Native ISOs are alive and well on the 5D3:
5D3 native ISO test on Vimeo

Quite a difference at 1250 too.

I work a lot with 160, so this is proof that the old rules still apply.

It seems to be doing something different at 1250 for sure.

Thanks so much for sharing this Andrew... very useful indeed.

Sure thing. Now I just need to figure out what the devil these noise reduction settings do...

My first thought was to wonder what options the mkIII has as far as noise reduction - it's clear it kicks in at 1250 as all the fine noise disappears at that point. Makes sense as I've noticed in other people's tests that noise seems to stop increasing beyond that point under normal shooting situations. Can you select different levels of noise reduction?

In the video that opens this thread, noise reduction remains disabled throughout. It does seem like something unique kicks off at ISO 1250, but it's not noise reduction.

There are four options in the "High ISO speed NR" menu on the mk3: Standard/Low/High/Disable. I'll put together and post some high ISO, high contrast shots to compare the different noise reduction settings.

Interesting. I notice in the manual it mentions that some noise reduction is always applied at all ISO settings, but that wouldn't explain the complete drop off of fine noise at 1250 unless the default settings change with ISO. They also mention that the high ISO setting can reduce shadow noise at lower ISOs - I'm looking forward to your tests to see how this all really plays out.

I think I see what you're referring to Evan - (pg. 143) under the heading High ISO Speed Noise Reduction
"This function reduces the noise generated in the image. Although noise reduction is applied at all ISO speeds, it is particularly effective at high ISO speeds."

I think they mean that when the High ISO NR is applied, it is applied at all ISOs and not just the high ISOs. For the lens cap test, the High ISO NR was set to Disabled, which I would understand to mean it's disabled at all ISO speeds.

That said, surely there is "normal" noise reduction being applied all the time and for all ISOs, and perhaps that does have to do with what we're seeing at ISO 1250. In any event, we should look over some real images - I'll post them here when they're ready.

One possibility is that noise reduction settings have a different effect on video than on stills. On the mkII they had no effect on video, it could be that on the mkIII there is always some level of high-ISO noise reduction enabled in video mode even when it's set to disabled in the menu. The user manual doesn't mention anything about differences between the two modes so I guess testing will be the only way to find out for sure.

I added some High ISO NR comparisons to the video, and added my thoughts to the video description. I'll shoot some comparisons of the High ISO NR settings on high contrast daylight shots soon.