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Focal lengths and EPIC recording resolutions (2k, 4k, 5k)

Asif Limbada

Asif Limbada

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Hi guys,

I have searched a lot here, but could not find the answer. As I'm new to "5K" cinematography with my EPIC-M, I did not have enough time to properly analyze this, but my 18mm RPP Focal length changed once I switched from 5k to 2k to test 300fps.

So here it goes:

1. Is there a multiplier to be considered when using S35 sensor at full 5k. (7d APS-C sensor has x1.6 more tele on lens compared to 5d's full frame)

2. If I need to shoot 2k 300fps, and need a wider view, how do I calculate the focal lengths of my RPP.

3. Can we conform the actual coverage of my lens in 2k mode, which I can see in 5k.

Again, I'm new to all this. But I'll catch up fast. Advice.
 
Hello Asif

A handy little app that runs on the iphone and Ipad is pcam pCAM Film+Digital Calculator

hear is a screen shot of the focal length match
pcam.html
 
Last edited:
Olivier Madar said:
I think something like that : 5 x 1024 / 2 x 1024 x 18mm --> 45 mm
Click to expand...

Can you please explain what does each variable represent in that equation. All I can understand is 18mm is the focal length of the lens and after the calculation of the equation 45mm is what Ill get from it.

So even at EPIC S35 sensor's, 5K FF mode, we are not getting the exact focal length of the lens???
 
Asif Limbada said:
Can you please explain what does each variable represent in that equation. All I can understand is 18mm is the focal length of the lens and after the calculation of the equation 45mm is what Ill get from it.

So even at EPIC S35 sensor's, 5K FF mode, we are not getting the exact focal length of the lens???
Click to expand...

You are always getting the exact focal length of the lens. The lens will always be an 18mm, nothing will ever change this. Your field of view may differ when you shoot with a smaller sensor but the 18mm will always be an 18mm. What you are doing is using less of the lenses coverage when you use less of the sensor. When you shoot 2k you are only using the center of the sensor, so you are using the center of the lens, think of it as the equivalent of "pushing in" on a frame in after effects.
when I go from 4k 16:9 on the red one to 2k 16:9, I just half the focal length of the lens in order to get the same field of view. So if I was using a 50mm on 4k, I'll use a 25mm on 2k in order to get the same field of view.

The best app for more complex focal length matching that I have found is Pcam.
 
Yes, don't confuse yourself Asif -- the focal length never changes. An 18mm is an 18mm whether on a tiny sensor camera or a giant sensor camera. What changes is the field of view, and you want to know what the equivalent focal length needs to be to maintain the same field of view of a certain focal length on a certain sized sensor.

As Shane says, the conversion is easy between 4K and 2K on the Red One because 2K mode uses half the sensor area of 4K, so you'd need to use half the focal length to maintain the same field of view when switching from 4K to 2K.

Full-Frame 35mm, such as on the Canon 5D, means a 36mm x 24mm sensor. It may be easier if you just compare horizontal view rather than diagonal view since then you don't have to deal with aspect ratio affecting the diagonal length.

"Super-35" is technically 24mm wide -- which is what the Red One is in 4.5K mode. So that's a 1.5X difference with FF35 (36 divided by 24 = 1.5.)

Various widths:
36mm = FF35 / Canon 5D sensor
30mm = entire M-X sensor
27.65mm = 5.1K area of M-X sensor (Epic 5K mode)
24.4mm = 4.5K area of M-X sensor
24mm = Super-35 film
22.3mm = Canon 7D sensor (APS-C)
22.12mm = 4K area of M-X sensor
16.59mm = 3K area of M-X sensor
11.06mm = 2K area of M-X sensor

I got these figures off of the internet so some may be off a tiny bit, but they are all close enough for you to calculate the difference in field of view. Just keep in mind that there is a whole other issue of vignetting when you put a lens designed to cover a smaller sensor onto a larger sensor. But that doesn't affect focal length calculations for field of view.

