Q2 monochrom auto-ISO

[Simon Blackley]

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I shoot in manual mode, either at ISO 100 or in auto-ISO.

When in auto-ISO, I notice that the camera never drops the ISO below 200, even when there is plenty of light.

Can anyone explain what's going on?

[jaapv]

That is correct as ISO 100 is a pull value with reduced dynamic range in the highlights. 

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[JimKasson]

8 hours ago, jaapv said:

That is correct as ISO 100 is a pull value with reduced dynamic range in the highlights. 

That is surprising to me. Here's what I get for EDR vs ISO on my Q2M.

 

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[jaapv]

well, in that case I have no explanation. Your measurement is confirmed by PhotonstoPhotos

 

 

 

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[Simon Blackley]

Surely Leica can tell us what the base ISO is, ffs?

[JimKasson]

It turns out there is some funny business at ISO 100. It looks like Leica is chopping off the right side of the histogram before nominal full scale. This is a different techniques than SOny, Nikon, and Fuji use for their fake ISOs.

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[JimKasson]

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I've looked into this situation some more, and it appears that ISO 100 is a fake ISO on the Q2 Monochrom.

 

https://blog.kasson.com/leica-q2-monochrom/leica-q2-monochrom-edr-fake-iso/

[junix]

This data is mentioned in the Technical Data sheet of the Q2M "Dynamic range 13 aperture stops for basic ISO 200", screen shot from the pdf:

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Now you can shoot in Manual mode at ISO 200

[JimKasson]

I don't get anywhere near 13 stops of DR at ISO 200. I get about 12 stops for EDR. What are Leica's test conditions?

Edited May 20, 2022 by JimKasson

[JimKasson]

25 minutes ago, JimKasson said:

I don't get anywhere near 13 stops of DR at ISO 200. I get about 12 stops for EDR. What are Leica's test conditions?

Bill Claff gets the read noise at ISO 200 as 3.117 LSBs.

https://www.photonstophotos.net/Charts/RN_ADU.htm#Leica Q2 MONOCHROM_14

2*log2(16383/3.117) = 12.36 stops.

Edited May 20, 2022 by JimKasson

[Ba Erv]

I’ve been confused about this for months.  Sean Reid also mentioned in his testing that base ISO was around 160…which doesn’t exist on the camera.  For me the battle with this camera has been preserving the highlights while monitoring the histogram. The highlight spike on the histogram seems to go from midrange to off with merely a 1/3 change of exposure compensation between ISO 100 vs 200.  There doesn’t appear to be any smooth transition.  Someone educate me, pleeeeese!

[JimKasson]

17 minutes ago, Ba Erv said:

I’ve been confused about this for months.  Sean Reid also mentioned in his testing that base ISO was around 160…which doesn’t exist on the camera.  For me the battle with this camera has been preserving the highlights while monitoring the histogram. The highlight spike on the histogram seems to go from midrange to off with merely a 1/3 change of exposure compensation between ISO 100 vs 200.  There doesn’t appear to be any smooth transition.  Someone educate me, pleeeeese!

Bill Claff has mentioned to me that the sensor might be saturating before clipping of the ADC at ISO 100. Determining the base ISO of the sensor under those conditions will depend on what deviation from linearity is allowed at the upper end of the photon transfer curve. If you allow a lot of nonlinearity, and Bill's conjecture is what's going on, then the base ISO is about 100*(16383/12521) = 131. Don't hold me to that calculation. It's late here.

Edited May 20, 2022 by JimKasson

[Ba Erv]

25 minutes ago, JimKasson said:

Bill Claff has mentioned to me that the sensor might be saturating before clipping of the ADC at ISO 100. Determining the base ISO of the sensor under those conditions will depend on what deviation from linearity is allowed at the upper end of the photon transfer curve. If you allow a lot of nonlinearity, and Bill's conjecture is what's going on, then the base ISO is about 100*(16383/12521) = 131. Don't hold me to that calculation. It's late here.

Thanks, correspondIngly it’s early here so I need another Red Bull and a few more espressos before math.