The 4k DOF Conundrum

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Bassman2003

Veteran
I have been watching the 4k revolution from a distance while filming with my HD cameras and taking notes. It seems to me that the best feature of 4k for normal video work is being able to re-frame or punch in. The more I think about this, the more I see a problem with blurring the background.

For example - if you are filming an interview, you might want to shoot at a nice portrait focal length and a wide aperture to blur the background. Lets say f2.8 on a C300. Now lets say you shoot that same interview with a 4k camera (C300 MKII) In order to be able to have options, you will probably want to shoot the interview much wider that before (knowing you can punch in). The problem is that your DOF is now going to be deeper given the much wider focal length. But if you are using a normal zoom lens, f2.8 is your limit. You are going to need to switch to a prime lens to get very shallow for the wide lens.

Just some thoughts. Anybody else see this? Kind of changes the lens landscape for these cameras imho.
 
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This ties into your issue. . .I'm still not a huge fan (though I see the practicality of it) this application of 4K and here's why: it's gonna look cheap. It just is. The difference between actually reframing (whether moving camera in or zooming/switching to tighter lens) and simply cropping in the same shot in post to make it tighter is a real, visible thing. If we're talking green screen, then, yeah, the gains are probably minimal for actually reframing rather than post-cropping, but with an actual background, it will look nicer to zoom in/move camera. I get that for time considerations ("the CEO only has 15 minutes to do this, can't do it twice") the post/crop thing is great, still think it looks kinda cheesy and cheap on a real background.
 
Bassman2003 said:
I have been watching the 4k revolution from a distance while filming with my HD cameras and taking notes. It seems to me that the best feature of 4k for normal video work is being able to re-frame or punch in.
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But is it though? I thought the idea was that the main advantage was capability of downrezzing for superior IQ. All the talk about re-framing, taking out shake and the like is just giving you tools to rescue bad footage - not a tool to actually generate good footage. It's not a expanded-framing-options device - it's a way to recover from framing mistakes and like in all such cases of repairing mistakes and rescue tools, there will be compromises and downsides. There is no substitute for getting the footage right in the first place. If you get a flat tire, sure you can use that skinny spare and it can save your bacon somewhat, but it's limited in range and you don't look upon that skinny spare as a regular tire to be regularly driven on, just a way to get you out of a jam.

Now, if you're talking about using 4K re-framing as a way of speeding up work and convenience when you have limited time, then it is again a tradeoff between speed and convenience vs IQ - think of it analogously to the idea of using a zoom vs primes - a zoom is far more convenient, but at the cost of slightly worse IQ compared to dedicated primes (of course, these days zooms have gotten very, very good IQ-wise). The convenience is not cost-free.
 
Thanks for your replies. I guess I am thinking of an interview situation where 4k with a wide frame would give you graphics as well as punch in capabilities if delivering in 1080p. I do not see this as lesser image quality or not "shooting it right" but more of getting the best out of the tools. The wide shot at say f1.4 compared to a tighter shot at f2.8 isn't necessarily going to look worse if you get the DOF calculations right and are delivering in 1080p. True, for a 4k delivery you are compromising.

But more of the focus of my post was that there are no f1.4 zooms! The video world has embraced still lenses and a lot of still zoom lenses as they give you options. Well 4k just took some of those options away in this scenario. Which means it is time to get that 35mm or 24mm f1.4 prime for interviews.
 
Bassman2003 said:
But more of the focus of my post was that there are no f1.4 zooms! The video world has embraced still lenses and a lot of still zoom lenses as they give you options. Well 4k just took some of those options away in this scenario. Which means it is time to get that 35mm or 24mm f1.4 prime for interviews.
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A Sigma 18-35mm f1.8 with a SpeedBooster gets you there. Or a Sigma 24-35mm f2.0 full frame with a SpeedBooster is still pretty darn fast. Hell, an f2.8 zoom with a SpeedBooster does really well.
 
