Auto-ISO or EC in Manual Mode - how does it work?

By Manual mode I mean that the aperture and shutter are set by the User and can not be changed by fancy stuff inside the camera.

I'm curious because my camera doesn't have Auto-ISO and EC is disabled in Manual.

EC changes ISO value.

Got it, I think, thank y'all both.

If i set an Exposure Value (aperture+shutter) and the scene wants more - then Auto-ISO (if it had) would increase the ISO number sent in metadata** to the converter thus increasing brightness pro-rata in the converted image, and EC (if enabled) would bias that number accordingly.

** or the setting of the PGA between the sensor output and the ADC.

I imagine that the ISO values so decided are not in the hard steps that one gets from the ISO knob.

In manual exposure mode, my Nikon cameras apply the exposure compensation (EC) setting to the in-camera lightness meter. If a scene would normally meter at 0, an EC setting of +1 will force a lightness meter reading of -1. An EC setting of -1 will force a meter reading of +1.

In manual plus auto-ISO, the camera selects an ISO that, combined with the f-stop and shutter speed selrcted by the photographer, will produce a lightness meter reading of 0. If the photographer also dials in an EC of +1, the ISO will double. An EC setting of -1 would result in the ISO being halved.

Thanks, Bill, looks like we were typing at the same time, but my thoughts appear to agree with you.

On my Nikon cameras Auto-ISO has six settings per stop rather than the two or three available when manually setting ISO. I'm not sure how "hard" they are, and for that matter I'm not sure how "hard" the aperture and shutter speeds are when I use those to automatically adjust the exposure (rather than the ISO).

Your SD9 doesn't really "need" it, because there is no (raw) point in using a higher ISO setting, as it does not reduce exposure-referred read noise or quantization.
Cameras that use higher analog gain at an early stage in the signal chain to gain on late-stage noise and/or quantization can benefit the user by upping the ISO setting, especially when no special highlight needs are present.

With the cameras I've used that have "EC" in M mode with auto-ISO, it is very, very, simple. With an "EC" of 0, the camera just chooses the ISO setting that seems to be well-matched to the "exposure value" (exposure time and f-number as a single parameter) and absolute metered exposure. It chooses the ISO setting that gives the same image "lightness" as the same "EC" would give in Av or Tv mode with fixed manual ISO. Using positive EC tells the camera to make lighter images; using negative tells the camera to make them darker. For Av and Tv with fixed ISO, that varies actual exposure. For auto-ISO with M, exposure in unchanged but varying ISO setting with "EC" varies the lightness.

It is an image lightness bias that is intended mainly for compensating for high- and low-key scenes, but you can use if for ETTR or extending headroom, if you wish.

Perhaps one of the simplest ways of looking at "EC" with auto-ISO in M mode is to think about full-manual. You point the camera at something, and you can turn the ISO dial and watch the meter change with ISO, because it is metering relative to intended middle grey for the ISO settings, not an absolute exposure meter. If you don't consider the key of the scene, and just turn the ISO dial until the meter says "0", this is exactly what the camera does when it automates ISO but the EC is set to "0". If you manually change the ISO such that the meter says "+1", this is the same thing as automation of ISO when you set the "EC" to "+1". IOW, the camera is just "turning the dial" for you; it is automating what you would do anyway, if you were doing full manual and turned the ISO dial to get the same value on the meter.

The main difference is that with full manual, you may be choosing it because light is constant and you don't want anything varying, and auto-ISO can not properly emulate that, as it will change the ISO as you point the camera from white subjects to black subjects, etc. If you're not doing that, then auto-ISO is often safer than full manual, because if you forget to change the EC value it is usually only 1 to 2 stops off of target, but with full-manual, if you forget to change the ISO with shade and sunlight, you can be much farther off target, by several stops.

I do not agree with the characterization that dialing in an EC setting "is automating what you would do anyway." One of the reasons I use full manual exposure mode is that I can use a metering reference to confirm my settings. Whether the reference is straw-hued summer grass (+0.6), a stand of Ponderosa pines (-0.3), or a neutral-toned field (0.0), if my chosen settings produce the desired reading for my chosen reference, anything else in the frame will also be captured with a desirable lightness. Unless the quality of light illuminating the scene changes, I don't care what the in-camera lightness meter is reading, it's aligned with my objectives. You even make reference to this in your next paragraph.

Auto ISO, by contrast, puts the camera in a mode where it's constantly checking and adjusting image lightness even when the light hasn't changed. Rather than focusing on composition and the moment, I'd have to be aware of what's being metered, how the metering mode may influence the results, and how the camera might respond. For me, this was what became so frustrating about working with auto ISO: I had a pretty good idea how the camera would respond but it was always a guess. In full manual, I know exactly how I'm responding to changing lighting and data input from the camera.

In my experience, it's more likely that a photographer shooting manual plus auto ISO will forget to adjust EC than a photographer shooting full manual will forget to adjust ISO. The photographer shooting full manual pretty quickly develops a habit of paying close attention to scene brightness in the OVF along with the in-camera lightness meter reading (DSLR) or to the lightness of the EVF image combined with the lightness meter reading. These in-camera references (along with subtle changes in scene brightness detected by the eye and body) are the photographer's most immediate & reliable indicators of a need to make a setting adjustment. A 1/3-stop change in the quality of light, scene brightness in the OVF, image lightness in an EVF, or the in-camera lightness meter reading will get the full manual photographer's attention.

I just watch the ISO setting in my viewfinder and use that as a guide. When it bottoms out I need to raise the shutter speed or (less often) set a higher f-stop; if the ISO is going too high then I do the opposite (if the situation allows it). I find that approach more readily visible and intuitive than monitoring the EC dial in the viewfinder, and vastly easier than responding to quickly changing conditions by reaching for the ISO button and the rear dial at the top of my cameras.