How to Compare Sensor Sizes and Crop Factor

A 35 mm f/1.8 lens on an APS-C body frames like a 52 mm on full frame. Multiply the aperture by the crop factor too and you get f/2.7, which describes the depth of field.

At which point somebody says the lens has become slower, and the argument starts. It has not, and untangling why explains the whole subject.

Three things, and crop factor applies to two

Framing — crop factor applies. A smaller sensor captures a smaller portion of the image circle, so the field of view matches a longer lens on a larger format. Multiply focal length by the crop factor.

Depth of field — crop factor applies. To frame identically on a smaller sensor you either use a shorter lens or stand further back, and both increase depth of field. Multiplying the f-number by the crop factor gives the equivalent.

Exposure — crop factor does not apply. f/1.8 admits the same light per unit area on any sensor. A meter reading of f/1.8 at 1/250 and ISO 400 is correct on every format there is.

The confusion comes from the fact that the smaller sensor collects less total light, because it has less area. Total light affects noise; light per unit area affects exposure. Those are different quantities and conflating them is the whole argument.

What the trade-offs actually are

Larger sensors give shallower depth of field at equivalent framing, and gather more total light, which generally means less noise at a given exposure. Both are real advantages and both are why full frame and medium format exist.

Smaller sensors give more depth of field at equivalent framing, which is an advantage in macro, landscape and product work where depth is the constraint. They also make long focal lengths cheaper and smaller — a wildlife photographer on a crop body gets reach that would cost several times as much on full frame.

Neither is better. They are different points on a set of trade-offs, and the format that suits depends entirely on what you photograph.

Phone sensors and marketing numbers

A phone’s actual focal length is a few millimetres. The number quoted in specifications is nearly always the full-frame equivalent, because the real one means nothing to most readers.

That equivalence is calculated the same way, with a crop factor several times larger than APS-C. It also explains why phone images have such deep depth of field naturally — the actual focal length is tiny — and why the shallow-background effect in portrait modes is computational rather than optical.

Lens coverage limits what fits

A practical constraint alongside the equivalence arithmetic.

Lenses project a circular image, and that circle has to cover the sensor. A lens designed for a smaller format projects a smaller circle, so mounting it on a larger sensor produces dark corners or a heavily vignetted frame — some cameras crop automatically to compensate, which discards the sensor advantage entirely.

The reverse works. A full-frame lens on a crop body covers the sensor with room to spare, using only the centre of the image circle, which is generally the sharpest part.

So lens compatibility runs one way, and a plan to buy crop lenses now and a full-frame body later means replacing the lenses too.

Common mistakes to avoid

• Applying crop factor to exposure, which produces a wrong meter setting.
• Comparing lenses across formats without converting both focal length and aperture.
• Assuming a larger sensor is better for every purpose, including macro where depth of field is the constraint.
• Quoting a phone’s actual focal length as though it were comparable.
• Forgetting that a crop body with a lens designed for it has no crop relative to that lens’s design — the equivalence is only for comparing across formats.

How to do it with Camera Sensor Size Comparator

The Camera Sensor Size Comparator converts framing and depth of field between formats.

1. Choose the two formats you are comparing.
2. Enter a focal length and aperture on the first.
3. Read the equivalent on the second — both numbers change.
4. Note the exposure line, which does not.

Other photography tools are in the tools directory.

Frequently asked questions

Does crop factor change exposure?

No. An f/1.8 lens gives the same exposure on every format. Multiplying aperture by crop factor gives the depth of field and total-light equivalent, which is a different question from how bright the image is.

Is a bigger sensor always better?

For low light and background separation, larger helps. For depth of field in macro and landscape, for reach, and for size and cost, smaller formats have genuine advantages.

Why do phone specifications quote a different focal length?

Because they quote the full-frame equivalent. A phone’s actual focal length is a few millimetres, which is why its images have such deep depth of field optically.

Final thought

Convert focal length and aperture together, and leave exposure alone. Half the arguments about crop factor are someone applying it to the one thing it does not touch.

Try the free Camera Sensor Size Comparator