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Alex Birrell

Red Epic W 8k Helium crop factor for anamorphic

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Hello!

 

Can anyone tell me definitively the crop factor when using 2x anamorphic lenses on the Red Epic W 8k Helium in 8k 6:5 mode?

 

I've seen lots of answers for similar questions on this forum but mostly for the 6k Red Epic prior to the new sensor or talking about the VV Red Weapon 8k so I'm getting a bit confused!

 

Thanks!

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8K 6:5 mode is 18.92mm x 15.77mm. 4-perf 35mm anamorphic projection aperture is 21mm x 17.5mm.

 

I assume when you say "crop factor" you mean in comparison to 4-perf 35mm anamorphic. 17.5mm divided by 15.77mm gives you a 1.11X crop factor if you compare vertical view. Same goes for 21mm divided by 18.92mm.

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RED has a fairly useful tool on their website that will provide the dimensions of any of their sensors, at any resolution window: http://www.red.com/tools/crop-factor For what it's worth, the 8K Helium and 6K Dragon sensors have almost the same dimensions, so if you've shot 2x anamorphic 6:5 with a Dragon then 2x anamorphic 6:5 on Helium should give you almost exactly the same perspective.


That said, crop factor confuses so many people, and it gets misused in all kinds of ways. I find it much more straightforward to determine the normal focal length for the format you're shooting.


Based on the dimensions David's provided, by doubling the horizontal dimensions we get virtual capture areas of 42mm x 17.5mm for 35mm 4-perf scope, which has a diagonal measurement of 45.5mm; and 37.84mm x 15.77mm for RED Helium 8K 6:5, with a diagonal of 41mm. The crop factor between the two works out exactly the same as David's calculation, 1.11x, but I wouldn't get hung up on the numbers - I'd think of a 40mm as a typical normal perspective, and from there decide if my normal lens for that movie would be wider or longer than that based on my approach to the story.

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Well, the whole idea is that it's less wide on a RED than it is on 4-perf film. ;)
If you want the numbers, out of PocketAC:
  • For 35mm scope, projected, the "normal" value of 45.5mm gives a 49.5 degree field of view.
  • You'd usually round up to a 50mm, for a 45.5 degree field of view - slightly tighter.
  • A 40mm would give you an FoV of 55.3 degrees.
  • For RED Helium 8K 6:5 2x, a 40mm lens gives you a 50.6 degree FoV - about 1 degree wider than the theoretical "normal" of 35 scope.
  • To match a 50mm lens on 35 scope, you'd need a 45mm lens on the RED (which doesn't exist in many lens sets).
  • That 50mm lens on the RED gives a 41.5 degree FoV - tighter.
  • To match the 40mm on 35 scope, the RED would need a 36.1mm lens. Rounding down to a 35mm lens, you'd get a 56.8 degree FoV.
This is in a perfect world of theory, where anamorphic lenses wider than 50mm don't start to get noticeable distortion in some cases.
In any case, shooting with a typical set of 2x anamorphic lenses on a RED means getting an FoV that's either slightly tighter or more wide than what you'd get with 4-perf 35mm film.
And I did say not to get hung up on the numbers. :)

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So shooting on the Helium at 8k 6:5 and aiming for a 2.40:1 final aspect ratio, how much of the image would have to be cut from the sides, or would the de-squeezed frame not be that much wider? 

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Don't the dimensions of the sensor come into play with regards to cropping the image though? Or do you, with the Helium 8k super 35 sensor in 8k 6:5 get the entire field of view of the given anamorphic lens with no crop? 

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Yes, the sensor isn’t 6:5 (1.20 : 1) it’s more like 1.9 : 1 so if you record 6:5, you’re cropping the sensor.

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Are you asking if a given anamorphic lens on the Helium at 8K 6:5 gives you the same field of view as it does on a 4-perf 35mm camera, which is about a 21mm x 17.5mm area for standard anamorphic? 

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The max Helium sensor size is 8192 x 4320 / 29.9mm x 15.77mm, which is a 1.89 : 1 aspect ratio.

If you are shooting in standard-squeeze 2X anamorphic for a 2.40 : 1 finish, that means the actual area used, whether film or digital, is 1.20 : 1, nearly square.  Most digital cameras have a wider sensor than that, so the limitation with anamorphic in terms of sensor size is usually the vertical dimensions since you will not be using the full width of the sensor (otherwise your 1.89 : 1 sensor would yield a 3.78 : 1 image once unsqueezed.)

Since anamorphic lenses were mainly designed for the 4-perf 35mm standard format where the contact print made off of the negative used a 21mm x 17.5mm projector mask, if you want to maintain the same field of view when shooting digitally, you'd need to use a camera with a sensor that is at least 17.5mm tall.  Trouble is that the Helium sensor is 15.77mm tall.

So if you record 8K 6:5, you are recording 5184 x 4320 pixels with a sensor area of 18.92mm x 15.77mm, a bit smaller than the 4-perf 35mm area (about a 1.11X crop factor, not major) so your anamorphic lens image will be a little cropped on a Helium compared to on a 4-perf 35mm camera.

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