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How Do Anamorphic Lenses Work?


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#1 Derek Stettler

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Posted 20 March 2012 - 09:14 PM

I've been reading a bit about anamorphic lenses and how they work, but I'm curious about a few things:

1. Are anamorphic lenses essentially designed to have specific, controlled astigmatism, where one meridian of the lens has a different focal length than the other? What's the difference between a lens with astigmatism and an anamorphic lens?

2. Related to the first question, if anamorphic lenses have two different focal lengths, how is it that the squeezed image projected onto the film is completely in focus? The affect of having two different focal lengths should mean that only one meridian would be in focus at a time, right?

3. When using a motion picture camera using anamorphic lenses, does the viewfinder have to be changed every time a different lens is placed on the camera? If not, then how is it that one single viewfinder can correctly decompress the image projected from different anamorphic lenses?

4. I've noticed that the characteristic anamorphic flare is always blue. I assume this is due to the antireflection coatings, but lens flare in spherical lenses can have different colors depending on the particular coatings used on different lenses. So does that mean anamorphic lenses all use the same AR coatings? If not, then why is the flare always or almost always blue?

Thank you so much. I'm a student who has, as yet, never used a motion picture film camera or anamorphic lenses, but I'd like to someday and so I want to learn about them and how they work.
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#2 Rob Vogt

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Posted 20 March 2012 - 10:13 PM

Anamorphic lenses do not have 2 different focal lengths.
They have one (in the case of primes).

When people say anamorphic lenses have 2 focal lengths they mean to say, for example, in the case of a 70mm anamorphic lens the horizontal angle of view would be equivalent to a 35mm spherical lens's angle of view. The vertical angle of view would be equivalent to a 70mm spherical lens's. But the depth of field would be the same as the 70mm (which is why they refer to it as a 70mm Anamorphic).

To answer your other question it depends on the camera. Some cameras and directors finders have optics built in that allow you to flip back and forth between academy and scope. Most of the time in a production, if you are shooting both anamorphic and spherical you would be shooting super35 so you have to change the ground glass and adjust the glow mask if need be to switch back and forth. Or you could have a custom ground glass made with multiple sets of framelines.

As for the blue flare Id guess it's because most of the time you are seeing Panavision anamorphics can account for the similar lens coatings, if that even is why it makes it flare blue. It could just be that that frequency of light has a higher rate of refraction from the internal mirrors. I've used some Russian anamorphics a while back and didn't have that blue flare effect, although we didn't try to flare the lenses that I recall.

I hope this helps clarify things.
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#3 Leo Anthony Vale

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Posted 22 March 2012 - 01:20 AM

amended 03-24-12

I've been reading a bit about anamorphic lenses and how they work, but I'm curious about a few things:

1. Are anamorphic lenses essentially designed to have specific, controlled astigmatism, where one meridian of the lens has a different focal length than the other? What's the difference between a lens with astigmatism and an anamorphic lens?


Astigmatism is a lens aberation, thus something to be corrected.
But cylinder lenses are astigmatic.

http://en.wikipedia....iki/Astigmatism

2. Related to the first question, if anamorphic lenses have two different focal lengths, how is it that the squeezed image projected onto the film is completely in focus? The affect of having two different focal lengths should mean that only one meridian would be in focus at a time, right?

An anamorphic system has two focal lengths, else no squeeze. The system is basicaly two parts, the anamorphic section and the spherical backing lens. Both sections have to be focused.
When both are set at infinity, there is no astigmatism in the image. If the backing lens is focused without focusing the anamorph, astigmatism shows up in the image. When focused at inf, the anamorph is afocal'

In a standard anamorphic sysym, The spherical lens focuses in the vertical axis and the anamorph

There are three main ways of focusing the system. The earliest systems focused by moving the neg and pos cylinders closer and focusing the spherical normaly. As the neg and pos elements get closer together the squeeze ratio lessens, resulting in the infamous anamorphic mumps .
The first systems used anamorphic attachment mounted infront of the main lens. franscope ads  --001.jpg
Both lenses had to be focused seperately.
Makes for tricky follow focus.
Later later, single units with the focus for both sections interlocked.
B&L C'Scopes were most probably the first.
b&l  35mm CinemaScope.jpg
B&L CinemaScope

The second type is Panavision's 'variable astigmatiser'. The anamorph remains set at inf, which eliminates the mumps; while the variable astigmatiser adjusts the astigmatism in the image.


Here's the patent, figure it out:

http://www.google.co...ved=0CD0Q6AEwAw

The next is used by most nonPanavision anamorphots today. The cylinder and spherical sections are permanently set at infinity.
A spherical colliminating section is added to the front of the system. Light rays from less than inf go in an come out parallel, i.e. 'focused at inf.'. In effect, a variable close up dioptre!



3. When using a motion picture camera using anamorphic lenses, does the viewfinder have to be changed every time a different lens is placed on the camera? If not, then how is it that one single viewfinder can correctly decompress the image projected from different anamorphic lenses?

Normally one would be using lenses that all have the same squeeze, so it's a moot point. Also one doesn't have to use a squeeze element in the viewfinder. One can get used to viewing a squeezed image and it'sharper.

4. I've noticed that the characteristic anamorphic flare is always blue. I assume this is due to the antireflection coatings, but lens flare in spherical lenses can have different colors depending on the particular coatings used on different lenses. So does that mean anamorphic lenses all use the same AR coatings? If not, then why is the flare always or almost always blue?


Panavision and Bausch&Lomb CinemaScope have blue horizontal flares. TohoScope usually had red flares, which were smaller than Panavision and C'Scope flares. HammerScope also had smaller and dimmer flares. I,ve seen red flares on other foreign
sytems.

Perhaps this post has left you even more confused. If so... I,m tired myself.


Camp on Blood Island.jpg mysterians_4_WA02746_L.jpg

in HAMMERSCOPE and in TOHOSCOPE,

---LV


APPENDIX
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