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Django Unchained bokeh


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#1 David Edward Keen

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Posted 19 November 2015 - 06:46 PM

i can't upload the pic but i also dont know how to erase an entry. thats my entry haha


Edited by David Edward Keen, 19 November 2015 - 06:54 PM.

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#2 David Mullen ASC

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Posted 19 November 2015 - 07:01 PM

You're probably referring to the vertical oval bokeh from the front anamorphic prime lenses?  Or the square-ish bokeh from the rear anamorphic adapted telephotos?


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#3 David Edward Keen

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Posted 20 November 2015 - 08:15 AM

Wow, it might be, but as important is I gotta learn about these anamorphic lenses, especially the rear anamorphic adapted telephoto lens. "Rear" and "front" describe the position of....?

Edited by David Edward Keen, 20 November 2015 - 08:20 AM.

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#4 David Edward Keen

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Posted 20 November 2015 - 08:19 AM

"Primo zooms equipped with rear-mounted anamorphic optics." (Google)

Rear of the lens?
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#5 Dom Jaeger

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Posted 20 November 2015 - 10:51 AM

Making a zoom lens that uses cylindrical (anamorphic) elements at the front or in the middle like anamorphic primes do seems to be a very difficult task for lens designers without compromising the image quality, so anamorphic zooms are almost always a normal spherical zoom with rear-mounted anamorphic optics. Sometimes such adapters are used with a telephoto lens to extend the range of a set of anamorphic primes, or give better quality than the long end of a zoom. You need space between the mount and the rear element to fit the optics, so only certain zooms and telephoto primes are adaptable.

 

The adapter takes the spherical image produced out the back of the lens and stretches it in the height dimension to create an anamorphic image that will intercut easily with anamorphic primes. Basically it's a focal extender in the vertical axis only. But because the result is simply a vertically stretched spherical image, when it gets unsqueezed it reverts to looking like a spherical image with normal bokeh.

 

By contrast anamorphic primes use cylindrical elements at the front and/or middle of the lens to capture and create an image which is compressed in the width dimension. They horizontally compress different planes of the object space at different ratios - the plane of focus might be a 2 times squeeze but out-of-focus areas further away will be more compressed - so when the image is unsqueezed the bokeh is still a bit oval. 

 

Theoretically you could just use a normal zoom and crop to 2.40 but that wastes time changing viewfinder formats while shooting, creates extra work in post and subtly changes the grain or resolution, so usually rear anamorphic optics are used. Even the latest Angenieux anamorphic zooms use a rear anamorphic group like this.  


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#6 David Mullen ASC

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Posted 20 November 2015 - 11:57 AM

Most of an anamorphic lens is a normal spherical lens.  It has a few extra elements to squeeze the image laterally, and often those elements look like a vertical cylinder of glass, round on one side. So when this cylinder is viewed straight on, the shape of the element is square.  Adapted anamorphic lenses usually have a small cylindrical anamorphic elements in the back; anamorphics built from the ground up often have larger cylindrical elements near the front.

 

You can think of most anamorphic primes, and in fact the earliest CinemaScope lenses, as normal lenses with a large wide-angle adaptor in front, only the adaptor just expands the field of view horizontally.  So you can imagine that in general, an anamorphic lens is going to be larger than the spherical lens it was built from.  Rear-adapted anamorphics get around this problem but at some loss of speed and you don't get most of the classic anamorphic artifacts like stretched bokeh.


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#7 David Edward Keen

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Posted 27 November 2015 - 05:26 PM

Thanks. This is interesting. Gotta check into it
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#8 David Mullen ASC

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Posted 27 November 2015 - 05:58 PM

Though this scene in "JFK" doesn't have out of focus points of light to make the bokeh more obvious, you can see the effect of mixing front-anamorphic primes with rear-adapted anamorphic (a telephoto or zoom):

 

Look at the background and you can see some classic vertical oval bokeh of regular front-anamorphic prime lenses:

jfk1.jpg

 

But some of the tighter shots were made with what looks like a rear-adapted telephoto anamorphic (note the square-ish bokeh):

jfk2.jpg


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