Re-framing is something that is done more by people making movies as far as I can see.

I don't think re-framing is one of the top benefits of 4k. Slight re-framing is great and will work perfectly and without issues if we're talking about 10-15 percent crops. Beyond that you do hit the issues mentioned here.
And I agree that a significant re-framing of something like interview shoots is not really going to give great results. Our eyes can spot when it's just a punch-in.

I also think that 4k to 2k downrezzing is probably it's greatest strength.
 
To me it's great to shoot a slightly wider shot in 4k and in post make the final framing But just as often I make the framing in camera and sometimes go tighter in post if I need to.
 
Everyone is gonna use 4k as a way to achieve whatever they want it to achieve.

I have been using 4k as a way to get more color and luminance information into 2K...being a Cmos sensor and all.

As as far as the re-framing, that can be useful but downscaling is an art. If you downscale by numbers/ratios that are not neatly divisible, the computer has to do some interpolation algorhythms that can result in loss of texture/information. This may not be an issue for most folks producing for web, but is a real thing for big screen and such. 4k down to 2k works fine. But UHD to 2k, or 4k to HD are not good ratios for downscaling.

There are times when I shoot 4k and plan to finish in 2K and do a 45 degree shutter angle for post stabilization. But when I do this, I try to plan out my margin for downscaling in such a way as to minimize its effects. That said, I never think of 4k as a way to punch in. But that's just me. I can certainly see the appeal of re-framing, but I prefer to just do it on the spot.
 
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I don't often shoot interviews, but when I do I find 4k really helpful. Generally I'm setting up two cameras to get enough coverage the interview can be cut as smoothly as possible. Having at least one of the cameras shooting 4k lets you jump from wider 4k shot, to 2nd camera shot, to cropped in 4k shot (and so on and so forth), gives you an ability to craft some much more visually interesting edits.
 
I'm not using 4K (on my FS7) for day to day jobs at all. 1080P works fine for that. When I shot my last feature I did shoot in 4K and the 2K downres shows it. I also used it for slight reframing, correcting horizon and an occasional slow pull out when I wished I had set up a dolly but did not.
 
If I have the time and space I will shoot interviews with 3 cameras.

As far as 4K goes, I find that acquiring in 4K for 1080 delivery generally gives me better results than just shooting with a 1080HD camera.
 
Bassman2003 said:
In order to be able to have options, you will probably want to shoot the interview much wider that before (knowing you can punch in). The problem is that your DOF is now going to be deeper given the much wider focal length.
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I'm surprised nobody jumped on this yet. DOF refers to the range of distance that appears acceptably sharp. Your aperture as well as your focal length to focus distance are the only two things that really affects this. So although you are shooting with a 50mm instead of a 100mm (or at 10 feet instead of 5 feet) when you punch/crop in later in post, your closeup will wind up with the same percieved DOF as you would have if you were shooting with with a longer lens (or at a closer distance.)

What your essentially saying is that because your finishing in 1080, 4K provides you with two camera angles so you are going to add a wide angle where normally you just had a medium/closeup. If you were shooting this interview with 2 cameras you would be stuck with the same issue where your wide angle has more perceived DOF. This isn't an issue of 4K, this is an issue of adding a wide shot to your interview!

On a side note, when possible, instead of opening up your aperture, you could just step back and use a longer focal length to get a narrower FOV on your background. This will actually afford you more DOF in your shot without sacrificing background blur. For an interview, this is ideal so that your subject isn't moving in and out of focus.
 
Josh Bass said:
The difference between actually reframing (whether moving camera in or zooming/switching to tighter lens) and simply cropping in the same shot in post to make it tighter is a real, visible thing.
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Josh, you're correct that moving the camera in is visibly different than cropping in post, however, zooming/switching to a tighter lens is the same thing as cropping in post from a optical standpoint. With both zooming in or cropping in you are narrowing your FOV.
 
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