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4:4:4 ETC with GH4


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Guest Ebrahim Saadawi

To get the highest quality image out of the GH4, is shooting 4K 4:2:2 10bit in the Atomos Shogun in ProRes HQ 10bit through the HDMI port. This the most pixel count and most colour information and least compression the camera has to offer. You take that ProRes 4K file and do what ever you want in 4K, but if you are going to crop/reframe remember to downscale the output to 1080p in the end render.

Remember that crop modes are only relevant when the camera is not giving you every pixel (like a 4K GH4 does or 21mp stills on a DSLR for example), when the camera downsamples/lineskips internally to a smaller pixel count than the area it's deriving the image from, then it's useful to use a smaller different part of the sensor in the center to get a crop effect without downsampling, but when it's already using the entire sensor and giving you the full resolution, there is no point, you take it and crop in post, that's why DSLRs don't have a crop mode for photography, because there is no point in cutting the edges in-camera if you can do the same in post more percisely. If your output is 1080p you can do some very heavy cropping in post in the GH4 4K files, all the way to a s16 size sensor and still have around 2K of resolution.

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If your output is 1080p you can do some very heavy cropping in post in the GH4 4K files, all the way to a s16 size sensor and still have around 2K of resolution.

Let's not confuse things here. You can crop right in on 4k files - 4k is 4x the size of 2k, you can actually crop into a quarter of the frame if you wanted to. That, however, does brings up issues like lens resolving power and all the rest of it.

It's not really equivalent to a S16 sensor, however - let's not confuse post cropping with sensor sizes.
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Guest Ebrahim Saadawi

Let's not confuse things here. You can crop right in on 4k files - 4k is 4x the size of 2k, you can actually crop into a quarter of the frame if you wanted to. That, however, does brings up issues like lens resolving power and all the rest of it.

It's not really equivalent to a S16 sensor, however - let's not confuse post cropping with sensor sizes.


It is equivalent to changing to a smaller sensor size.

To OP: I explained the highest quality way to get a crop out of the GH4 above. Anything else would lose quality. And no the GH4 does not record or output a 4:4:4 signal, maximum is 4:2:2. Some people claim that if you have a 4K 4:2:0 video and downscale it to 1080p it becomes 4:4:4 1080p. But anyhow the highest quality crop mode you can get from the GH4 is recording 4K 4:2:2 10bit externally and cropping in post to the desired image.
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Guest 560a4aedcb80685284629074497fdc75

Tests have shown that etc in 1080 mode is superior to cropping the 4k image in post. I'll link a video when I'm home later.

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Is there anybody who understands my idea. It is to get high magnification video with i.e. 100-300mm lens and 444 fullhd. Better than normal ETC. Better than cropped 4k. Internally.

 

The act of converting 4k output to 444 will reduce it to 1080p. If you perform a digital zoom, you will have to bring the resolution even lower. It will be the exact same quality as if you digitally zoomed 1080p footage anyway, except it will be 444, which you won't notice.

 

And trust me, the added work in using an external recorder is a drop in the ocean compared to the hell you will put yourself through by adding all these processes in post!

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I tested a little 4k+2x digital zoom vs fullhd+ETC. Both read 100% sensor pixels but 4k 2x seems to be sharper. So if I want the best magnification 4k is better than ETC.

I dont know if it does 2x digital zoom before or after 4:2:0 color sampling. Seems that the colors are not improving very much compared to 100% ETC. The sensor is noisy and a bit blurred anyway at 100%.

 

Sample frames:

 

http://***URL removed***/forums/post/54842900

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It was a quick test in bad light. Iso was 200 and there is lots of sharpening and added color. I tried to figure if color res is better. I must test more in good light. Still seems that 4k 2x is the way to go if a high magnification is the goal. But no 60P then.

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Guest Ebrahim Saadawi

You have a sensor that contains 4096x2160 pixels (it's a bit bigger for stills but lets simplify). When you set the camera to ETC, the sensor uses the centre 1920x1080 pixels, throwing put all the remaining ones. Using a smaller sensor on the same lens gives that magnification effect, plus deeper depth of field.

In 4K mode, the camera recods the entire pixels, which gives you the centre 1080p pixels plus the remainin ones outside. This way you can throw them away later and use only the center portion again by cropping.

Now in terms of colours, the GH4 records internally in 4:2:0 8bit in bith the 1080p etc and 4K. It outputs a 4:2:2 10bit signal also in 4K. So your best chance to get highest quality colour is to shoot in 4K 4:2:2 10bit and crop in post. This will give you a 1080p 10bit 4:2:2 ETC image. Also if the GH4 can ouput 4:2:2 10bit 1080p in ETC mode, you can use that, bypassing the post-cropping step.

Using the 4K mode though will give you more freedom in cropping choice.

If you want to do it internally the best way is shooting 4K and cropping. From my tests there is no advantage in using the ETC mode unless you're not planning to do any post work.

The internal 4K 4:2:0 8bit is a magnificant image by the way and you don't really need much more than that for anything. But since you're asking for 4:4:4 then the 10bit 4:2:2 4K shogun suggesstion had to be pointed out. It's 2 thousand dollars.

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1. 4k center crop
 
2. 4k 2x dig zoom downsampled to fullhd
 
3. ETC fullhd
 
4. best JPG center crop 1920x1080
 
5. RAW (ACR) center crop 1920x1080
 
I noticed that the sensor debayering ruins fine color detail even in RAW photo and levels the differences of different modes. GH4 4:2:0 4k video and ETC is very close to best camera JPG in 100% sensor readout. 4:4:4 RAW is sharper but not perfect.
 
If I use 4k/2x dig zoom I get a little sharper result that cropping 4k or using ETC.
 
I think that Samsung NX1 with 6.5k sensor debayering and downsampling is maybe far better than GH4 in 4k video. I think that even 100% RAW is worse in GH4. We will see.
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