Pls show examples where increasing brightness in post better than increasing ISO.

Started Oct 1, 2013 | Discussions thread
Jack Hogan
Veteran MemberPosts: 4,025
Re: You are welcome
In reply to Karl Persson, Oct 3, 2013

Karl Persson wrote:

Jack Hogan wrote:

But the X-axis is different, as is the slope at lower EVs. And I don´t see how multiplying ISO1600 values by 4 get you to ISO6400 values (in the graph). Please elaborate (I dont use RAWdigger).

On the D800e ISO 6400 raw values are really ISO 1600 raw values times four

Hi Karl,

Those are histograms of the bottom couple of hundred levels of the raw data from four D800 DPR studio scene captures at different exposure (hence the different slopes and y-axis but that is not what that image is there for).

Doubling ISO aotbe results in an approximate doubling of the mean raw values. As long as the amplification is done on the analog signal from the sensor before A/D conversion every integer value after amplification will be filled in because the signal fed to the ADC appears continuous as a result of igain and dithering - much like raising the volume on your radio. The histogram would be full, as it is at ISO800 and ISO1600ish*.

If on the other hand the signal from the sensor is held constant and doubling is applied after it has already been converted to a set of digital integers, the resulting raw values would look something like this:

10,11,12,13,14 at ISO1600 would become 20,22,24,26,28 (every other value present) at ISO3200 and 40,44,48,52,56 (every forth value present) at ISO6400.

It is just a straight multiplication of the ISO1600 integers and gaps will result as shown in the raw histograms above. It would also make no difference whether you applied the multiplication in-camera (by raising the ISO) or in-computer (by moving the 'Exposure Compensation' slider to the right in a well behaved raw convertr) because the signal from the sensor is not involved - you are just acting on the same digital data in both cases. The raw data would simply be the ISO1600 information multiplied by two or four in the example.

Well, in fact there would be a littledifference. What would happen to values 5000,5001,5002,5003 captured at ISO1600 when pushed to 6400 with the same exposure? 20000,20004,20008,20012. Oops, they would be clipped, since saturation for the D800 is around value 16000  But if you shot at the lower ISO the information would be captured - and you may decide that you want to squeeze it into your final image in the end.

So if you are in the field, shoot raw and you have maxed out aperture and exposure time given your artistic constraints (DOF, sharpness, bokeh, blur, detail in highlights) - and you feel the urge to push the ISO to 6400 to make the image pleasantly bright on your D800's screen - keep in mind that you have the option to back down to ISO1600 and gain two additional stops of DR without any IQ penalty whatsoever. Those two additional stops of DR may come in handy in some situation. The downside is that the image may look darker than you are used to on the camera screen and you may have to adjust the capture's brightness to your liking during raw conversion.



*You guessed it, those missing values in the ISO1600 histogram mean that the D800 stops using analog amplification a little before ISO1600

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