Focal Length Equivalent

What is Crop Factor?

Crop factor (also called focal length multiplier) describes the ratio of a 35mm full-frame sensor's diagonal to a given sensor's diagonal. It tells you how much a smaller sensor "crops" into the image circle of a lens compared to full frame.

The Formula

To find the equivalent focal length on a different sensor:

1. Convert the source focal length to its 35mm equivalent: FL35mm = FLsource × Crop Factorsource
2. Convert from 35mm to the target sensor: FLtarget = FL35mm / Crop Factortarget

Combined: FLtarget = FLsource × (Crop Factorsource / Crop Factortarget)

Common Crop Factors

• Medium Format (GFX): 0.79x — larger than full frame, wider field of view
• Full Frame (35mm): 1.0x — the reference standard
• APS-H: 1.3x — used in some older Canon pro bodies
• APS-C (Nikon/Sony/Fuji): 1.5x
• APS-C (Canon): 1.6x
• Micro Four Thirds: 2.0x
• 1" Sensor: 2.7x
• Smartphone (1/2.3"): 5.6x

What Changes, What Doesn't

Crop factor affects the field of view (the angle of view captured) and the apparent magnification, making a lens behave as if it were a longer focal length on full frame. However, the actual optical properties of the lens do not change:

• Field of view: Changes with crop factor (the primary effect)
• Depth of field: Shallower on larger sensors at the same framing
• Perspective / distortion: Does not change (depends only on camera-to-subject distance)
• Maximum aperture (light gathering): Does not change

Practical Examples

• A 35mm lens on APS-C (1.5x) gives the same field of view as a 52.5mm on full frame
• A 50mm lens on Micro Four Thirds (2.0x) gives the same field of view as a 100mm on full frame
• A 63mm lens on Medium Format GFX (0.79x) gives the same field of view as a 50mm on full frame

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