Another ISO 12232 metric of digital camera sensitivity

[Doug Kerr]

The HTML code that (in my email client, Thunderbird) came from the LaTeX code that described this equation:

is this:

<math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msqrt><msub><mi>x</mi><mn>1</mn></msub></msqrt><mo>+</mo><mfrac><msup><mi>n</mi><mn>3<br>
</mn></msup><mn>2</mn></mfrac></mrow></semantics></math><br>

The original LaTeX code for this equation seems to have escaped, but I suspect it was this:

\sqrt{x_1}+\frac{n^3}{2}​

Best regards,

Doug

 

[Ted Cousins]

ISO 12232:2019, Amendment 1:2020 added a new metric of digital camera sensitivity, the encoding-relative sensitivity (ERS).

This is defined in terms of the photometric exposure required to produce a digital output signal of the value that is said to be equivalent to L*=50 (presumably referring to the L*a*b* color space coordinate) for the encoding system that is in use.

This seems to be, for all practical purposes, equivalent to the ISO SOS, but takes a different approach to referring to the digital output value.

Equivalence to SOS: Functionally, ERS acts similarly to Standard Output Sensitivity (SOS). However, ERS utilizes the CIELAB L* scale to standardize output reference values across varying image encoding systems and gamuts.

best,

Ted.

 

[Doug Kerr]

Hi, Ted,

You said the 'bot said:

"Equivalence to SOS: Functionally, ERS acts similarly to Standard Output Sensitivity (SOS). However, ERS utilizes the CIELAB L* scale to standardize output reference values across varying image encoding systems and gamuts. "

I think that is a very good description of the situation.

Thanks,

Best regards,

Doug

 

[Ted Cousins]

Hi, Ted,

You said the 'bot said:

"Equivalence to SOS: Functionally, ERS acts similarly to Standard Output Sensitivity (SOS). However, ERS utilizes the CIELAB L* scale to standardize output reference values across varying image encoding systems and gamuts. "

I think that is a very good description of the situation.

Out of interest, I went to Bruce's calculator, converted L*=50, a*=0, b*=0 to a couple RGBs:

Lab to ProPhoto:

Lab to sRGB:

Interestingly different, eh?

best,

Ted.

 

[Doug Kerr]

Hi, Ted,

Indeed.

The ISO SOS "sensitivity point" is based on a digital output defined in terms of a value (118 for 8-bit representation) in RGB color space, but seemingly not a specific one.* The REI "sensitivity point" is defined in terms that has a "fixed" implication of relative luminance (namely L*=50 in the L*a*b*color space).

*One might think that this was in terms of the IEC 61966-2-1 color space ("sRGB") but seemingly not.

As you have shown, for the sRGB color space, a relative luminance indicated by L*=50 in the L*a*b*color space has RGB values of a bit over 118.

Thanks.

Best regards,

Doug

 

[Ted Cousins]

Hi, Ted,

Indeed.

The ISO SOS "sensitivity point" is based on a digital output defined in terms of a value (118 for 8-bit representation) in RGB color space, but seemingly not a specific one.* The REI "sensitivity point" is defined in terms that has a "fixed" implication of relative luminance (namely L*=50 in the L*a*b*color space).

*One might think that this was in terms of the IEC 61966-2-1 color space ("sRGB") but seemingly not.

As you have shown, for the sRGB color space, a relative luminance indicated by L*=50 in the L*a*b*color space has RGB values of a bit over 118.

Thanks.

Best regards,

Doug

 

[Ted Cousins]

Hi, Ted,

Indeed.

The ISO SOS "sensitivity point" is based on a digital output defined in terms of a value (118 for 8-bit representation) in RGB color space, but seemingly not a specific one.* The REI "sensitivity point" is defined in terms that has a "fixed" implication of relative luminance (namely L*=50 in the L*a*b*color space).

*One might think that this was in terms of the IEC 61966-2-1 color space ("sRGB") but seemingly not.

And yet CIPA DC-004 is quite clear in that regard to SOS:

Y ＝MAX x γ{ 0.2126 γ -1 {R/MAX}＋0.7152 γ -1{G/MAX}＋0.0722 γ -1{B/MAX} }
MAX is the normalization coefficient（＝maximum digital output level: 255 for 8-bit system）γ{ }, γ -1{ } denote sRGB gamma characteristics and the inverse transformation (linearization) characteristics defined by IEC61966-2-1.

As you have shown, for the sRGB color space, a relative luminance indicated by L*=50 in the L*a*b*color space has RGB values of a bit over 118.

A bit under 119, actually ...

best regards,

Ted.

 

[Will Thompson]

What does martial arts have to do with photography?

Chuck Norris
David Carradine
Bruce Lee

???

 

[Doug Kerr]

Hi, Will,

What does martial arts have to do with photography?

Chuck Norris
David Carradine
Bruce Lee

???

I give up.

Best regards,

Doug

 

[Will Thompson]

Someone mentioned Chuck!