"Native" ISO ?

[Paz_Pazzaz]

I've been reading about some new cameras that have 'Dual' - 'Native' ISO.

My camera is supposed to have 'Native ISO' of 400. I'm guessing that anything shot either above or below ISO 400 would have worse image quality than something shot AT 'Native '400' ISO.' Is that right? Or only something shot above ISO 400?

Is 'Native' ISO something that came along with digital cameras? I don't remember the term from film days. I've done a good bit of reading, a lot of material seems to contradict other material... and how anything could have 'Dual - Native' is beyond me.

[Bob]

This is a bit of an over simplification, but I hope it will get the idea across. Adjusting the ISO does not change the sensitivity of the sensor. Instead, it amplifies the signal the sensor produces when it is illuminated by a given amount of light. ISO is a gain control. There are actually two amplifiers involved in this. The first is an analog amplifier on the sensor chip. After the onboard analog amplification, the signal is converted from analog to digital and then there is a digital amplifier following that. ISO affects both.

In a dual native ISO sensor, instead of one on-chip analog amplifier, there are two. The second amplifier is stronger than the first and optimized for lower light levels. Which amplifier is used depends on your ISO setting. If it is below the second (higher) native ISO, the lower gain amplifier is used. If it is at or higher than the second native ISO the higher gain amplifier is used. So, you actually do have two different signal paths and two native ISO settings.

As for quality, that's tricky. I don't like the term native ISO as it over simplifies what's going on. Changing the ISO not only affects noise, it affects dynamic range and the distribution of stops that can be recorded above and below middle gray. Native ISO isn't always the best setting for a given lighting situation. I'm not going to go into that aspect. Suffice it to say that as you increase the ISO from the first native ISO towards the second native ISO, the image will be brighter and generally more noisy until you hit the second native ISO and then the second amplifier kicks in and the image quality improves again.

[Steve Grisetti]

I also wondered about this. Thanks, Bob!

[Paz_Pazzaz]

Bob,

Thank you for your easily understandable explanation. I get it now. Suppose I'll have to run some tests to see how my camera works best.

After I (quite recently) learned that my 'native ISO' should be 400, I've begun to use it as a default setting. Previously I always tried the lowest possible or else let the camera decide based on a range I can choose in camera settings. I had from 200 (lowest available) to 3200. The camera often chooses a setting that blows out info. So far I've been pleased with the 400 setting and I've changed either aperture or shutter to make the image look better while I'm shooting.

Paz