The short answer is that there is a 1.25X difference in size between 4K on the Red One and 5K on the Epic. So your 18mm, assuming it doesn't vignette is going to look more wide-angle on the Epic in 5K mode than it does on the Red One in 4K mode -- it will look like you were still shooting in 4K but had switched from an 18mm to a 14.5mm in terms of the increase in field of view. But remember, you didn't change focal length, just view... I'm only talking about what you'd have to switch to on the 4K camera to match the view of the 18mm on the 5K camera. Or, if you liked the view of the 18mm on the 4K camera, you'd have to switch to a 22.5mm lens on the 5K camera to maintain the same field of view.
 
Last edited:
Shane Kelly said:
You are always getting the exact focal length of the lens. The lens will always be an 18mm, nothing will ever change this. Your field of view may differ when you shoot with a smaller sensor but the 18mm will always be an 18mm. What you are doing is using less of the lenses coverage when you use less of the sensor. When you shoot 2k you are only using the center of the sensor, so you are using the center of the lens, think of it as the equivalent of "pushing in" on a frame in after effects.
when I go from 4k 16:9 on the red one to 2k 16:9, I just half the focal length of the lens in order to get the same field of view. So if I was using a 50mm on 4k, I'll use a 25mm on 2k in order to get the same field of view.

The best app for more complex focal length matching that I have found is Pcam.
Click to expand...

Perfect. If I shoot anything on 2k with the EPIC, FoV is more than halved compared to 5k.

Ill get the app, just in case :-)
 
David Mullen ASC said:
Yes, don't confuse yourself Asif -- the focal length never changes. An 18mm is an 18mm whether on a tiny sensor camera or a giant sensor camera. What changes is the field of view, and you want to know what the equivalent focal length needs to be to maintain the same field of view of a certain focal length on a certain sized sensor.

As Shane says, the conversion is easy between 4K and 2K on the Red One because 2K mode uses half the sensor area of 4K, so you'd need to use half the focal length to maintain the same field of view when switching from 4K to 2K.

Full-Frame 35mm, such as on the Canon 5D, means a 36mm x 24mm sensor. It may be easier if you just compare horizontal view rather than diagonal view since then you don't have to deal with aspect ratio affecting the diagonal length.

"Super-35" is technically 24mm wide -- which is what the Red One is in 4.5K mode. So that's a 1.5X difference with FF35 (36 divided by 24 = 1.5.)

Various widths:
36mm = FF35 / Canon 5D sensor
30mm = entire M-X sensor
27.65mm = 5.1K area of M-X sensor (Epic 5K mode)
24.4mm = 4.5K area of M-X sensor
24mm = Super-35 film
22.3mm = Canon 7D sensor (APS-C)
22.12mm = 4K area of M-X sensor
16.59mm = 3K area of M-X sensor
11.06mm = 2K area of M-X sensor

I got these figures off of the internet so some may be off a tiny bit, but they are all close enough for you to calculate the difference in field of view. Just keep in mind that there is a whole other issue of vignetting when you put a lens designed to cover a smaller sensor onto a larger sensor. But that doesn't affect focal length calculations for field of view.

The short answer is that there is a 1.25X difference in size between 4K on the Red One and 5K on the Epic. So your 18mm, assuming it doesn't vignette is going to look more wide-angle on the Epic in 5K mode than it does on the Red One in 4K mode -- it will look like you were still shooting in 4K but had switched from an 18mm to a 14.5mm in terms of the increase in field of view. But remember, you didn't change focal length, just view... I'm only talking about what you'd have to switch to on the 4K camera to match the view of the 18mm on the 5K camera. Or, if you liked the view of the 18mm on the 4K camera, you'd have to switch to a 22.5mm lens on the 5K camera to maintain the same field of view.
Click to expand...

In that case , all I need to do is divide 36mm, with widths of the resolutions required.

Ill be back with more questions, as I start experimenting with my EPiC :-)
 
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