Zoom 4K-clips in 1080-timeline or shoot with optical zoom?

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Hardy-

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Zoom 4K-clips in 1080-timeline or shoot with optical zoom?

PostSun Jul 31, 2022 12:35 pm

Hi, a low-budget question ...

I want to buy a BMPCC 4K camera and a lens for it, both for a sum below 1800 €.

In Resolve, in timeline and delivery, I work with 1080p.

As for the lens, I'm happy with a focal length of circa 40 mm. Sometimes I need to zoom that by 2x.

So, I want to switch between two settings: Zoom 1x and 2x.

Now, for the 2x situation I see two options:


(A) Shoot in 1080p and use a variable-zoom-lens that is set to 2x.

... or ...

(B) Shoot in 4K and use a non-variable-zoom-lens (1x), and later in the 1080-timeline, scale up this 4K footage to its original size, so that I get a 2x zoom without quality loss in the 1080 delivery.


I understand that either option has its advantages and disadvantages.
• Comparing a variable-zoom-lens with a non-variable at the same price, the latter has a better quality.
• The optical 2x setting needs more light.
• Zooming 4K in a 1080-timeline incorporates no such problems, obviously. And it's more flexible in post.

Any opinions?

Thank you.
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Mike Ambrose

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Re: Zoom 4K-clips in 1080-timeline or shoot with optical zoo

PostSun Jul 31, 2022 1:33 pm

A 2x digital zoom is not the same as optical zoom.


Digital zoom is essentially cropping. Optical zoom changes focal length and fundamentally will look different than crop.

Personally I'd go optical every time. It's also easier to frame as you know exactly what you're getting. (Granted, with the digital zoom you can slide around your frame after the fact.)
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Hardy-

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Re: Zoom 4K-clip in 1080-timeline or shoot with optical zoom

PostSun Jul 31, 2022 1:44 pm

Thanks, but my question is not about "digital zoom" which is, of course, worse than optical zoom.

My question is about using 4K footage on a 1080-timeline.

When I zoom out the 4K by 0.5x, I fit the complete 4K image in a 1080 frame.

When I zoom in the 4K from 0.5x to x1, that same 4K image will apear twice as large in the 1080 frame. That 1x factor is not a "digital zoom".

I just want to compare the quality of that "trick" to a 1080-footage shot with a lens with variable zoom.
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Mike Ambrose

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Re: Zoom 4K-clips in 1080-timeline or shoot with optical zoo

PostSun Jul 31, 2022 2:27 pm

Regardless of word choice, my point still stands- changing the focal length will look different than a crop.
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Hardy-

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Re: Zoom 4K-clips in 1080-timeline or shoot with optical zoo

PostSun Jul 31, 2022 2:58 pm

Thank you for your hint.

Just to be sure we're on the same page: Let me give two examples.

Example 1:
Variable zoom lens (30 to 60 mm) is in use, set to 60 mm (2x), camera produces 1080p footage.
1080p footage will be uncropped in Resolve's 1080p timeline. The timeline viewer shows a ball whose top touches the upper image edge and whose bottom touches the lower image edge.

Example 2:
Fixed focal length lens (30 mm) is in use (1x), camera produces 2160p footage.
2160p footage will be cropped in Resolve's 1080p timeline. The timeline viewer shows a ball whose top touches the upper image edge and whose bottom touches the lower image edge.


Mike, you're saying example 1 will provide a better image quality? (Regarding noise, sharpness, vignetting etc.)

I understand that there will be differences. But what about the price factor? Fixed focal length lens -- versus -- variable zoom lens. For the same price, the former would provide a better quality, wouldn't it? I'm just wondering how much the lens type affects the above comparison.
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Mike Ambrose

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Re: Zoom 4K-clips in 1080-timeline or shoot with optical zoo

PostSun Jul 31, 2022 4:16 pm

It's definitely going to be subjective on which is better.

