Crop Factor
Crop factor compares a sensor's diagonal measurement to full frame's 43.27mm diagonal, defined as full frame's own diagonal divided by the sensor in question. Full frame is exactly 1.0x; APS-C runs roughly 1.5x (Nikon, Sony, Fujifilm) or 1.6x (Canon); Micro Four Thirds is close to exactly 2.0x. Sensors physically larger than full frame, like medium format, get a crop factor below 1.0.
Multiplying a real focal length by crop factor gives its full-frame-equivalent field of view. The same multiplier applied to aperture gives a depth-of-field-equivalent f-number — a genuinely different, separate calculation from field of view — but crop factor never changes exposure: an f/2.8 lens needs the identical shutter speed and ISO for correct exposure on any sensor size.
Frequently Asked Questions
Does a bigger crop factor mean a worse camera?
No — a higher crop factor gives free telephoto reach and generally more forgiving depth of field at matched framing, real advantages depending on what's being photographed, not a straightforward downgrade from full frame.
Why does medium format have a crop factor below 1.0?
Because it's physically larger than full frame, the reference point — a crop factor below 1.0 means a longer, not shorter, real focal length is needed to match a given full-frame field of view.