Focal Factor for 2k and 3k

[Nonsense]

When I'm shooting at 2k or 3k, anyone know how I calculate the focal length of my 18-50 RED lens? I'm just trying to describe to a client how we will be using losing some lens width when we down rez to 3k or 2k.

 

[filip kovcin]

i beleive this is simple...

i beleive this is simple...

try this:

4K :2K = X mm: Y mm

if you know X - it should be simple.

i.e. if you use 50mm with your zoom lens - shooting 4K, it SHOULD be 2x50mm =100mm in 2K.

so, if my calculation is correct - if shooting 3K, it will look like this:

4K:3K = 50mm : Y mm

1.333 =50mm : Y mm

Y mm = 50mm x 1.333

Y mm = 66,666mm

simple way:

2K - focal length x 2
3K - focal length x 1.666
4K - focal length x 1

do you think this is correct?

filip

 

[David Mullen ASC]

4K to 2K cropped is easy -- you lose half the view. Your 18mm lens is still an 18mm lens, just that it has half the field of view in 2K cropped mode. You could say it has now become a 36mm lens... though that's not really an accurate way of thinking about it because if you put the 18mm lens on a Super-8, Super-16, 65mm, or IMAX camera, it would still be an 18mm lens.

Or to put it another way, you'd have to cut the focal length in half to maintain the same field of view when cropping the image in half -- so you'd want to switch to a 9mm lens in 2K cropped mode to maintain the same view as an 18mm lens in 4K mode.

As for 3K, well, that 75% of 4K (3/4's obviously)... so you'd need to use a 13.5mm lens in 3K cropped mode to maintain the same view as an 18mm lens in 4K mode. Or you could say that your 18mm lens now has the same view as a 24mm lens... (if I did the math right...)

 

[Nonsense]

Thanks David! Thats what I thought, but just wanted to make sure.

 

[Lars Xang]

wow David that made me finally understand all this.

 

[Richard Bradbury]

I did the homework on this recently and came up with a conversion table for both field of view and depth of field matching between 4K, 3K and 2K. It's set up to go in the lid of a box of superspeeds.

Example: You're shooting on a 50mm lens at T2.8 in 4K at 25fps.
To get 60fps with the same fov/dof you switch to 3K, and you put on a 35mm lens at T2.
To get 120fps with the same fov/dof you switch to 2K and you put on a 25mm lens at T1.4.

 

[David Mullen ASC]

Now try and match all of those focal lengths with only a set of Ultra Primes...

 

[filip kovcin]

I did the homework on this recently and came up with a conversion table for both field of view and depth of field matching between 4K, 3K and 2K.

Richard,

can you kindly explain (maybe if you feel this is not interesting for all - send me a P.M.) to us what formula did you use to calculate things in your tables.

also - if you can - maybe to suggest best match scenario - using existing lenses - not i.e. 24,4mm lens...

thank you,

filip

 

[David Mullen ASC]

It's pretty simple math since 2K (2048 pixels across) is half the horizontal view of 4K (4096 pixels across) and 3K is 3/4's of 4K. Though I'm not sure where 24.4mm came from dividing 50mm in half...

My point though is that unless you are using a zoom, more than likely you'll just use the nearest number in your prime lens kit. For example, in the Ultra Prime line-up, there's a 24mm and a 28mm, but not a 25mm.

 

[Richard Bradbury]

Quite right David. I did the calculations having worked out the actual sensor size for each format, rather then the somewhat simpler method you suggested, and I was slightly out!oster_oops:

I've amended the table above in the name of accuracy, but it won't affect lens choice when switching format. You'll always have to go for the nearest option available. If your shot is so critical that you need to match a fov down to a fraction of a degree then you probably shouldn't be changing lens anyway.

 

[Eric Lange]

I try to base my calculations on actual sensor size rather than just the number of pixels, as different sensor sizes have different pixel pitches or pixel sizes. Some of the sensors we use range from 5 micron "pixels" to 9 micron. The RED ONE literature states 29 micron square pixels which is a pixel size of approximately 5.4 micron by 5.4 micron. Does anyone know the exact sensor size and pixel pitch of the scarlet?

Cheers E.