Field of view factor for 4K to 3K, 2K?

[Noah Kadner]

Quick question- how does the field of view change from 4K to 3K and to 2K. Let's say I have a 50mm lens in 4K, what would that look like in 3K?

My field of view is narrower because I'm shooting a smaller slice of pixels off the sensor, right? So would that 50mm lens now be more like a 70mm? And what about in 2K?

thanks,
Noah

 

[Jason Ramsey]

I'm completely talking out of my rear here, but from what I've read you would just multiply the lens focal length by the following, would you not?

This is from RED.com

Given 35mm 36mm x 24mm = 1.0X
RED 4K 22.2mm x 12.6mm = 1.62X
RED 3K 16.65mm x 9.36mm = 2.16X
RED 2K 11.1mm x 6.24mm = 3.24X

so a 50mm on 35mm would be:
RED 4k it would = 81
RED 3k it would = 108
RED 2k it would = 162

equivalent...

is that right?

Difference in crop factor from RED 4k to RED 3k would be 1.333333 (I think). So, taking your above question if you considered 50mm on RED 4k.. that would be about 66.6 at RED 3k. 100mm at RED 2k (2x crop factor from 4k).

I'm actually just interested in the question, so tried to figure it out myself, but am not a numbers or tech person, so someone else will have to confirm the above or shoot me down.

later,
jason

 

[Daniel Browning]

someone else will have to confirm the above or shoot me down.

It's all spot on, Jason.

 

[Noah Kadner]

The 35mm to 4K conversion factor sounds pretty steep for a 50mm lens- are you sure about that? What effect does the aspect ratio have?

-Noah

 

[Daniel Browning]

The 35mm to 4K conversion factor sounds pretty steep for a 50mm lens- are you sure about that?

The reason it's steeper than you might expect is that the recording area of 4K is smaller than the total sensor, thanks to the lookaround feature. The upcoming RED ONE 4.5K feature (no horizontal lookaround) will give a ~1.4X crop factor.

What effect does the aspect ratio have?

It depends on your point of view. I prefer to compare as though both formats will be cropped to match the widest horizontal angle of view. That is, to imagine that FF35 will be cropped from 36x24mm to 36x20.25mm for comparison with RED ONE 4K (1.78:1 aspect ratio). If instead you crop the RED ONE to match the 3:2 aspect ratio, the crop factor is around 1.9X.

 

[Noah Kadner]

So then it goes without saying if you're working in 2K you'd better have some really wide angle lenses. To get a 50mm-equivalent perspective you'd need around a 17mm lens?

-Noah

 

[Sidney L. Plaut]

this was what I was taught here on reduser :

"Multiply the size of each pixel, which is 5.4 microns on the Red one, by the number of pixels in each direction. This will give you the size of the crop. So for 4K 16:9 that's, .0054(moving the decimal 3 places to convert from millionths to thousandths) x 4096 = 22.12mm. Then, .0054 x 2301 = 12.43mm. So, 22.12mm x 12.43mm with a 25.37mm diagonal. Versus 24.89mm x 13.98mm with a 28.55mm diagonal, for s35 cropped for 16:9. This gives a Field of View crop factor of about 1.125.

So to get the same FOV as the 50mm in super35, you'd have to use a 44mm on the Red in 4K. This lens doesn't exist in any sets that I know of as a prime, so you'd either have to use your 50mm and move back a foot or two, or use a 40mm and move forward a foot or two. So, not enough to really worry about.

If you were shooting Academy 35mm, which is 21.95mm horizontally, cropped for 16:9, there is really no difference between this and 4K 16:9 on the Red One." end quote.

 

[Ed Watkins]

I had a converter for this built into my iPhone app (it was called Lens2Lens), I'm working on putting it back in. Should be back in iSee4K in a few weeks.
It's a relative calculator, i.e you choose the lens and f stop you have on one camera and it would tell you the lens and f stop you would need on a different format camera to approximate the DOF (hyper focal distance) and FOV.

 

[Douglas Underdahl]

Unless you are building sets and need to know very exact fields of view based on camera positions, then use this:

In 35mm, a normal lens is about 35mm. Some folks like 30, some 35, some 40.

In 16mm, a normal lens is generally about 16mm focal length, could be 20, maybe 14.

