Blackmagic Forum

Does anyone know the math operation to calculate the 2K variation of a specific aspect ratio (assuming I'm already working in 4K)? Sorry, math isn't my strong suit and the aspect ratio we're going for on this end isn't a preset/industry standard.

What I've managed to figure out (and I'd also appreciate a confirmation on this) is that if I want an aspect ratio of 1.62:1 in 4K, then the resolution should be 4952x3048.

This is as far as I got, but don't feel I've quite arrived.

Any help here would be rewarded with good vibes.

A bonus question, and this comes from a confusion over how cropping occurs, but let's say I want to frame shots for this aspect ratio in the future, what is a suggested practice for figuring out the parts of the reference screen to block off? In the old days I'd just put tape over the top and bottom and wall - roughly framing for wide screen. Is there a way to add custom reference guides to a BM camera?

Thanks again!

-E

4K = 4096
UHD = 3840

For 4K, a 1.62 aspect would be 4096 x 2528.
For UHD, you're looking at 3840 x 2368.
2K would be 2048 x 1264.

ericnorcross81 wrote:Is there a way to add custom reference guides to a BM camera?

I'm not aware of any. Cinematography forum might know better.

Jim Simon wrote:4K = 4096
UHD = 3840

For 4K, a 1.62 aspect would be 4096 x 2528.
For UHD, you're looking at 3840 x 2368.
2K would be 2048 x 1264.

Thank you, I'll take a look at these.

-E

ericnorcross81 wrote:... A bonus question, and this comes from a confusion over how cropping occurs, but let's say I want to frame shots for this aspect ratio in the future, what is a suggested practice for figuring out the parts of the reference screen to block off?

You need to use Frame Guides set On in the Project Settings of the BMPCC4K.

Exit Project Settings.

Tap on the Screen Settings upper left corner,

Select the Frame Guide option.

You’ll see your current default setting such as 2:39:1 with left and right arrows.

Tap on the actual current number, 2.39:1.

That will go to a screen where you enter 1.62:1.

Tap on Update and you’re done.

NB: It’s not a permanent change that will appear in your Project Settings if you select any different aspect ratio via the menu.

Aspect ratio to resolution math is a strange hobby of mine. And the methodology is quite straight forward.

In general. You take the numbers and multiply/divide them with whatever side of the resolution that gives an answer that is useful.

For example. Your image is 4096x2160 (aka DCI 4K), you want to crop it to 2.4:1. You can't multiply the height with 2.4 to get the width because that would give a width that is too wide with 5184. So instead you divide the width with 2.4 to get the height which gives you 1706. So the result is 4096x1706.

Likewise. Your image is 4096x2160 and you want that 1.62:1 crop. If you divide the width with the ratio your height will be too high with 2528. So that's when you multiply the height with the ratio to get the width. So you get the width 1333 (I would round it down to a nicely evenly divisable 1332).

Wether to divide the width or multiply the height can be known by looking at the source images ratio. In the case of 4096x2160 that equates to just about 1.90:1. You get that number by dividing width with the height. If the crop ratio is higher than the source ratio. You divide the width. If the crop ratio is lower than the source ratio. Then you multiply the height.

So. To your question. For a 2K image to be cropped to 1.62:1 . We do the same. (Going on the assumption that 2K is 2048x1080)

1.62:1 is lower than 2K's ratio of roughly 1.90:1. That means we can multiply the height of the source resolution by the ratio to get the width.

1080*1.62=1749.6. Round that to an evenly divisable 1750. So your final cropped resolution will be 1750x1080.

Now. Do not get me started on how to change the aspect ratio while getting the number of total pixels to be roughly the same (I have my reasons for pursuing this question!). That is a rabbithole that I need more time for preparation before I lure unsuspecting newcomers down its slippery slopes.

And getting custom cropmarks is one of the many reasons I've been looking into buying an BMPCC6K...

jmalmsten wrote:Aspect ratio to resolution math is a strange hobby of mine. And the methodology is quite straight forward.

In general. You take the numbers and multiply/divide them with whatever side of the resolution that gives an answer that is useful.

For example. Your image is 4096x2160 (aka DCI 4K), you want to crop it to 2.4:1. You can't multiply the height with 2.4 to get the width because that would give a width that is too wide with 5184. So instead you divide the width with 2.4 to get the height which gives you 1706. So the result is 4096x1706.

Likewise. Your image is 4096x2160 and you want that 1.62:1 crop. If you divide the width with the ratio your height will be too high with 2528. So that's when you multiply the height with the ratio to get the width. So you get the width 1333 (I would round it down to a nicely evenly divisable 1332).

Wether to divide the width or multiply the height can be known by looking at the source images ratio. In the case of 4096x2160 that equates to just about 1.90:1. You get that number by dividing width with the height. If the crop ratio is higher than the source ratio. You divide the width. If the crop ratio is lower than the source ratio. Then you multiply the height.

So. To your question. For a 2K image to be cropped to 1.62:1 . We do the same. (Going on the assumption that 2K is 2048x1080)

1.62:1 is lower than 2K's ratio of roughly 1.90:1. That means we can multiply the height of the source resolution by the ratio to get the width.

1080*1.62=1749.6. Round that to an evenly divisable 1750. So your final cropped resolution will be 1750x1080.

Now. Do not get me started on how to change the aspect ratio while getting the number of total pixels to be roughly the same (I have my reasons for pursuing this question!). That is a rabbithole that I need more time for preparation before I lure unsuspecting newcomers down its slippery slopes.

And getting custom cropmarks is one of the many reasons I've been looking into buying an BMPCC6K...

--
This is very inspiring though, and I love that this is your hobby. I see a podcast episode in our respective futures hahaha

rick.lang wrote:

ericnorcross81 wrote:... A bonus question, and this comes from a confusion over how cropping occurs, but let's say I want to frame shots for this aspect ratio in the future, what is a suggested practice for figuring out the parts of the reference screen to block off?

You need to use Frame Guides set On in the Project Settings of the BMPCC4K.

Exit Project Settings.

Tap on the Screen Settings upper left corner,

Select the Frame Guide option.

You’ll see your current default setting such as 2:39:1 with left and right arrows.

Tap on the actual current number, 2.39:1.

That will go to a screen where you enter 1.62:1.

Tap on Update and you’re done.

NB: It’s not a permanent change that will appear in your Project Settings if you select any different aspect ratio via the menu.

--
I'll try this, thank you!