An In-depth Discussion of M + Auto-ISO (Very Long!)

Preamble

The reason I am writing this piece is not to convince everyone that they should convert to an auto-ISO user, but simply to explain why auto-ISO is a very good feature. Seeing that many people tend to dismiss this feature without really understand how to use it, and discourages others at the same time, I feel it calls for a proper discussion.

Introducing Auto-ISO

Auto-ISO is a feature that lets the camera change ISO automatically. The first canon SLR to have real auto-ISO is 7D. Canons before 7d had a non-working auto-ISO that always stuck on ISO 400. Since 7d, auto-ISO would select the ISO that would give you correctly exposed final image based on the calculations of the camera's metering system, in combination with chosen shutter speed and aperture.

Personally I think auto-ISO is the most useful in M mode, I'll explain why and then explain it's application in Av mode. But before we do we will first discussion some surrounding issues.

The Nature of ISO

In digital photography, increasing ISO is increasing the level of amplification to the signal that sensor has received, and base ISO is simply leaving that signal free of any additional amplification. Amplification is either done by hardware (or sometimes called analog) or by software. Canon APS-C SLRs are only capable of hardware amplification at full stops, e.g. ISO 200/400/800 etc. 1/3 and 1/2 stop increments are done by in camera software and you can achieve the same effect in post processing.

Software amplification is manipulation to the image file after hardware amplification is finished, and it does not achieve the same image quality as hardware amplification. the reduced quality is easily demonstrated by comparing two images taken with the same shutter speed and aperture under the same lighting, one shot at ISO 100 and underexposed by 3 stops, then increase exposure by 3 stop in raw processing software, the other correctly exposed at ISO 800, it would be immediately apparent to the viewer that the ISO 800 image have much less noise as well as better contrast and colour accuracy.

This is a very important point to remember, because it means by manually setting ISO, what you are dictating is the amount of amplification the camera will apply to the image after it is captured. Whereas controlling aperture and shutter speed is controlling how that image is to be captured.

Earlier I have said that these in-camera 1/3 stop ISO increments are done by software and that software is not ideal. While this is true in reality 1/3 stop push or pull makes hardly any real impact to image quality, so for all intended purposes it should be overlooked.

On a side note, in the current line up of canon APS-C cameras the upper limit is ISO 6400. ISO 12800 is selectable but is software pushed from ISO 6400 and is not available in Auto-ISO (because you can just push your ISO 6400 is post processing). I do not know if the reason for this is whether it is limited by hardware amplification technology or that at this level, image quality is already destroyed so badly, hardware amplification would produce no better result than software.

Camera Metering

Regardless what some people would like to claim, the human eye and brain adapts to different lighting conditions way too well so that we are unable to correctly judge the actually amount of light surrounds us. Camera engineers recognised this long ago and invented electronic metering system (which they put in cameras). the metering system judges the scene for us and calculates the required shutter speed and/or aperture and/or ISO. If we use these calculated values to shoot the picture we will get a balanced looking image, we call it "correctly exposed image". (for the sake of simplicity we will skip the variation caused by different metering mode, it is also not material to this discussion)

Over the years the metering system improved gradually. As of 2011, the metering system from the world's largest and best camera manufacture is very advanced and very accurate. Majority of camera users - those who use Av, Tv, P, or greenbox mode, are quite happily relying on the metering system. The only people that don’t use it are those who use M mode in its classic fashion (i.e. manual ISO). They rely on their "brain and experience" when it comes to metering. As far as I can see, they are not producing "more correctly exposed" images, and if I was forced to make a guess, I would say they produces a lot more unusable images than the rest of us when they try to find out the correct settings in a given scene.

Now, if by using Av mode, where the user dictates aperture and ISO value, we trust the metering system to determine the correct shutter speed that would deliver a correctly exposed image, then there really is no reason as to where the user is to dictate aperture and shutter speed, why we would not trust the same metering system, using the same calculation formula, to determine the correct ISO that would deliver a correctly exposed image. Such mode is M + auto-ISO.