Of course there are slight changes when going super 35, or super 16, or 2/3 video, but most DPs like to pick a main lens to shoot most of the feature with, usually their normal lens. For comedy, a wider lens is often used as a normal (like see BLUES BROTHERS), and for romance or drama, a longer lens is often used. A 50mm is a favorite for over the sholder CUs to make stars look better and less "bulbous".

Your film can have a coherent look throughout by picking a standard lens and shooting most everything with it, which is why a lot of DPs don't like zooms, but rather move the tripod or dolly in and out to change from wide to medium to close ups.

I've never heard of DPs refer to a crop factor. They know what the normal and wide and telephoto lens is for each format by heart. It's pretty easy to just remember the above paragraph. So I think the sooner you stop using this term on set, the better. You are more likely to hear someone say "slap on a 2 inch" or "put on the 18".

Also, you'll find a lot of DPs have a favorite stop and in 35mm it usually hovers around 4.

 

[Noah Kadner]

Unless you are building sets and need to know very exact fields of view based on camera positions, then use this:

In 35mm, a normal lens is about 35mm. Some folks like 30, some 35, some 40.

In 16mm, a normal lens is generally about 16mm focal length, could be 20, maybe 14.

Of course there are slight changes when going super 35, or super 16, or 2/3 video, but most DPs like to pick a main lens to shoot most of the feature with, usually their normal lens. For comedy, a wider lens is often used as a normal (like see BLUES BROTHERS), and for romance or drama, a longer lens is often used. A 50mm is a favorite for over the sholder CUs to make stars look better and less "bulbous".

Your film can have a coherent look throughout by picking a standard lens and shooting most everything with it, which is why a lot of DPs don't like zooms, but rather move the tripod or dolly in and out to change from wide to medium to close ups.

I've never heard of DPs refer to a crop factor. They know what the normal and wide and telephoto lens is for each format by heart. It's pretty easy to just remember the above paragraph. So I think the sooner you stop using this term on set, the better. You are more likely to hear someone say "slap on a 2 inch" or "put on the 18".

Also, you'll find a lot of DPs have a favorite stop and in 35mm it usually hovers around 4.

Right- but this is the heart of the question. In 35mm what you say is all true. But with a RED the digital sensor is not the same area as a 35mm piece of film. So a 50mm lens is not going to give you the same shot on a 35mm camera as it would on a RED because, it does not shoot the same frame. That's the field of view factoring I'm trying to get at. If you were to walk onto a RED shoot coming from a 35mm shoot and say, put up the 50mm- you'd find it a bit tighter framing than you expected and might back up a few to a wider lens to get the equivalent, right?

Noah

 

[Douglas Underdahl]

so a 50mm on 35mm would be:
RED 4k it would = 81
RED 3k it would = 108
RED 2k it would = 162

Also, you can see that this crop factor leads to confusion that will mark you as inexperienced. A 50mm does not ever turn into an 81, or 108, or a 162. It's always a 50mm no matter what format you use it on. On a RED at 4k it would be a slightly longer than normal lens. On 3k it would be something of a mild telephoto. On 2k it would be a medium telephoto, at the middle range of a lot of popular zoom lenses for 16mm that go from about 10mm to 100mm.

So the 8mm Peleng that I modified to PL mount is a crazy wide angle (fish eye) in 35mm, a super wide in 3k, and a very wide lens in 2k.

Now, some lenses like the older 8mm Ziess in Arri Standard mount only "cover" 16mm. So they vignette or look like a porthole (circle with black edge like when Porky Pig said "That's All Folks) if you use them in 3k or 4k or 35mm. So coverage is not the same as focal length. The Peleng has the same focal length but larger coverage that can be used for Academy 35mm.

 

[Douglas Underdahl]

Right- but this is the heart of the question. In 35mm what you say is all true. But with a RED the digital sensor is not the same area as a 35mm piece of film. So a 50mm lens is not going to give you the same shot on a 35mm camera as it would on a RED because, it does not shoot the same frame. That's the field of view factoring I'm trying to get at. If you were to walk onto a RED shoot coming from a 35mm shoot and say, put up the 50mm- you'd find it a bit tighter framing than you expected and might back up a few to a wider lens to get the equivalent, right?

Noah

First, you have to define a 35mm piece of film.

A 35mm piece of film can be full frame 35, like what you used to put into your Nikon still camera, or Super 35mm, or 35mm academy.

A normal lens is about 50mm on a full frame Nikon. Actually, that's the lens that they usually sold with those cameras but it's probably a bit wide angle. A normal lens on Super 35 is probably about 40, could be 35 depending on what you like. On Academy which is smaller due to the allowance for the sound track, it's also about 35, and as I say, some like 35, some like 40, or 30 or even 50. It's all personal taste and what you feel the script needs.

