You are right but this is only true if you are recording a scene with a narrow dynamic range like the one you posted.

For example, if a scene covers only 5 stops, the raw file might contain a range of values (in a 14-bit raw file) between:

1. 512 through 16383 at ISO 1600
2. 256 through 8191 at ISO 800
3. 128 through 4095 at ISO 400
4. 64 through 2048 at ISO 200
5. 32 through 1023 at ISO 100

All of these can be made to look the same after you adjust only the Exposure slider during post processing. All of the noise levels and colors will be the same because the physical exposure is the same. As CaptainC points out, you need to be using a modern ISO invariant sensor.

The only difference will be in the appearance of the JPEG that comes straight from the camera. For the range of raw values I listed above, it is likely that only #2 will look right. #1 will be overexposed and #3, 4 and 5 will look underexposed by 1, 2 and 3 stops.

For a scene with a wider DR you can't really get away with this much variation. However, since colors are only really strong around middle gray +/- 1 stop and clear details +/- 2 stops, the important parts of any scene will still cover less than 5 stops.

I was unaware until recently that the camera exposes everything based on aperture and shutter speed alone. The ISO you dial in is a simple after the fact amplification of the signal, and it happens during the processing, not the exposure itself. So taking a shot with ISO 1600 or 200 makes no difference if you are shooting raw. You can apply the 3EV boost using your ISO dial or pushing the exposure 3EV during conversion of the raw file. Here are 2 jpegs, one was taken at ISO 200 and one at 1600. The 200 jpg straight from the camera was black, and the 1600 jpg was properly exposed. Aperture and shutter were the same both shots. the raw 200 was pushed 3 EV during conversion from raw to jpg. Bet you can't tell the difference!

You say:
"the camera exposes everything based on aperture and shutter speed alone. The ISO you dial in is a simple after the fact amplification of the signal"

"The camera doesn't care it shoots everything at its native ISO and only applies the amplification after the fact."


So:
I set my camera to aperture priority, ISO 200, and f/5 and aim it at a target: the camera sets shutter at 1/50.

Keeping everything else the same I now set the ISO to 1000. Based on your comments I would expect the shutter to stay at 1/50.

It doesn't, it goes to 1/250. And it shows that value in the viewfinder before I take the shot.

This doesn't seem to track with your assertions. What am I missing?

Edit: Camera is Nikon 5300 set to record Raw.

You are letting the camera set the shutter speed. But the EXPOSURE is going to be the product of the aperture and the shutter speed. The camera does not shoot the photo at any other ISO but the native ISO of the camera. The ISO you dial in, or set in a menu, is processed in AFTER the shutter closes - it is written into the raw file, and appears in the processing of the jpeg or tiff which is the readable file. In other words the instructions for brightness are added into the raw file after the exposure is made.

OK, I understand that the sensor sensitivity does not change. But Cat says the camera sets the exposure (aperture / shutter) independently from the ISO setting. But in my example, when I changed ISO the camera changed the shutter speed. So the camera, for the same image, gave a different exposure to the sensor depending on ISO setting.

If the ISO change (i.e. the gain change) is applied only after the exposure, as you all seem to imply, why did the camera change the shutter speed when I changed ISO?

Maybe it's time for a beer.....

Extremely interesting!

For folks like me, who spent a lifetime dealing with film and A.S.A., ISO, din, Weston and personalized exposure indexes, it my be difficult to relate all of that directly to digital/electronic sensitivity ratings. Of course, physically speaking, there is quite a difference between silver halide/chemical technology and the physics of the movement of electrons in a circuit. Film, with its emulsion issues of speed, grain, acutance and the enormous variations of all of theses factors as they are effected by as many variations in processing is a science and sometimes a form of alchemy or witch's brews in itself. Digital photography is more stable, repeatable and predictable- at least to me.

In a practical usage sense, however, it seems to me that are cretin correlations or parallels of the two technologies that hold true regardless of their theoretical differences such as format/sensor size and an increase of nose/grain as the exposure index is increased and how theses phenomena relate to the quality of the final image, especially in critical scenarios or requirements such as where high degrees of enlargement are indicated.

In my own professional/commercial my practice is simply to shoot everything in RAW, oftentimes in manual mode and selection ISO speeds accordingly. Mostly I opt for the lowest speed that is practical but have not experienced any serious quality losses at moderate to higher speeds. I seldom have the need to go as high as 1600 let alone higher. Am I missing something?

Oh!- this is in no way a facetious question. Down here in "commercial land" we get so involved in workaday stuff and production routines that we can miss out on some of the newfangled good stuff and latest discoveries or theories. Please elaborate!

My question is, regardless of where the "amplification" takes place in the digital chain of events, is the any difference in photographic results and does this discovery debunk the common knowledge or conventional wisdom of RAW vs. Jpeg usage, noise issues or anythg in post processing?