Calculate the required focal length for your camera based on sensor size and desired field of view.This formula applies to horizontal field of view only. For other angles, the math changes.
Last updated: March 2026 | By Patchworkr Team
Focal length is the distance in millimeters from the optical center of a lens to the camera's sensor when focused at infinity. It determines how much of a scene is captured (field of view) and how large subjects appear in the frame.
A shorter focal length (e.g., 24mm) provides a wider field of view, capturing more of the scene but making subjects appear smaller. A longer focal length (e.g., 200mm) provides a narrower field of view, magnifying distant subjects but capturing less of the scene.
The relationship between focal length and field of view depends on sensor size. Smaller sensors effectively "crop" the image, making the same lens appear to have a longer focal length. This is why we use "full-frame equivalent" values to compare lenses across different camera systems.
Find the focal length needed for a 60° field of view on an APS-C sensor:
Crop factor is the ratio between a full-frame sensor (36mm) and your sensor. APS-C has ~1.5× crop (Nikon/Sony) or 1.6× (Canon), meaning a 50mm lens acts like a 75-80mm on full-frame. It affects field of view, not actual focal length or optical properties.
Yes, longer focal lengths produce shallower depth of field at the same aperture and subject distance. However, the main factors are aperture (f-stop), subject distance, and sensor size. A 50mm f/1.8 has much shallower DOF than 50mm f/8.
The 'normal' lens approximately matches human eye perspective. It's roughly equal to the sensor diagonal. For full-frame (43mm diagonal), that's 50mm. For APS-C (28mm diagonal), it's 30-35mm. Normal lenses provide natural perspective without distortion.
Full-frame equivalent provides a common reference point when comparing lenses across different sensor sizes. A 25mm lens on Micro Four Thirds (50mm equiv.) captures the same field of view as a 50mm on full-frame, making it easier to choose lenses.
Not exactly. Perspective compression is caused by distance to subject, not focal length. Longer lenses force you to stand farther away for the same framing, which compresses depth. You'd get the same compression with a wide lens if you stood at the same distance (and cropped).
85-135mm (full-frame equivalent) is ideal for portraits. It provides flattering perspective, comfortable working distance, and shallow depth of field for background blur. Wider lenses (35-50mm) work for environmental portraits. Avoid ultra-wide for close-up faces (distortion).
Yes! Full-frame lenses work on APS-C but with a narrower field of view due to crop factor. A 50mm full-frame lens on APS-C acts like 75mm (1.5× crop). Some lenses designed for APS-C won't cover full-frame sensors (vignetting/black corners).
Focal length is a fixed optical property (e.g., 50mm). Zoom lenses have variable focal lengths (e.g., 24-70mm), allowing you to change field of view without changing lenses. Prime lenses have one focal length but often offer better image quality and wider apertures.
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