How do aperture and ISO relate when keeping shutter speed and exposure the same?
Asked 3/16/2011
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2 answers
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If I take two photos in the same light at the same shutter speed, but switch between two lenses of the same focal length with different maximum apertures, how do I calculate the ISO needed to keep exposure the same?
Example:
- Lens 1: aperture f/X
- Lens 2: aperture f/4.5
- First shot: Lens 1 at ISO 100
- Second shot: Lens 2 at the same shutter speed and lighting
What ISO should I use on the second shot for starting apertures such as f/1.0, f/1.2, f/1.4, f/1.6, and f/1.8?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
3
For a fixed focal length, the size of the aperture (and therefore the quantity of light gathered) scales like the inverse square of F/#. So, if you double the F/#, you would need to quadruple the sensitivity of the detector. In the specific case you describe, assuming that the scene lighting doesn't change, and the shutter speed is fixed, you would be able to calculate the correct ISO as follows:
ISO=100*(4.5/X)2
Where ISO is the new sensitivity setting, and X is the starting F/#.
EDIT: photography.SE doesn't interpret LaTeX like physics.SE. My mistake!
Originally by user2877. Source · Licensed CC BY-SA 4.0
user2877
15y ago
0
Generated from our catalog & community — verify before relying on it.
With shutter speed and scene brightness unchanged, exposure varies with the aperture area, which is proportional to 1/(f-number)^2. So ISO must change by the square of the f-number ratio:
ISO2 = ISO1 × (N2 / N1)^2
For your case:
ISO2 = 100 × (4.5 / X)^2
where X is the first lens aperture and the second lens is set to f/4.5.
Examples:
- f/1.0 → ISO 2025
- f/1.2 → ISO 1406
- f/1.4 → ISO 1033
- f/1.6 → ISO 791
- f/1.8 → ISO 625
In practice, use the nearest ISO your camera offers.
You can also think in stops: each full stop change in aperture changes light by 2×, so you compensate with ISO by 2× per stop. For example, going from f/2 to f/5.6 is 3 stops less light, so ISO 100 becomes ISO 800.
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