Here's a great example of the difference you'll see. (In this case the photographer is backing up with each longer focal length but it still exemplifies the difference focal length has on the look and feel of a shot).

Image

If you consider cinema, people are often shooting on prime (non-zoom) lenses due to the downsides that comes with a zoom lens. People who buy zoom are often doing so for the versatility (being able to change focal length during live usage). It's really going to come down to what you value and what your end goal is with your content.
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Hardy-

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Re: Zoom 4K-clips in 1080-timeline or shoot with optical zoo

PostSun Jul 31, 2022 6:03 pm

The geometric effects (big nose, small ears etc.) are clear to me regarding great focal length & distance versus short length & distance. Hitchcock's dolly zoom trick also demonstrates this nicely.

The geometry shouldn't be significantly different between my two examples.

In my two examples the distances are equal, say, two meters. Different are just the focal lengths and the image resolutions that the camera generates.

I would say, the variable lens zoom too is nothing but a "crop" function, the cropped material here just being lense glass rather than pixels. The useable lens glass area gets smaller (cropped), and at the same time the lens magnifies that smaller area for the projection on the sensor. (As the useable glass area is smaller, it also needs more light.)

For a portait, for instance, the distance ratio from the camera to the model's nose, and from the camera to the model's ears, are in my two examples equal. That is, in either example, the distortion is equal.

Example 1 (60 mm lens on 1080p):
Camera -> -> -> -> Model's nose
Camera -> -> -> -> -> -> Model's ears

Example 2 (30 mm lens on 2160p):
Camera -> -> -> -> Model's nose
Camera -> -> -> -> -> -> Model's ears

In both examples, the model's head appears in the same geometric shape. In example 2 the model is just smaller in relation to the image frame.

The head only changes its form when the camera distance varies, regardless of the focal length.

E.g. when the distance cam-to-ears is twice as great as the distance cam-to-nose ...

Camera -> ->Model's nose
Camera -> ->-> -> Model's ears
... then the nose will be much larger than the ears.


E.g. when the distance cam-to-ears is almost equal to the distance cam-to-nose ...

Camera -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> Model's nose
Camera -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> Model's ears
... then the nose will be almost as large as the ears.


The only thing I'm wondering about is the difference in the picture quality regarding 2160p/fixed focal length versus 1080p/variable zoom.
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Howard Roll

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Re: Zoom 4K-clips in 1080-timeline or shoot with optical zoo

PostSun Jul 31, 2022 8:04 pm

Hardy- wrote:I would say, the variable lens zoom too is nothing but a "crop" function, the cropped material here just being lense glass rather than pixels. The useable lens glass area gets smaller (cropped), and at the same time the lens magnifies that smaller area for the projection on the sensor. (As the useable glass area is smaller, it also needs more light.)


You could say it, but you'd be mistaken. Most zooms are sharpest in the middle of the range. Whether it's sharper at full wide vs full tele would depend on the lens. By your logic they're sharpest at the wide end and softest at the long end which isn't true. Don't take my word for it see for yourself.

https://www.the-digital-picture.com/Rev ... Crops.aspx

Downscaled images will always have more resolution than a crop.

Good Luck
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Hardy-

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Re: Zoom 4K-clips in 1080-timeline or shoot with optical zoo

PostMon Aug 01, 2022 12:16 am

Thanks for the tip. I believe you. I'm just asking :-)

So, back to my initial post: You would say option (A) is better than (B).

As you say, the variable zoom has one best focal length while its other zoom settings are less sharp.
However, a lens with a fixed focal length is optimized for just that length. I thought that such a fixed lens may be better than a variable lens at its shortest focal length setting.

With this dilemma in mind: When using a fixed lens, I could vary the zoom later in the 1080p timeline by scaling the 2160p footage from x0.5 to x1.0. I thought this method might provide 100% sharp delivery pictures at x0.5 and at x1.0 (two sharp settings) -- while the method with the variable lens recorded on 1080p might provide 100% sharpness just at one zoom setting.

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