Originally posted by Class A So you're saying "measurement error"?

That's pretty obvious, isn't it?

The 4-digit precise DR figure 14.12 EV at ISO 70 and 13.70 EV at ISO 91 are derived from the 0dB luminosities. You can compute it for yourself (the SNR numbers appear when hoovering with the mouse):

14.12 vs. 14.127 = -log_2(0.008%) + log_2(4992 x 3284/8M)/2 (ISO 80)

13.70 vs. 13.668 = -log_2(0.011%) + log_2(4992 x 3284/8M)/2 (ISO 100)

So, the difference in EV is log_2(0.011%)-log_2(0.008%) = log_2(11/8) = 0.46.

BTW, the published EV difference is 0.42 = log_2(111/83) giving an indication of possible luminosity measures prior to rounding (0.0083% and 0.0111%). However, all luminosities are probably rounded to 0.001% for a reason.

Therefore, the EV differences are within log_2(106/84)=

**0.34** and log_2(114/76)=

**0.58** from the measured data.

The expected EV difference between ISO 100 and 79 (1/3 stop less) is

**0.33, **the expected EV difference between ISO 70 and 91 is log_2(91/70)=

**0.38**, the published EV difference is

**0.42**, the EV difference from published SNR data is

**0.46**. The published difference (0.04) from expectation is small meaning that they measure a bit better than the rounding to 0.001% would suggest.

But there is absolutely no reason to believe that the deviation of DR vs. ISO from expected behaviour (at ISO 80) is from anything but measurement uncertainties.

Originally posted by Class A So you're saying "measurement error"?

Do you have an idea why it is an "extended" setting?

Because "80" is an ISO setting Pentax has never used before?

I'm sure, 80 was a compromise between two schools of thought: "real photographers don't care about the minimal difference between 80 and 100 and it is confusing" vs. "the minimal difference is what will make the K-5 shine".

So, they've put 80 where it belongs too: into the geek section aka extended setting.

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Last edited by falconeye; 11-08-2010 at 02:45 AM.
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