The RED sensor is VERY similar in size to Academy (check our Brook's FAQ 2nd page). Super 35 is very recent in motion picture production and full frame 35 (Vistavision) wasn't used much except in films like PATTON and for shooting background plates for Star Was. Most Zeiss superspeed sets are 18, 25, 35, 50, and 85 and are based to be used on Academy cameras.

So if you were coming on a RED shoot coming from a 35mm shoot, if it was Academy and your RED was set to 4k 2:1 or 16:9 then you'd be pretty much bang on in terms of what lenses you'd been using all along. David Mullen said he shot a lot of Super 35 and then went to RED for Manure and he thought the dolly grip was laying tracks too close, but he realized that he was basing his placement of camera and dolly on the Super 35mm ground glass in his director's finder and it took him a week to realize it was his fault and to not get mad at the grip! I was going to suggest popping his ground glass out and drawing some new narrower side lines on it with pencil or sharpie (comes off with lacquer thinner).

You could put a 50mm on and back up but 50mm gives quite a different look than 30mm. Lots of shooters put zooms on and trombone around instead of moving the camera and staying with one focal length and it's one reason a lot of films have a lazy TV look but of course it's faster and if you have to shoot ten pages a day . . .

 

[David Mullen ASC]

If you are switching from 4096 to windowed 2048 pixels across in recording, then the difference in FOV is easy to calculate -- you lose half the horizontal view, aka your 50mm lens in 4K mode now looks like a 100mm in 2K mode. Or to put it in another way, in 2K windowed mode, you'd have to switch to a 25mm to maintain the same FOV as the 50mm in 4K mode.

In terms of switching between RED versus 35mm cine shooting, the recorded 4K RED area of the sensor is similar in width and in horizontal view to 35mm Academy / standard 1.85, not Super-35.

 

[Douglas Underdahl]

Yes, Super 35 extends over into the area traditionally used for the optical track on release prints, so it's larger than 35mm Academy.

David, what focal length do you generally consider as your normal lens for Academy/RED? Stop for interior work?

 

[David Mullen ASC]

I don't really believe in the whole "normal" lens concept, I just believe that there's a certain range of medium focal lengths where perspective is relatively undistorted -- for 35mm cine shooting, that's usually in the 35mm to 50mm lens range.

Otherwise, the difference in view between a 50mm lens on a RED or 35mm Academy camera versus a Super-35 camera is not a big deal other than that any director's finder you use should have the same markings.

Lately it seems I keep lighting to around a f/2.8 for most interiors -- in other words, I light to the widest aperture that I think most lenses will behave well at, to capture and balance with the most amount of natural light. But I'm discovering just how shallow-focus that is lately because of watching this stuff live on an HD monitor and seeing focus pullers struggling with it, especially on longer lens shots.

 

[Douglas Underdahl]

2.8 seems to be nice for interiors, picks up natural light and practicals well but I'm with you, 4 is a stop the AC can live with. Which is why I'm not so hot on the idea of going with the full frame Epic or Scarlet stuff, unless the ASA is considerably higher.

 

[Noah Kadner]

What about a conversion multiplier coming from 1.85- the one we have at the start of this thread is super35 right? ps found this:

Higher res version here: http://en.wikipedia.org/wiki/Crop_factor

Noah

 

[Douglas Underdahl]

1.85 is just an aspect ratio. You can do a 1.85 extraction from any format, 16, super 8, RED 2:1, Academy. It's not an actual measurement expressed in inches or millimeters.

As for crop factor or multiplier, I suggest that you stop using these terms and instead simply commit to memory the standard (not wide angle, not telephoto) focal length for each format that you intend to use.

 

[Ed Watkins]

As for crop factor or multiplier, I suggest that you stop using these terms and instead simply commit to memory the standard (not wide angle, not telephoto) focal length for each format that you intend to use.

I agree with you on the crop factor, it makes no sense to constantly be referring to your current lens in relation to another format, it just gets confusing and unwieldy.

 

[Noah Kadner]

As for crop factor or multiplier, I suggest that you stop using these terms and instead simply commit to memory the standard (not wide angle, not telephoto) focal length for each format that you intend to use.

Ok, then in your opinion what's a typical standard focal length in 4K, 3K and 2K?

-Noah