G6 vs newest sensors: Real difference in high ISO noize

Started Mar 5, 2014 | Questions thread
Anders W Forum Pro • Posts: 21,468
Re: G6 vs newest sensors: Real difference in high ISO noize

Øyvin Eikeland wrote:

Ulric wrote:

BTW, when you look at the "Measured ISO" that DxOmark use, don't assume that it is more real or more correct than the manufacturer stated ISO. It is just defined differently. Look at their measurements within their own coordinate system and you'll be fine.

Hi, I have problems interpreting the DXO graphs. Some help would be appreciated. DXO is apparently calculating the "real" iso sensitivity of the sensor by exposing the sensor to a known intensity and then calculate the sensitivity from the raw files. Is this correct?

For each camera ISO, DxO measures how much exposure the sensor can take before you get highlight clipping in RAW. This in turn determines what they call "measured ISO". If the lowest "measured ISO" of one camera is 100 and that of another 200, it means that the first camera can take twice as much exposure (one EV more) before you see any clipping in RAW.

Comparing DR, SNR, etc. at the same "measured ISO", as DxO does, effectively means that cameras are compared at the same exposure and the same level of RAW file saturation (kept constant at the clipping point of the sensor). Other sites (if they know what they are doing) also compare at the same exposure but at different levels of RAW file saturation (if the camera ISOs are differently calibrated with regard to measured ISO).

The points in the graph are then plotted against measured ISO and not against what you set on the camera. The right-most data-point for the OM-D E-M5 is taken with the camera iso set to 25600, right?


The measured ISO for this setting is only 11848. Therefore the data point is plotted at 11848 along the X-axis. What I do not get is this: How can I find the dynamic range (or SNR) you will get if I set the camera to an ISO of 25600?

If you expose up to the clipping point of the RAW file, the DR of the image would be the value indicated in the graph (6.61 EV). If you expose less than that it would be correspondingly less (one EV less DR for each EV less exposure).

The difference between the E-M5 with a camera ISO of 25600 and a measured ISO of 11848 and another camera, X, with a camera ISO of 25600 and a measured ISO of 25600 is that at the same exposure (and the meter of the two cameras would normally suggest the same exposure) the ADU values in the RAWs of camera X would be slightly more than one EV closer to the clipping point than that of the E-M5.

Consequently, when camera X is at the clipping point of the sensor, the E-M5 is slightly more than one EV below the clipping point. When the E-M5 is at the clipping point, camera X is has slightly more than one EV worth of clipping.

If the dynamic range in the raw-files are indeed 6.61EV, isn´t it unfair to plot the point just below 12800?

No, it isn't unfair. But I am not sure I understand your reasoning here. Why would it be unfair?


Øyvin Eikeland

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