"Pros always use Manual exposure, A and S are for amateurs ..

Started 6 months ago | Discussions thread
sluggy_warrior Senior Member • Posts: 1,350
Re: "Pros always use Manual exposure, A and S are for amateurs ..

Michael Fryd wrote:

Actually, while many think of the ISO setting as controlling the camera's sensitivity, that's not actually what it does.

"Sensitivity" is the ability to "sense" something. When it comes to digital cameras it refers to the ability to detect photons. Down deep, what a sensor is really doing is counting photons. The number of photons counted is the same whether the camera is set for ISO 100 or ISO 128,000. If the photon wasn't seen by the sensor at ISO 100, it won't be seen at ISO 128,000.

What the ISO setting does do is alter how the collected data is processed. At low ISO a particular photon count might map to a dark grey. At a higher ISO that same photon count might map to bright white. The sensitivity of the sensor is not altered. No matter what the ISO it still counts the same number of photons.

While I don't like Michael's lengthy wording style that confuses readers unnecessarily, I have to agree on differentiating the terms "exposure" and "brightness". "Exposure" should be the amount of light the sensor has been "exposed" to, and that depends only on Aperture and SS. You can also control the lighting condition, but that's not a camera setting.

With films, ISO was part of the triangle, but it's usually fixed until the next film roll.

With ISO-less/ISO-invariant sensors, ISO is no longer part of the whatever-you-call-it triangle. I set my ISO setting to Auto and read its value as an indicator whether I could/should add more exposure (by adjusting aperture and/or SS) for a cleaner image. I usually shoot underexposed -2/3EV to avoid clipping, and bring up brightness in post.

When pointing at a scene, the camera would evaluate the correct brightness (e.g. ETTR). With Auto-ISO enabled, the camera still tries to find the combination of A and SS (exposure) to meet that brightness. But if there's no such combination that can satisfy the current constraints (e.g. already at max aperture of the lens, and SS is already at minimum defined by user to avoid shake), then it will increase ISO as last-resort option to meet the expected brightness. If you don't allow ISO to be raised automatically, the camera has no choice but to let the image be dark, and you can increase brightness in post with noise just like how the camera would raise the ISO.

I'd blame the software for the confusion as well. For example, darktable has a module named "exposure" but what it does is adjusting brightness and black point (measured in standard EV unit).

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