Focal Length Changes 4K > 2K

[Tim Fassnacht]

Hey All,

I know I have seen a post with an in-depth field of view comparision when going from 4K to 2K (2:1, 16:9) on here at some point but I can't seem to track it down. Does anyone know where such a post exists?

For times savings sake- your run of the mill "essentially doubles it" responses can be omitted here.

Thanks

Tim

[Greg M]

Technically the focal length does not change, you are just cropping out the center of the image. The depth of field is the same.

[wedowee]

I think I did a post a while back. If I didn't, I know at least I did field of view comparison test and can post it soon.

[Gunleik Groven]

You're probably refering to Brooks RED FAQ.

http://www.reduser.net/forum/showthread.php?t=1487

[wedowee]

Here is a LOMO lens at 22mm. The first one is 2k, then 3k, then 4k. I have other size lenses I can post if you'd like (18, 28, 35, 50, 75, 300mm). They are in 2:1 format.

I kept the camera stationary and shot with the same lens at 2k, then changed the project to 3k, then 4k. Recorded about 10 seconds each time. Exported a .jpg in smaller dimensions for posting here. I didn't worry about focus or exposure.

These are build 15 snapshots, but field of view won't be affected by build changes.

[wedowee]

Here are photos from the 50mm lens. Same procedure as above, camera in same place as above.

Notice the 22mm 2k shot (shown in the post above) is about the same framing or field of view as the 50mm 4k (since 22 is almost half of 50). So if you have this exact framing in mind for the shot, you can use the 50mm in 4k if you need all the resolution, or use the 22mm in 2k if you don't need all the resolution or you want to use faster frames per second for slo-mo, etc.

I plan on laminating these as a frame of reference for renters who want to rent my Red (whose name is FRed).

[wedowee]

Let me know and I'll post all lenses.

Just an elementary observation - Say you wanted to render the above 3 shots on a timeline in 2k, the field of view would remain exactly the same, but if you stretched it to the same size as a 4k render (by grabbing the corner of the image and stretching it in FCP for example), it would be pixelated. That's why some who shoot 4k choose to add simulated camera motion in post (a pan, tilt, or zoom in), as there is plenty of data there.

[Alexander Nikishin]

Basically, when going from 4K to 2K you effectively double your focal length.

In other words, an S35 24mm Master Prime is a 24mm in 4K and a 48mm in 2K.

In 3K, the crop factor is 1.3x the focal length....... A 24mm is effectively turned into a 32mm lens.

Hope that helps.

[Manuel Wenger]

...adding to Alexanders post, this is true if you calculate it back to the 4K focal lenghts. A S35mm 24mm MP is 24mm in 4K a 24mm in 2k but behaves like a 48mm in 4K at 2K, because a 24mm on 4K would have the same FOV as a 12mm on 2K.

On a practical case it means if you are planning to shoot wide shots in 2K (the only reason you´ll go for wide on 2K would be slomo) you have to look for S16mm wide lenses.

Manuel

[Hans von Sonntag]

...adding to Alexanders post, this is true if you calculate it back to the 4K focal lenghts. A S35mm 24mm MP is 24mm in 4K a 24mm in 2k but behaves like a 48mm in 4K at 2K, because a 24mm on 4K would have the same FOV as a 12mm on 2K.

On a practical case it means if you are planning to shoot wide shots in 2K (the only reason you´ll go for wide on 2K would be slomo) you have to look for S16mm wide lenses.

Manuel

Well put. And yes, the focal length of the lens won't be altered by the recording format only field of view changes.

Hans

[Johan Pabon]

Technically the focal length does not change, you are just cropping out the center of the image. The depth of field is the same.

To be correct I think: The deph of field depends on three factors: the focal length, the diafragm and the size of the chip.
With the diafragm and the focal length being a constant factor, when changing from 4K to 2K the deph of field will change because the effective size of the chip is changing.

[Chris Kenny]

The size of the sensor doesn't change depth of field. The only things that matter are aperture and focal length. The reason why people say Super35 has shallower depth of field than (say) 1/3" video is because you use longer lenses to achieve equivalent field of view with larger formats, and longer focal length means less depth of field.

(And lots of people don't actually understand the relationship between focal length, subject distance, and depth of field, but that's another discussion.)

[Greg M]

The depth of field remains the same from 4K to 2K. You are just looking at the center of the 4K image thru the same lens.