Started Dec 20, 2013 | Questions thread
ZodiacPhoto Senior Member • Posts: 1,988
Re: You're still missing something

texinwien wrote:

Ysbrand Galama wrote:

But if you want to improve the S/N-ratio, why not reduce the ISO instead of lengthening the exposure time? A lower ISO has a better S/N-ratio

No, it absolutely does not. ISO is not a part of the exposure equation. Shutter speed, aperture and scene brightness are the only three variables in the exposure equation. If you hold those three variables constant and lower ISO, you will not have an improved SNR.

That is absolutely true for an ISO-less camera. However, for a camera with analog pre-amplifier between the sensor and converter (ADC), reducing gain also increases S/N ratio, because sensor's thermal noise is less amplified. Now, how much it matters in a particular camera, depends on many variables - a simple 10 min. test can show. Take a series of photos of the same dimly-lit scene (a.g., your room) with the same aperture and shutter speed, but at different ISOs, and adjust to the same brightness in post-processing. Start from high ISO, like 3200, to eliminate highlights cropping. Compare the noise, and make your own conclusions. This is the best way to know YOUR camera.

Also, ETTR can be used with JPG shooting. For example, a scene with plenty of mid-tones, and some shadows, but without much of meaningful highlights. The "proper" histogram will show most information in the mid-tones, with shadows clipped on the left, and not much going on in highlights. If you use ETTR to move mid-tones to the right, you can rescue shadows and reduce noise. of course, you will have to reduce brightness in post-processing.

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