Why do 5Diii JPEG File Sizes Decrease So much Between ISO 500 & 640

[MichaelTheMaven]

Hey guys, I thought I would throw this out there because many of you are so knowledgeable. I've been doing some tests on the ISO Noise reduction and am noticing something strange between ISOs 500-640, the file size gets smaller. (Significantly). This only happens with ISO noise reduction on, doesn't happen in RAW or video.

First of all- Are you guys able to reproduce this?
Second of all- If it isnt compression, what the heck could it be? (As far as I know and as discussed on an earlier thread, file size should get larger with more ISO noise, not get smaller).

I posted some of my results and thoughts on my bloghttp://www.michaelthemaven.com/?postID=2287&why-canon-5diii-owners-should-save-jpegs-if-shooting-with-isos-over-500 but thought I would throw this out here.

Thanks for the help!

M

[prestonpalmer]

Theoretically, file size should increase as ISO increases.  Id like to see what others have to say.  I assume the noise reduction algorithm has something to do with it.

[LetTheRightLensIn]

Hey guys, I thought I would throw this out there because many of you are so knowledgeable. I've been doing some tests on the ISO Noise reduction and am noticing something strange between ISOs 500-640, the file size gets smaller. (Significantly). This only happens with ISO noise reduction on, doesn't happen in RAW or video.

First of all- Are you guys able to reproduce this?
Second of all- If it isnt compression, what the heck could it be? (As far as I know and as discussed on an earlier thread, file size should get larger with more ISO noise, not get smaller).

I posted some of my results and thoughts on my bloghttp://www.michaelthemaven.com/?postID=2287&why-canon-5diii-owners-should-save-jpegs-if-shooting-with-isos-over-500 but thought I would throw this out here.

Thanks for the help!

M

combo of 1/3 below ISOs being cleaner with more DR than ones 2/3 below and probably moreso with the switching on of heavy-handed 5D3 NR, why I avoid standard NR at all costs, too plasticky at any ISO, even at ISO100.

[Bosman]

Consider shooting without NR, if your images need help do that in post but most will say they look great w/o NR. I tell the people who shoot for me to turn it off because it makes skin muddy and plasticy, more soon the Nikon cameras to be honest, but I have seen the 5dm3 NR and it looks pretty good, still its better to leave that detail control in your hands.
Your file size increases as you go higher in iso. I have no idea why this is but it just is and i am ok with that. Your lower file size is nothing to be concerned about. 

[PhilDrinkwater]

combo of 1/3 below ISOs being cleaner with more DR than ones 2/3 below and probably moreso with the switching on of heavy-handed 5D3 NR, why I avoid standard NR at all costs, too plasticky at any ISO, even at ISO100.

I would suspect this is the case.