Equivalent Aperture Calculator

f/1.8 on 1.5× APS-C frames like f/2.7 on full frame for depth of field and field of view. It is still f/1.8 for exposure and noise on that smaller sensor. Use this when someone says ‘just get the 50 /1.8’ and you are not on the same format.

Inputs

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Full-frame equivalent aperture: 2.70f/
DoF stop delta vs full frame: 0.58stops

Recommended gear

Fast wide prime
If you want full-frame f/1.8 DoF on APS-C, you need about f/1.2, not another f/1.8. Buy the speed you actually need.
Typical range: $200–$800
IBIS camera body
Format is a body choice. Equivalent aperture only makes sense once you know that body’s crop.
Typical range: $800–$2,500

Calculation method

Multiply the taking f-number by crop factor to get the full-frame f-number that would match field of view and depth of field. The DoF stop delta is log2 of the crop: 1.5× is about 0.58 stops more depth, 2× is one stop. Light per unit area on the sensor is still set by the real f-number; ISO and shutter do not get a crop bonus.

Exact formula

N_eq = N × crop. dofStopDelta = log2(crop). Limits: Exposure on the sensor is still N; T-stops, speed boosters and pixel-size noise are omitted.

Why portraits look different

Same 50 mm f/1.8 on APS-C is a tighter crop and more DoF than on full frame. To match a full-frame 50/1.8 look you want about a 33 mm f/1.2 on 1.5× — which is why those lenses exist.

Video T-stops

T-stop is transmission. Equivalent aperture here is still f-number geometry. A T1.5 cine lens on S35 is not ‘the same’ as T1.5 on full frame for DoF.

Frequently asked questions

Do I set f/2.7 on the lens?

No. You set f/1.8. f/2.7 is how you would describe the DoF to a full-frame shooter. The camera meters f/1.8.

What about noise?

A smaller sensor generally needs more ISO for the same shutter and f-number in the same light, so more noise. That is a different argument than this DoF conversion.

Speed booster again?

A 0.71× adapter multiplies both crop and aperture. Apply the adapter first, then this calculator on the resulting crop and f-number.

Disclaimer — no professional advice, no liability

Equivalent aperture is a DoF/FOV comparison, not a license to underexpose. The meter still wants the real f-number. Confirm the look with a test shot on your body.

These calculators, pages and any related content are provided “as is” and “as available”, solely for general educational and informational purposes. They are simplified mathematical estimates, not a light-meter reading, a studio plot or a shooting prescription, and they do not constitute professional photographic, cinematographic or optical advice.

Results can be wrong, incomplete or inapplicable to your camera, lens, sensor, filter, lighting or use case. You must independently verify every figure, assume all risk, and confirm settings on a test shot before a job that can miss the image, damage gear or injure someone.

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Equivalent Aperture Calculator

Inputs

Results

Recommended gear

Calculation method

Exact formula

Why portraits look different

Video T-stops

Frequently asked questions

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Field of View Calculator

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