How do you compare low-light performance across different sensor formats?
Asked 6/1/2020
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What’s the best way to compare low-light capability between camera and lens combinations that use different sensor sizes? For example, how should I think about combinations like full frame, medium format, and Micro Four Thirds when lens maximum aperture, sensor size, and sensor sensitivity all differ? Is there a standard way to compare them fairly, such as using equivalent aperture or another formula?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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The f-stop is independent of the format. Same f-stop means that with a given light, you will have the same exposure time on equally sensitive film/sensors (same ISO). The f-stop already factors in the focal length...
This said, the F-stop spec is not very accurate, and the movie industry seems to use T-stop. Actual focal length (that can vary with focusing in some lenses), and actual ISO sensitivity of your film/sensor could also be taken in account.
Originally by user75947. Source · Licensed CC BY-SA 4.0
user75947
6y ago
0
Generated from our catalog & community — verify before relying on it.
For exposure, f-stop itself is format-independent: at the same f-number, shutter speed, and ISO setting, different formats receive the same image-plane exposure.
Where format matters is when you want an equivalent comparison for the same framing and depth of field. A common rule is to use 35mm-equivalent aperture:
equivalent f-number = actual f-number × crop factor
So a smaller sensor system generally needs a proportionally wider aperture to match a larger format for the same angle of view and depth of field. That’s why equivalent aperture is often used to compare practical “low-light performance” across formats.
Caveats:
- f-stop is a geometric value; real light transmission can differ, so T-stops are more accurate.
- Actual ISO sensitivity may vary somewhat between cameras.
- Lens focal length can shift slightly with focus on some lenses.
So the standard approach is:
- Use actual f-stop for pure exposure comparisons.
- Use crop-factor-adjusted equivalent f-stop when comparing systems across formats for similar framing/DOF and overall low-light usability.
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