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Vignette on Cooke Varotal 20-100


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I was doing some tests around my apartment with a Cooke 20-100 Zoom on an Alexa Mini (4K UHD 16:9) and noticed a hard vignette at the 20mm end on the frame right side.  When I zoom in a little bit, the vignette becomes more of a shading which is closer to what I expected. Stopping down didn’t help (shot is at F/3.1)  and playing with rear filter holder didn’t change anything. 

Is this related to being centered for Academy rather than Super 35? Or is it likely due to not being maintained?  The lens has been sitting on a shelf for a long time, hasn’t really been serviced, but operated okay for my tests, no binding or catching, etc  

-TristanA5AE5711-F400-4419-9C7C-9269CED336AF.thumb.jpeg.67ee72b27eda645a1c6fc3e2d8b7d51b.jpeg

 

Edited by Tristan Noelle
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  • Tristan Noelle changed the title to Vignette on Cooke Varotal 20-100
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Some years ago I measured the image circle diameter of a 20-100 Cooke Varotal to be under 29mm at the wide end. That's too small for some of the higher resolution modes on an Alexa Mini. 4K UHD 1.78 needs an image circle of 30.29mm for instance. If the zoom has the front mask, removing it may give you a little more coverage. See the image circle requirements for Alexa Mini modes here:

https://www.arri.com/resource/blob/31908/14147b455c90a9a35018c0d091350ff3/2021-10-arri-formatsandresolutionsoverview-3-4-data.pdf

A zoom (or any lens) is never centred for Academy or S35, that's a film camera mount setting. It's possible the lens tracking adjustment is off, or the Alexa mount is slightly off-centre to the sensor.  Or there is some distortion or cocking in the zoom somewhere, the mount shim ring is not flat etc. 

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1 hour ago, Dom Jaeger said:

Some years ago I measured the image circle diameter of a 20-100 Cooke Varotal to be under 29mm at the wide end. That's too small for some of the higher resolution modes on an Alexa Mini. 4K UHD 1.78 needs an image circle of 30.29mm for instance. If the zoom has the front mask, removing it may give you a little more coverage. See the image circle requirements for Alexa Mini modes here:

https://www.arri.com/resource/blob/31908/14147b455c90a9a35018c0d091350ff3/2021-10-arri-formatsandresolutionsoverview-3-4-data.pdf

A zoom (or any lens) is never centred for Academy or S35, that's a film camera mount setting. It's possible the lens tracking adjustment is off, or the Alexa mount is slightly off-centre to the sensor.  Or there is some distortion or cocking in the zoom somewhere, the mount shim ring is not flat etc. 

Thanks for your insight, Dom, that all makes sense. The Alexa PL mount seemed solidly in place, but it was a rental so who knows. If I can get the lens serviced before I shoot again, I’ll see if that changes anything.  

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