Panasonic GH4 Review

[Jay Turberville]

Andrew, the math has been explained that even though only the luma could possibly be 10-bits from an 8-bit 4K to 1080 downsample, in practice all that's going to happen is a low-pass filter and any additional luma variance will come from noise; not the same thing as capturing and directly storing 10-bits from the sensor.

No, you can trade resolution for levels of tonal gradation.  The simple proof of this is in every B&W halftone image you've ever seen printed. And actually, that's true for every B&W photo as well.  A B&W negative is fundamentally not continuous tone, but instead is a fine distribution of opaque silver clumps and clear emulsion.  The appearance of tonal gradation comes from the size and distribution density of the film grain. The same is true for prints except that there is an opaque base underneath the emulsion.

 

Saying that "all that's going to happen is a low pass filter filter and any additional luma variance will come from noise ..." is, I think, missing the point.  The low pass filter is a key factor in converting an image with fewer tonal levels into one with more tonal levels.  That's part of the process by which a truly two level (black ink & white paper) printed image become viewed as a "continuous" tone image.  As you reduce the image magnification, the detail in the halftoning dots becomes too small for the eye to distinguish separately.  Your eye acuity acts as a low pass filter. And the result is that you see a smooth blending of tones rather than just black and white specs.

 

Now, whether a downsampled GH4 4K image delivers a 10 bit luminance channel or not is a tougher question to answer.  And frankly, I don't have the math and image sampling theory skills to really tackle it well.  But my bet is that if you low pass filter the image correctly you will see a real improvement in the luminance bit depth. I'd also bet that it doesn't make it all the way to 10bit.  I think it will fall short because the 4K image doesn't really have 4k of luminance information to begin with.  It really only has about 3K (3072x1648) of luminance detail available for downsampling. If you downsampled to 720p, then I think you'd probably get your 10 bit luminance.

Oh, and for those who might try to test this, please be sure to apply some kind of additional low pass filtering (gaussian blur or someting similar) prior to downsampling.  Most downsampling algorithms are geared toward producing (artificially) sharp looking images and don't use enough low pass filtering to produce an image with per-pixel detail similar to the original.  They tend to let a pretty significant amount of aliasing (low frequency artifacts) to occur. I'd experiment with pre-blurring the image by somewhere between .5 and 1.0 pixels. (We went through this with the staff at DPReview.com years ago in regard to downsampling and dynamic range.  They kept incorrectly claiming that downsampling didn't improve dynamic range when in fact it does - if you downsample correctly.  They didn't low pass appropriately and image noise that should have been reduced with a low pass filter was passed through as aliased noise.) 

 

[yeo0]

I am curious:

 

1. I am curious as to comparisons of screen grabs between the C100, GH4 and C500.

 

2. I would like to know the Exact Flatness of C100 vs C300 vs GH4, from in-cameras, and otherwise.

 

3. Most importantly, how much Flatter does the Cinema D profile, with everything dialled down (-5 Highlights, +5 Shadows + 15 Master Pedestal) make the profile?

[johnnymossville]

I am curious:

 

1. I am curious as to comparisons of screen grabs between the C100, GH4 and C500.

 

2. I would like to know the Exact Flatness of C100 vs C300 vs GH4, from in-cameras, and otherwise.

 

3. Most importantly, how much Flatter does the Cinema D profile, with everything dialled down (-5 Highlights, +5 Shadows + 15 Master Pedestal) make the profile?

 

 

I'll shoot a clip in a few hours with those settings and post some screen grabs here for you.

[johnnymossville]

Here's a really poorly done test done in a hurry (Lunch Break, I was hungry!), but I hope it shows somewhat how Cinelike-D compares to the standard profile on the GH4

 

 

https://drive.google.com/file/d/0B7dqeHPkjs-RTEFrX2F4cnozV2M/edit?usp=sharing

 

 

[bigmouthmedia]

Has anyone with the 12-35/35-100 2.8 lenses notice that when you use autofocus (or do the half shutter) that the focusing almost 'shimmers' ? The image looks like it almost shifts imperceptibly.   What would that be ?