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How exactly do anamorphic elements themselves work? Is there a difference between front and rear?


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#1 cole t parzenn

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Posted 24 March 2016 - 02:21 AM

I've heard of a rear anamorphic adapter (Duclos) but only one - anyone know if it was single focus?


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#2 Dom Jaeger

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Posted 24 March 2016 - 08:59 AM

There have been rear anamorphic adapters for decades, most zoom shots in anamorphic movies have been done with a rear adapter on a spherical zoom. They simply take the normal spherical image formed out the back of a lens, stretch it vertically and mask the top and bottom. Basically like a doubler in only the vertical axis, hence the focal length of the lens is doubled to match the equivalent anamorphic focal length. There is no focussing of the adapter required, beyond the initial calibration. Once unsqueezed, the picture returns to a normal spherical image.

Front anamorphic lenses, with cylindrical elements in front of the stop, are completely different, and compress a wider field of view along the horizontal axis during formation of the image. They have effectively two focal lengths, one horizontal and one vertical, although the longer vertical focal length is the one the lens is referred by. Due to the different depths of field in the vertical and horizontal planes and an elliptical entrance pupil, after unsqueezing the image retains some compression in the out of focus areas.
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#3 cole t parzenn

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Posted 24 March 2016 - 10:43 AM

Thanks. Why do front anamorphic elements have to be focused along with the rest of the lens? What image would a front anamorphic element alone form?


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#4 Sebastien Scandiuzzi

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Posted 24 March 2016 - 04:01 PM

Thanks. Why do front anamorphic elements have to be focused along with the rest of the lens? What image would a front anamorphic element alone form?

 

For true anamorphic lenses, in the way we are referring to them (since anything that distorts the image and then needs another element or an acute angle to see the image correctly is by definition anamorphic), you don't need to re focus the front element. I think your thinking of an anamorphic adapter for spherical lenses which do need separate focusing for the adapter and the lens- quite annoying. 

The image a front element would form is a little difficult to answer but in essence it would form a distorted view for whatever is receiving the image. 


Edited by Sebastien Scandiuzzi, 24 March 2016 - 04:04 PM.

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#5 cole t parzenn

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Posted 24 March 2016 - 05:51 PM

I had read about the focus issue in an old thread on early anamorphics. In any case, why do anamorphic adapters need to focus?


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#6 Sebastien Scandiuzzi

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Posted 24 March 2016 - 08:10 PM

I could be wrong, its been a while. I think the lenses your referring to weren't true purpose built anamorphic lenses. From what I remember older anamorphic lenses were rehoused spherical projector lenses with anamorphic adapters and since the screen didn't move a double focusing lens wasn't a big deal, you set both of them once and walk away from the projector. But since film lenses need to be able to track a moving subject it made the cheaper rehoused projector lenses much harder to use. Which partly answers your second question, they wouldn't need constant adjustment if nobody moved which of course isn't the case. But the reason why you need double focusing when using an adapter is because the 'take' lens (spherical in this instance) was engineered with only the internal lens elements in mind so when you add an external distorted element, it changes where the light rays converge (focal point) throwing focus off. 

 

Hope that helps. Be interesting to hear other peoples thoughts since I'm not 100% on the history of double focusing lenses. 


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#7 cole t parzenn

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Posted 24 March 2016 - 11:12 PM

That does help. David is probably 110% on the history of anamorphic lenses but he seems to be busy, right now.


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#8 Sebastien Scandiuzzi

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Posted 25 March 2016 - 12:03 AM

I know, right? In other forums, I'm always saying "I'm sure David Mullen will have more to say about this..."! Wish he had his own forum, that would be great. 


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#9 David Mullen ASC

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Posted 25 March 2016 - 05:22 AM

A lot of your questions would be answered if you read John Hora's article on anamorphic lenses in the ASC Manual... he talks about the early CinemaScope lens and how the anamorphic elements had to be focused separately from the spherical elements, similar to what happens today with attached front-anamorphic adaptors, which essentially was what the original Hypergonar anamorphic was built by Dr. Cretien in the late 1920's, brought back to shoot the first CinemaScope movie, "The Robe" (watch it on blu-ray on you'll see some odd focus problems at times.)

 

Getting it to all focus at the same time involves internal cams, etc.  From the article:

 

The original CinemaScope system was based on designs of Henri Chretien and consisted of a conventional prime lens that formed the image, and a supplementary afocal wide-angle attachment on the front. This attachment was essentially a reversed Galileon telescope, similar to viewing through a set of “opera glasses” in reverse. A large front concave negative element is seen through a positive convex element. This is essentially a supplementary wide-angle adapter. The anamorphic effect is achieved by grinding these lenses as sections of a cylinder rather than as sections of a sphere, as is the case with conventional optics. The power of this attachment is zero in the vertical axis and 1⁄2 in the horizontal, axis resulting in a “squeeze” factor of 2. Placed in front of a 50mm prime, for example, the lens system would behave as a 50mm in the vertical plane and as a 25mm in the horizontal plane. Thus, the lens behaves as if it were two different focal lengths simultaneously.
The anamorphic attachment had to be adjusted to bring the horizontal field into focus at the same plane as the vertical field which had been brought into focus by the spherical prime lens. With the introduction of integrated designs, these two focus adjustments are automatically done by precision mechanical coupling within the lens assembly. For example, when the focus is changed to a closer object, the prime lens which forms the image must be moved forward to obtain focus. Additionally, the anamorphoser’s position and spacing must be simultaneously adjusted to bring the horizontal rays into focus at the same plane as the vertical.

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#10 cole t parzenn

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Posted 31 March 2016 - 11:10 PM

A lot of your questions would be answered if you read John Hora's article on anamorphic lenses in the ASC Manual...

 

 

I'll look that up - thanks!


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