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Porthole on Arri VP on Alexa mini ?


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It's just a question of whether they would vignette in 3.2K or 3.4K Open Gate mode, they would be fine in standard 2.8K (2K or HD output) mode.

 

Thank you very very much David !!! (it's the cropped frame you suggested me on this http://www.cinematography.com/index.php?showtopic=75594&p=489741 )

 

Please give me some infos about the porthole effect .. i'm a bit confused

This happen to digital sensor with size BIGGER than the standard 35mm film sizen of 20.96mm ??

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All 35mm cine lenses were designed to cover standard 35mm (21mm), most cover Super-35 (24mm wide) but on some, there is more corner fall-off. I used the classic Cooke 20-100mm zoom in Super-35 on a TV show and there was a feint vignette effect, not unpleasant, it was just brighter in the center of the image, dimmer in the corners if you looked for it. Sometimes you can color-correct the effect out in post if the fall-off isn't too extreme.

 

A lot of digital cinema cameras now though have sensors that are wider than 24mm so some 35mm cine lenses (particularly wide-angle ones or the wide end of a zoom) show more vignetting problems, to the point of having black shadows in the corners of the frame.

 

All lenses project a circular image out the back of the lens and the film gate or sensor crops this to a rectangle. If the image circle isn't bigger than the rectangle, you see darkening in the corners of the frame, sometimes a fast obvious edge of the image circle, sometimes a more gradual fall off. Some lenses tend to have this porthole effect where the center of the image is brighter than the sides, and depending on how much the sensor sees out to the edges of the image circle, it can be more obvious or less obvious.

 

An extreme example would be the HAL POV shots in "2001" where a super wide-angle lens made for a 35mm camera was put onto the larger 65mm camera, creating a circular mask around the image.

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Thank so much David for your extended analysis !!

 

Tell me something more please ..

 

I have read that the digital sensors (instead of film) requests the light rays going ''straight'' on them ..tele-centric not on an angle

 

And that some lenses like the old standard primes .. 'cause of their design.. send their edge-rays on an angle .. and the digital sensors (instead of film) can not capture these rays .. so they give a viggnet effect

 

That happens on the edge-rays OUTSIDE the era of the of the standard 35mm frame ??

 

If mean if i use standard primes on Alexa .. on my 20.96mm width frame .. will i have that problem ??

 

Thanks again David !! I appreciate your help !!

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