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MX 2K more noisy than 4K?

J

Jon B.

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Has anyone had this experience with the MX sensor?

I was shooting last night in an incredibly dark stage performance and wanted to test slow motion. I set the configuration to 2K 16:9 iso 1600 and immediately saw a noisy blue image. I opted out of the slow effect because it was a live performance (although in hindsight I should have taken a quick grab).

The image was incredibly noisy in the blue channel. It was considerably more noise than with the settings 4K 16:9 iso 1600.

Anyone else with this?
 
There is a simple explanation to this.
When you shoot 4k and downsample to 1080, you also "downsample" the noise (decrease the areal of the noisy pixels) to 1/4th.

When shooting 2k you see the noisy pixels bloom in an almost 1/1 ratio @ 1080.

The downsampling is hiding a lot of the noise.

SO
The cam has the same noise carracteristics, the downsampled image has not.

If you in addition to this shot @ Tungsten, the blue channel will be gained. Try to develop @ 5000 and grade that (it will look yellow). That will probably be less noisy.

Cheers

Gunleik
 
That seems a bit right, although I did some frame grabs today in a quick and dirty test and am having trouble uploading the two images here.

The tifs I grabbed, both 2K and 4K, have a different look in the shadow/blacks. The 2K just slightly more blue noise. I will be shooting again tonight and will grab a couple comparison shots.
 
What would be nice was an r3d grab (from RC-X)

You most host it somewhere else, though. Reduser doesn't allow for files of that size.

(a 4k r3d would be about 1.6 mb)
 
If you punched in on a piece of film, the grain would be enlarged along with the image. The same thing is napping here, but it is a sensor, not a strip of film, and it is noise, not grain. 2k is just punching in and throw g away the excess.
 
I did snapshots in rcx exporting full res tifs. I zoom into the 4K and create the same frame as the 2K. When I view, essentially at the same framed image as the 2K, I see more blue noise. Both r3ds were taken seconds from each other, the difference being 2K and 4K. The colors in 2K are a little more blue.

Too bad I can't post it now, but later if I still haven't figured out the reason, I'll find a way to get these images up there.

I noticed this first under extreme low-light situation so it was more noticeable.
 
Well, I think Andrew and I pretty much have descridbed the reason, and it is in sync with what you see, and actually should expect.

Cheers!

G
 
Shouldn't a 2K inner frame of the 4K grab resemble precisely another 2K grab of the same shot? I didn't realize when shooting 2K that blue noise was boosted?.. this is what I am seeing.

There must be more going on then a cropped sensor.
 
The cropping = 4x the size of the noisy areas @ 2k compared to downsampling 4k to 2k.

Thus, shot @ 2k viewed @ 2k will give more noise.

If you took the 2k image and scaled it down to 1k (1024 x 512 or 576 depending on ratio) with full debayer, the image noise should be comparable to the scaling from 4k to 2k - given that you have a good scaling algorithm.

If you in addition was shooting highspeed, that would lead to a worse compression scenario, probably not decreasing noise...
 
Thanks for the replies,

I am running 30.6 and will be looking to get some test shots to post tonight. Granted this is my first shooting with the MX
 
Shooting in 2k is roughly similar to S16mm film and 4k is more similar to 35mm. You can't defeat physics. When your shooting 2k you are magnifying a smaller portion of the chip and therefor magnifying the grain more. I would never recommend 2k in a low light situation.
 
I was intending to shoot slow motion and in fact need to use 2K.

Anyway, using the exact same chip to capture for the 4K and the 2K. What I am saying here is that the portion taken out of the 4K image to match the exact frame from the 2K image, the 2K is blue noisy. Yes, the 'grain' is magnifying but the overall colors in the shadows is more blue.

Also, zooming into a 2K frame on the 4K image is also magnifying the pixels, therefore matching the 2K shot. The 2K has more blue in the exact same shot set-up. The only difference should be that the sensor is cropped, not that the blue channel is more noisy.

I will post pics later but I wanted to check if anyone else had this issue in the meantime.
 
Increase in perceived noise would be expected (as described above).

The blue shift is curious, though. White balance?
 
Michael Olsen said:
Increase in perceived noise would be expected
Click to expand...

Right,

just like perceiving more noise in the 4K image when zoomed in. It should match the 2K image when doing this but it doesn't. I zoom into the 4K image to match the 2K image's frame and the 4K is a cleaner less blue shadow; same frame of view.
 
I had not engaged the varispeed yet and the shutterspeed on both 2k and 4k were the same.

I took some test shots today in daylight conditions and the difference was not as noticeable as last night under tungsten but it was still there.
 
Interesting. I haven't done exactly what you describe.

A fun test would be to shoot 4K-16:9 @ RC 28 & 36 vs 2k @ RC42 under lowlit tungsten...
 
I'm not sure anyone else mentioned this, but in 2K you are not downsampling anything. You are only using the center portion of the 4K imager to capture the images.

This means that 2K is not "scaled from 4K it is only "cut out" of the 4K. So you are using only 1/4 the image area to capture to r3d.

Words that are wrongly used here are "magnification" and "downsmapling" as niether of these things occur when in 2K mode.

By making the image area smaller it allows for higher frame rates and the ability to "overclock" the sensor from what it would be able to do in 4K.

Why does it look like crap in low light. Easy. You are using 1/4 of the sesonr capacity to capture your image, the center area is all that is used. Also you are overclocking this area to get the speed you want to achieve and chances are you are wide open, indoors.

The rule for speed is one stop of extra light needed @ 48fps, 64fps, 72fps, 96fps. That is four stops at 96fps. If you had to "crank" the ISO out to 1600 then you were probably capturing in very low light.

You might probably have been in tungsten lighting as well, whcih is just another barrier to a decent image on CMOS sensors.

The reason daylight looks better is because you are dealing with so much more light.

David
 
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