How to figure out the aspect ratio of a squeezed anamorphic lens?

[Abdul Rahman Jamous]

Let's say that I'm shooting by using Alexa mini and Orion 40mm anamorphic prime lens. How to know the dimensions of the squeezed image that occupies the sensor, knowing that my recording format is  open gate  3.4K?

[Abdul Rahman Jamous]

[Abdul Rahman Jamous]

Or in another words.  A crop lens wouldn't fill a full frame sensor, so how I may know if the anamorphic lens that I'm using would fill a full frame sensor or not. Is this information would be given to me by the lens provider? in that case why does an Orion anamorphic lens has two different sensor coverage, one for ALEXA and one for RED?

[Dom Jaeger]

According to those specs, the Orion lenses have an image circle diameter of 31mm, so they will cover all the Alexa mini aspect ratios in anamorphic mode:

Note the bottom sensor image has some of the various aspect ratios you might want to extract - 16:9, 2.39:1 or 4:3 - from anamorphic mode (which uses the full sensor height), and each of the diagonals (or image circle diameters) that are required to cover them. It's the number after the Ø  symbol.  An Orion lens will cover all of them. Each of the rectangles pictured will be unsqueezed to twice the width (with 2x anamorphics) for the final aspect ratio.

The sensor coverage specs for the Orions is giving you the largest format choice possible on an Alexa (a 4:3 extraction needs the largest image circle - 29.70mm) and on a RED (Monstro 8K 6:5 anamorphic mode). They are different because each camera has different sensor sizes and capture areas, but all you need to know is that the lenses will cover anything up to those choices.

[Dom Jaeger]

5 hours ago, Abdul Rahman Jamous said:

Let's say that I'm shooting by using Alexa mini and Orion 40mm anamorphic prime lens. How to know the dimensions of the squeezed image that occupies the sensor, knowing that my recording format is  open gate  3.4K?

For Open Gate:

The 2.39 Scope 2x frame is only slightly larger than in anamorphic mode, requiring an image circle of 28.30mm. The Orion lenses still cover it. Note that because you are only using the central portion of the sensor, the resolution is only 2.6K (2631 photo sites across) not 3.4K.

[David Mullen ASC]

No point in shooting 3.4K Open Gate since that is a 1.54 : 1 aspect ratio -- for 2.40 and a 2X squeeze, the only area you use is 1.20 : 1 sensor area, so you won't come near using Open Gate's width.  Now perhaps in Arriraw, you record Open Gate no matter what but you will be cropping to sides way down later. The anamorphic lens doesn't have to cover the 28.25mm width, it just has to cover the 18.17mm height, which it will.

[Abdul Rahman Jamous]

Thank you very much for patiently answering my questions. Obviously I need to improve my technical knowledge.

[Abdul Rahman Jamous]

Okay... let's say that hypothetically I still want to shoot in open gate format. according to the charts you gave to me. If I'm using an anamorphic lens that has the squeeze factor of  2x, then the dimensions of the squeezed image would be 21.71 x 18.17 mm (please tell me that I got this thing right LOL) so what if I used an anamorphic lens that has a different squeeze factor (1.3x or 1.5x) How I can figure out the dimensions of the squeezed image?

 

 

[David Mullen ASC]

But 21.17 x 18.17 is not Open Gate then so I’m not sure I understand your question. Open Gate is 28.25 x 18.17. Are you talking about after you’ve cropped the Open Gate recording in post to 21.71 x 18.17? 

Divide 21.71 by 18.17 and multiply by 2 and you get 2.39 : 1 as the aspect ratio.

You have to work backwards from your intended final aspect ratio to determine the sensor area used depending on the squeeze ratio of the lens. 
 

So if 2.39 is your intended aspect ratio, and your squeeze is 1.3 and your sensor max size is 28.25 x 18.17mm (1.55 : 1), you can figure out the width used. Divide 2.39 by 1.3 and you get: 1.838.

So the aspect ratio of the sensor used is 1.838 : 1 but because the image has a 1.3X squeeze, it becomes 2.39 : 1 when unsqueezed.

But your sensor is only 1.55 : 1 max, not 1.838 : 1 so that tells you that you are going to use the max width of the sensor but not the max height (this ignores the issue as to whether the Hawk 1.3X lenses were designed to cover a 28.25mm wide sensor.) 28.25 divided by 1.838 is 15.37. So if you use a sensor area of 28.25 x 15.37mm (1.838 : 1 aspect ratio) and the lens has a 1.3X squeeze, the final image would be 2.39 : 1 once unsqueezed.

[Abdul Rahman Jamous]

noooo ?

 

The aspect ratio of my de-squeezed image is absolutely not my concern here.

Hopefully the illustrations below would help me to make my question more clear.

[David Mullen ASC]

I think you’re missing the point, the final unsqueezed aspect ratio is CRITICAL to understanding this issue. Those red areas above all assume you want a final aspect ratio of 2.39 : 1 once unsqueezed (at least the first one does).

[David Mullen ASC]

If you don’t care about the final aspect ratio, then what’s the point of the question, it’s easy to figure out what you’ll get if you put a 1.5x lens and use all of Open Gate for the image. What your red area seems to be asking is what area of Open Gate is used if the lens is 1.5x and you want the final unsqueezed image to be 2.39 : 1.

[David Mullen ASC]

Think of it this way. Let's say you don't crop the Open Gate area at all, you use all 28.25mm x 18.17mm and put a 1.5X anamorphic lens on it (assuming the lens covers that large an area).

Since 28.25mm x 18.17mm is a 1.55476 : 1 aspect ratio, if you put a 1.5X squeeze on the lens, the unsqueezed image is 2.33214 : 1 (1.55476 x 1.5).

Do you want a 2.33 : 1 image?  If so, use the whole Open Gate sensor area.

Or do you want 2.39 : 1?  If so, you're going to have to trim some of the top & bottom to make the shape more widescreen.  So that means you'd use the full width of Open Gate (28.25mm) but not the full height, you'd trim 18.17 to 17.73mm.  28.25mm x 17.73mm = 1.59333... : 1 aspect ratio, multiply by 1.5X to get rid of the squeeze and you get 2.39.

With your second drawing, you could only get less than a final aspect ratio of 2.33 : 1 (max sensor area) if you planned on using a 1.5X lens but the full 18.17mm height of Open Gate -- so how wide that red box is depends on what aspect ratio that is less wide than 2.33 : 1 that you want to get to.

[David Mullen ASC]

Now if you're asking what horizontal area an anamorphic lens with an image circle of 31mm covers, I don't know - the image circle is a vertical oval I guess... so I don't know if that figure is the longest diagonal of the oval.

[David Mullen ASC]

Ask yourself why is the red box in the first drawing 21.71 x 18.17mm.  Why use 21.17mm out of a potential 28.25mm?  When you understand that, then you can figure out the rest.

[Abdul Rahman Jamous]

12 minutes ago, David Mullen ASC said:

Now if you're asking what horizontal area an anamorphic lens with an image circle of 31mm covers, I don't know - the image circle is a vertical oval I guess... so I don't know if that figure is the longest diagonal of the oval.

This is my question... 

[David Mullen ASC]

You'd have to test or put the lens up on lens projector, lens circle issues are not an exact science.

[Abdul Rahman Jamous]

Thank you very much!