Myth or truth: is raising RAW exposure the same of raising ISO?

[ntx]

Hi CiC,
not a bizarre question today, but I would like to investigate a (sort of) myth in photography.

Sometimes I hear photographers saying that raising the ISO settings is the same of shooting in RAW at the lower ISO settings and adjusting the exposure in the RAW developer.
That means shooting at ISO 400 is like shooting at ISO 100 giving a +2 stop exposure while developing the RAW. Taking care of the image quality, is that true?

Beside this, I wonder how the sensor achives higher ISO settings... are they achived by multipling the digital data or by amplifing the analogue signal?

Bye
Jenner

[Colin Southern]

is that true?

Hi Jenner,

In a word, no.

Beside this, I wonder how the sensor achives higher ISO settings... are they achived by multipling the digital data or by amplifing the analogue signal?

The latter. Providing that shot is correctly exposed, you'll get a LOT less noise by raising the ISO.

[_GUI_]

The latter. Providing that shot is correctly exposed, you'll get a LOT less noise by raising the ISO.

Watch out Colin,

This used to be true (and of course remains so) in some cameras, for example older Nikons and specially Canons. A poorly exposed shot at ISO100 will be much noisier once exposure is pushed in the RAW development than a correctly exposed shot at ISO1600 (assuming the same aperture and shutter on both). But lately appeared sensors, such as the Sony that Pentax K5, Nikon D7000 and Sony A580 share, behave close to an iso-less sensor. This means they provide negligible advantage in image quality (i.e. noise) for pushing ISO to get a correct exposure.

I already talked about this in the forum. Believe it or not, but a shot 5 stops underexposed in the Pentax K5 at ISO100 and corrected +5EV in pp provides nearly the same amount of visible noise as shooting at ISO3200 with the same aperture and shutter. So on these cameras, if the scene contains highlights information (bright skies on landscapes, light sources or windows on interiors,...), it is more recommended to stay at base ISO getting a general RAW underexposure than pushing ISO, since that would blow the highlights with almost no noise advantage.

So in newer cameras, raising exposure in the RAW developer and pushing ISO on the camera are becoming the same thing from the IQ point of view.

Beside this, I wonder how the sensor achives higher ISO settings... are they achived by multipling the digital data or by amplifing the analogue signal?

They are achieved in both ways. From base ISO up to ISO1600/ISO3200 (depending on the camera), ISO is achieved through analogue amplification. From that point, ISO is just a digital multiplication of the RAW data (that is why the pure RAW histograms are plenty of holes).

The lately appeared Fuji X100 even doesn't care of mutiplying (and this is very clever since it preserves highlights information at no cost); ISO3200 on this camera is just ISO1600 plus some metadata telling the RAW developer 'hey! this is supposed to be ISO3200, so display it to the uninformed user 1 stop brighter than it really is'. Tricky games taking place under the hood.

Regards

[MarkE]

This is very interesting. Now you have identified a topic for me to explore/experiment with

But, you still have exposure considerations such as action-stopping, DOF, etc which drives ISO when the image is taken.

[JK6065]

As I would also answer this question with no (as Colin did) it seems that in a mere technical approach there's something to say for both, as Gui replied. I'm certainly not saying his answer is wrong, so I should suggest you make sure you know what's best for your camera. Therefore you'd always have the right answer for your case.