EV0 in raw histograms

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RawDigger writes the following on their website (emphasis by me):

automatic calculation of grid zero (gray point position) and setting grid step to 1EV. The zero is located at the level that is 3 stops (8 times) lower than the maximum pixel value rounded up (ceiling) to the nearest power of 2. For example, if the maximum value of all pixels in the photo is 3000, then the nearest power of two greater than this value is 4096 and the EV0 value for this photo will be automatically set to level 512.

For a 16-bit raw file, the EV0 line is put a pixel value of 8192 (2^13), for a 14-bit file it’s at 2048 (2^11) etc. I have a simple question: why? It feels arbitrary, considering the definition of EV needs information from the scene, and not only the camera.

But perhaps I’m missing something obvious. Please enlighten me :slight_smile:

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I think the maximum value is meant with regard to the actual image not the maximum possible value of the camera.

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It doesn’t make sense to me why it wouldn’t just use 3 stops below the maximum possible value.

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Maybe my question was too short. I meant: why 3 stops? Why is the EV0 line at 2^{\mathrm{bitdepth}-3}?

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Reading the web page carefully, EV0 is set 3 stops down from the power of 2 that is at or higher than either the maximum value in the image, or the theoretical maximum for the camera.

Yes, this is abritrary. So is setting it at 18% of a maximum, but this is a similar setting, about 2.5 stops down from maximum.

In this example, EV0 has no connection at all to the exposure made by the camera.

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Thanks Alan for confirming my suspicion!

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Looking more closely at the graph on the linked page, it has an obvious bug. Perhaps the RawDigger people wouldn’t call it a bug, but a “simplification”.

EV 0 is at a value of 512.
EV -1 is at a value of 256.
EV -2 is at a value of 128.
And so on down to:
EV -8 is at a value of 2.
EV -9 is at a value of 1.
EV -10 is at a value of 0.

Oops. EV -10 should be at a value of 0.5. Then EV -11 should be at 0.25. And so on, down to EV -infinity at a value of 0. A graph that goes down to -infinity isn’t reasonable (EDIT or relevant as there can be no values between 0 and 1), so I can see why they have made this simplification.

And I see that if you don’t want EV 0 to be at a value of 512, you can change it.

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RawDigger has an overexposure offset (and possibly an Adobe one) on by default.

Defaults! :angry::fist: … :rofl:

@snibgo You know, you could just edit your entire post to fix the numbers.

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Sorry, I don’t understand? Fix what numbers?

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Here.

I skimmed your post. Sorry, if I misunderstood.

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The fix needed isn’t in my post, but in the graph at RawDigger Histograms | RawDigger

The graph claims falsely that EV -10 represents a pixel value of zero. In fact, EV -10 represents a pixel value of 0.5.

To correct the graph, either change “0” to “0.5”, or remove “-10” from the top-left.

It is, I admit, a pedantic point.

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Oh, I see what you mean now. :+1:

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OT - old mathemetician joke:

At the end of the first day of a mathemeticians’ conference, there is a long line of mathemeticians at the bar near the convention center. The mathemetician at the head of the line orders a beer. The one behind him immediately asks for a half glass of beer. The next one orders a quarter glass.

The bartender holds up his hand and draws two glasses of beer, placing them on the counter saying, “You guys need to learn your limits.”

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