Start · Module 1Topic · 5 min

Sensor sizes

Lesson 4 of 12

MODULE 1 ROADMAP

The short version

Sensors come in different sizes, and a smaller sensor records a smaller middle part of the picture a lens makes. That is why the same lens looks more zoomed in on some cameras. The crop factor is simply a number that tells you how much.

The picture-crop idea

Imagine printing a holiday photo, then cutting out just the middle with scissors. The middle piece looks like a closer, more zoomed-in shot, even though you never moved. A smaller sensor does exactly that: it records the middle of the picture the lens makes and ignores the edges.

A green glen with a winding stream and a track leading between steep hills under a cloudy sky — **Figure 2.** Same lens, same spot, three sensors. A smaller sensor simply records a smaller middle part of the picture, so it looks “zoomed in”. The Micro Four Thirds box is a squarer shape because that sensor is 4:3, not 3:2.

The crop factor tells you how much smaller a sensor is than full frame, and so how much tighter it frames with the same lens. Full frame is 1×. Most APS-C cameras are about 1.5× (Canon’s APS-C is about 1.6×), Micro Four Thirds is 2×, a 1-inch type sensor is about 2.7×, and phone main cameras are roughly 3× to 6×. Medium format is bigger than full frame, so its crop factor is less than 1: a 44 × 33 mm sensor is about 0.79×.

Figure 1. Every rectangle is drawn to the same scale, so you can see how much bigger each format is. The large-format inset is shrunk to a quarter so it fits.

Using it

To compare a lens across cameras, multiply its focal length by the crop factor. A 50mm lens on a 1.5× APS-C camera frames like a 75mm lens would on full frame. People call that the equivalent focal length. The lens itself has not changed at all; only the slice of the picture you keep.

Good news for phone owners: phone specs usually quote focal lengths that are already equivalents, such as “24mm”. You can compare them straight with full-frame lenses.

Why “inch” sizes mislead. Names like 1-inch or 1/1.3-inch are old labels from video cameras, not measurements. A “1-inch type” sensor is only about 13 × 9 mm. Look up the size in millimetres.

Lenses and formats. A lens is designed to light up a circle big enough for a certain sensor size, its image circle. A full-frame lens covers full frame and anything smaller. Medium- and large-format cameras need lenses with bigger image circles. A lens made for a smaller sensor, used on a bigger one of the same mount, gives dark corners or a crop mode, and on some systems it will not fit at all.

Easy traps

Using 1.5 for every APS-C camera. Canon’s is about 1.6.
Reading “1-inch” literally. It is a label, not a size.
Thinking a smaller sensor changes perspective. Perspective depends only on where you stand (Module 18).

Go deeper

Where the numbers come from. Crop factor compares sensor diagonals. The diagonal is √(width² + height²): 43.3 mm for full frame (36 × 24) and 28.2 mm for a 23.5 × 15.6 mm APS-C sensor, so 43.3 ÷ 28.2 = 1.53, rounded to 1.5. Canon APS-C (22.3 × 14.9 mm) works out at 1.61; Micro Four Thirds (17.3 × 13 mm) at 2.0; 1-inch type (13.2 × 8.8 mm) at 2.73; and 43.8 × 32.9 mm medium format at 0.79.

Shapes. Micro Four Thirds and most medium-format sensors are 4:3, while full frame and APS-C are 3:2, so the equivalence is exact only along the diagonal.

Working backwards. To get a given look, divide by the crop factor: an 85mm full-frame look on Micro Four Thirds needs 85 ÷ 2 = 42.5mm.

Finn tip: Write your crop factor and your lenses’ equivalents on a strip of tape inside your camera bag. Maths is harder when you are cold and the light is going.

Key takeaways

A smaller sensor records a smaller middle part of the lens’s picture, so it frames tighter.
Equivalent focal length = focal length × crop factor; phone specs usually quote it already.
Medium format has a crop factor below 1; bigger formats need lenses with bigger image circles.

Try it now · 10 minutes

Take one photo, then crop it on your phone to the middle two-thirds of its width. That is roughly what an APS-C sensor would have recorded with the same lens. Crop to half the width for the Micro Four Thirds view. You have just done crop factor with your thumbs.