Focal Length Calculator
Calculate the focal length needed for a desired field of view, or to frame a subject at a given distance.
Required Focal Length
49.5 mm
How to Calculate Required Focal Length
Focal length determines how much of a scene a lens captures and how large a distant subject appears on the sensor. This calculator works two ways: from a desired field of view and sensor size, or from a subject's distance and how large you want it to appear on the sensor.
Example
To get a 40° horizontal field of view on a full-frame sensor (36mm wide), you need a focal length of 36 ÷ (2 × tan(20°)) ≈ 49.5mm — close to a standard 50mm lens. Separately, to make a 1.8m tall subject fill 20mm of the sensor from 10m away, you'd need a focal length of (20 × 10,000) ÷ 1,800 ≈ 111mm.
Common Use Cases
- Choosing a lens to achieve a specific angle of view for landscape, portrait, or architectural photography.
- Working out how much reach (telephoto focal length) you need to fill the frame with a distant subject.
- Planning a shot when you know the subject distance and desired framing.
FAQs
How is this different from a field-of-view calculator?
A field-of-view calculator starts from a known focal length and tells you the resulting angle of view. This calculator works the other way: you specify the field of view or framing you want, and it tells you the focal length required to achieve it.
Why does sensor size matter?
A smaller sensor captures a narrower slice of the same lens's image circle, so it needs a shorter focal length than a full-frame sensor to achieve the same field of view — this is often described using a camera's "crop factor."
Which mode should I use?
Use the field-of-view mode when you know how wide a scene you want to capture. Use the subject-framing mode when you know how far away your subject will be and how large you want it to appear in the frame, such as for wildlife or sports photography.