Sensor sizes and crop factor

The common formats

Full frame measures about 36×24 mm, the same as a frame of 35 mm film. It is the reference everything else is compared to.

APS-C is about 23.5×15.6 mm in most brands (crop factor about 1.5×) and about 22.3×14.9 mm in Canon cameras (about 1.6×). Micro Four Thirds is about 17.3×13 mm, a 2× crop with a squarer 4:3 shape.

A 1-inch type sensor, used in premium compacts and drones, is about 13.2×8.8 mm (about 2.7×). The name is a confusing leftover from old video tubes: nothing on it measures an inch.

Phone main cameras range widely, roughly 1/2.5-inch to 1-inch type, so crop factors of about 7× down to under 3× (approximate). Medium format digital starts around 44×33 mm (about 0.8×) and goes larger.

What crop factor means

Crop factor is the ratio of the full-frame diagonal to your sensor’s diagonal. A smaller sensor sees only the middle of what the same lens would show on full frame.

Multiply the focal length by the crop factor to get the full-frame equivalent field of view. A 50 mm lens on APS-C (1.5×) frames like a 75 mm lens on full frame; on Micro Four Thirds it frames like 100 mm.

The lens itself does not change. A 50 mm lens is always 50 mm; only the view it gives on that sensor changes.

Light gathering and noise

At the same exposure settings, every sensor gets the same light per square millimetre. But a bigger sensor has more square millimetres, so for the same framing it collects more total light.

More total light means a cleaner image, because random noise matters less when there is more signal. As a rough guide, full frame has about 1 stop of advantage over APS-C and about 2 stops over Micro Four Thirds (approximate; sensor generation and processing matter a lot).

This is why phones work so hard with multi-frame stacking at night: their tiny sensors start with far less light, so software merges many frames to catch up.

Depth of field, lens size and cost

For the same framing and f-number, a larger sensor gives shallower depth of field. A full-frame camera at f/2.8 gives roughly the background blur of Micro Four Thirds at f/1.4.

Smaller sensors need shorter focal lengths for the same view, so telephoto lenses can be much smaller and lighter. A 300 mm lens on Micro Four Thirds frames like 600 mm on full frame at a fraction of the size.

The flip side: to match full frame’s shallow depth of field, small-sensor lenses need very bright apertures, which grow large and expensive. Bigger sensors also cost more to make, so bodies and lenses usually cost more too.

Equivalence, said carefully

Equivalence is the idea that you can match the look of two formats by scaling focal length and f-number by the crop factor, and ISO by the crop factor squared. For example, 25 mm f/1.4 ISO 400 on Micro Four Thirds is roughly equivalent to 50 mm f/2.8 ISO 1600 on full frame in framing, depth of field and noise.

It is a useful comparison tool, not a promise. Real lenses differ in sharpness and rendering, real sensors differ in efficiency, and aspect ratios differ, so treat equivalent values as approximate.

Exposure itself does not change: f/2.8 is f/2.8 on any sensor for brightness. Only depth of field and total light scale.

Megapixels versus pixel size

Megapixels count pixels; they say nothing by themselves about sensor size. A 50-megapixel phone sensor and a 50-megapixel medium format sensor are very different things.

Packing more pixels onto the same sensor makes each one smaller. Individual small pixels are noisier, but when you view the whole image at the same print size the difference largely evens out, so a modern high-resolution sensor is usually not much noisier overall (approximate).

More megapixels help with large prints and cropping. They also make files bigger and show every bit of camera shake and lens softness, so higher resolution asks more of your technique.