 39.6 degrees.

What focal length is considered 'normal'?

A 'normal' lens has a focal length approximately equal to the diagonal of the sensor, which is about 43mm for full-frame. In practice, 50mm is considered the standard normal lens for full-frame cameras. For APS-C cameras, normal is about 30-35mm, and for Micro Four Thirds, it's about 25mm. Normal lenses produce perspective similar to human vision.

What are the common sensor sizes and their crop factors?

Full frame sensors are 36×24mm (1.0× crop factor). APS-C sensors are approximately 23.5×15.6mm (1.5× for Nikon/Sony, 1.6× for Canon). Micro Four Thirds sensors are 17.3×13mm (2.0× crop factor). One-inch sensors are 13.2×8.8mm (2.7× crop factor). Smartphone sensors vary widely but typically have crop factors between 5× and 7×.

What is the difference between wide-angle and telephoto lenses?

Wide-angle lenses (below 35mm on full frame) capture a broad field of view and are ideal for landscapes, architecture, and interiors. Telephoto lenses (above 70mm) have a narrow field of view and high magnification, making them perfect for sports, wildlife, and portraits. Normal lenses (40-60mm) fall between the two and approximate natural human perspective.

Additional Resources

Focal Length - Wikipedia Angle of View - Wikipedia Crop Factor - Wikipedia

Share This Calculator

Include calculator values in URL

Embed on Your Website

Add this calculator to your website