Switching between resolutions. Epic Dragon files quality

[Alberto Guglielmi]

A newbie question. I am going to shoot a short video that requires higher FPS for some shots. And I might want to take advantage of crop factor for other shots that require a very long lens. So I am thinking of switching amongst resolutions instead of shooting it all at 6K.

Being a newbie in the RED world, I am not sure what issues I can run into for quality of files and matching purposes in editing.

I assume the constant I want to keep is compression ratio (8:1 should be good enough for this).
Second constant is trying to keep ISO consistent as much as possible.

With the above in mind:
- If I move from 6K 24fps (1/50th) to 4K 100fps (1/200th) or the steps in between, will it show differences in quality of footage? Can I match them equally?
- Should I consider shooting all at 5K for the base, and use 4K strictly for the one or two shots I need a higher fps? Limiting variances of resolution.
- How much differences in ISO if I step from 800 down to 320?

Thanks for your help. It is not the easier sensor to learn.

 

[Anton Shavlik]

Lower resolutions have relatively larger noise and, well, less resolution, so I'd say that you can lower the compression ratio to help with that a little. Maybe aim for a constant data rate, or when shooting highspeed go as low as the camera allows (which will be more than 8:1 at 4k 100fps unless you have 512gb minimags). The biggest visual difference with lower compression is a finer noise pattern, and better high frequency detail. Even then, for most shots you'll need to go a lot higher than 8:1 to lose significant detail at 6k.

Also if you're shooting 800iso at 6k it can look quite clean, but at 4k the noise will be magnified 2.3x larger and you'll need to be a little careful. ISO and noise will rely mostly on how your scene is exposed, but in my experience 320 on the skintone is very clean, while 800 has a slight texture at 4k, which can be easily removed with software. Both will be clean if you have the lowlight OLPF.

edit: also most lenses will perform best at 5k, as that's the 'standard' sensor size for cinema. That will give you more options for compression and frame rates, make your longest lens a little longer, and lessen the challenge of matching shots captured at 4k.

 

[Michael Tiemann]

The short answer to your question is this: do tests. Resolution, ISO, noise, compression, lens character, persistence of vision, etc., each have their own unique, non-linear effects on what you may perceive as a match. By the same token, these variables can be manipulated or managed to make things appear to match even when the math says otherwise. There is no formula or recipe that you can simply look up and apply to get a "correct" result.

The advice given above (think about matching data rates rather than specific compression / resolution numbers) is sound. Shooting the camera as clean as possible (ISO 320 with STH OLPF) so as to minimize the artifacts of noise is sound. But if the lens for your principal photography is a 35mm and the lens for your slo-mo is a 300mm, you are already in very different territory when it comes to matching. A test will tell you what gaps you have to bridge, and what creative approaches give you a solution. Test, test, test. Then test again. Then shoot.

 

[Jan Balster]

I know this only partially answers your question but I did a test to determine "compression vs resolution" for a live show I recently prepped for. I figured I'd share to add to the conversation.

The test was done with a Dragon, Canon 24-70 F/2.8L (at various focal lengths to match framing). Ingested in PPro and output as QT ProRes422HQ.

DropBox Link: https://db.tt/DcuT5i8N (1GB)

Bottom line is test, test, test.

Hope this helps,

- Jan

 

[Alberto Guglielmi]

Thanks guys. This was all very helpful.
I never considered the magnification of noise happening with crop factors. Ps I do have the lowlight OLPF.