Does aperture equivalence change with sensor size the way focal length/field of view does?
Asked 7/23/2017
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If you compare the same framing across formats, does the f-number itself scale with sensor size? For example, is a 25mm f/1.2 Micro Four Thirds lens equivalent to a 50mm f/1.2 on full frame, or more like a 50mm f/2.x? I'm trying to understand the difference between exposure equivalence and depth-of-field equivalence when comparing MFT and full frame.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
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No, f/stop does not vary with sensor size.
Nor does focal length vary with sensor size. The lens remains totally unaffected by the sensor.
HOWEVER, the field of view that the cropped sensor can see and capture is seriously affected in the smaller sensor. We might imagine that to be a lens difference, but it is only a sensor difference.
Exposure does "vary" with lens focal length, therefore the whole idea of inventing f/stop numbering is that f/stop does not vary.
f/stop = focal length / aperture diameter.
A lens twice longer has an aperture of twice diameter, for same f/stop number, and same exposure.
So regardless of the "size of the lens" (diameter or focal length), the same computed f/stop number represents the same exposure.
This is complicated slightly in that each glass-air surface in the lens has slight transmission losses, which is greater in lenses with more glass elements. However, modern lens coatings reduce this loss to a small factor, easily negligible in still photo cameras.
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
9y ago
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The f-number itself does not change with sensor size. A lens stays a 25mm f/1.2 lens regardless of what sensor is behind it.
What changes with sensor size is mainly field of view, and when comparing same framing across formats, depth of field.
For your example:
- Exposure equivalence: a 25mm f/1.2 on Micro Four Thirds gives the same exposure as a 50mm f/1.2 on full frame, assuming the same shutter speed and ISO.
- Depth of field equivalence: for the same framing and viewing size, a 25mm f/1.2 on MFT is roughly equivalent to a 50mm f/2.4–f/2.5 on full frame.
Why: f-number is focal length divided by aperture diameter, so it already accounts for lens size and determines image brightness at the sensor plane. Sensor size does not alter that.
So the short answer is:
- Same exposure: 25mm f/1.2 MFT ≈ 50mm f/1.2 FF
- Similar field of view and depth of field: 25mm f/1.2 MFT ≈ 50mm f/2.4 FF
There is no single “full equivalence” number across formats; it depends on whether you mean exposure, angle of view, or depth of field.
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