Why is anamorphic bokeh oval?

[roccoforte]

Logically it doesn't make sense to me that your bokeh remains elongated even after you squeeze! Are all circular shapes in the background stretched too?

 

What am I missing here??

 

Thanks...

[Julian]

Because the anamorphic lens projects an oval image onto your taking lens.

If you cut out some shapes on a piece of paper and put it on front of your lens, the bokeh will take that shape: http://www.diyphotography.net/diy_create_your_own_bokeh

 

The anamorphic lens is like a piece of paper with an oval cut.

It won't affect real objects, just the out of focus highlights.

[richg101]

you'd expect the ovals to disappear after de-squeeze but from what i see, the lens is stretching the image more on the out of focus areas than the in focus areas.  in the same way normal bokeh is bigger the further you are away from the out of focus area.  de squeezing removes the artifacts from in focus areas but out of focus areas are distorted more.

[roccoforte]

Yes - In my mind the oval-shaped bokeh should only be visible when you watch the footage in its pre-squeezed state. (i.e. when people are tall, and soccer balls are egg-shaped) But it remains oval even after it's been squeezed down into its rectangular aspect ratio. 

 

Logically, shouldn't the bokeh also squeeze down back into a circle?

 

But yes, I see it now; that it isn't uniformly stretched. Very interesting.

 

Thanks.

[dahlfors]

As much as I understand about the physics of light & cameras, I think Julian is correct here. The way bokeh appears should not have anything to do with the squeezing of an anamorphic - it only has to do about another feature of an anamorphic: light from out of focus areas travelling through the ovally formed hole.

 

Here's a video from Kai at DigitalRev: http://www.youtube.com/watch?v=H5jjr_vKruU

He cuts out his own bokeh shapes from paper - even his own name is used as bokeh :)

[dahlfors]

Also, on this topic: The vid-atlantic cinemorph filter is basically just an oval for adding the oval bokeh and a glass that adds flares to your footage: http://vid-atlantic.com/cinemorphic.html

[JohnBarlow]

Seems to me that no one has come up with the correct answer so far.....

:P

[QuickHitRecord]

Something I've noticed: anamorphic bokeh has a tendency to be more oval at the edges of a frame and more true-circular near the center. Clearly, this is a result of the curvature of the glass, but what focal length / distance / diopter combination gets you consistent ovals across the frame?

[JohnBarlow]

Shot with a 2/90mm at 1 metre, aperture reflections are consistent across the FOV

 

 

 

 

Something I've noticed: anamorphic bokeh has a tendency to be more oval at the edges of a frame and more true-circular near the center. Clearly, this is a result of the curvature of the glass, but what focal length / distance / diopter combination gets you consistent ovals across the frame?

[QuickHitRecord]

Shot with a 2/90mm at 1 metre, aperture reflections are consistent across the FOV

 

 

 

Very nice. Which lens? Diopter?

[JohnBarlow]

Very nice. Which lens? Diopter?

 

its a SK Cinemascope from the 90's, dioptr is a +1 from China

 

[Jaybird1091]

Actually, the ovaline shape of anamorphic bokeh has NOTHING to do with the shape of the front element. If it did, OP is right--the ovaline shape would be reversed after the de-squeeze in post.

Instead, the shape is due to one of the weird properties of shooting anamorphic. When you shoot with, say, a 100mm 2X anamorphic lens, you get a crop factor of effectively 50mm....in the left-right axis. In the up-down axis, however, you're still shooting effectively 100 mm. Since 100mm gives bigger boken than 50mm, your bokeh will be bigger in the up-down axis than in the left-right axis. Hence the oval shape.

[Brian Caldwell]

Not quite.  You haven't accounted for the huge difference between front and rear anamorphics, both of which have different focal lengths in the horizontal and vertical directions.  I posted the explanation here back in June:  

 

[andyson]

I saw his Zeiss late 1960 primes, which version, mount, etc where these

 

คาสิโน 

 

 

[Awcine]

All of these answers are wrong...  anamorphic lenses are designed to squeeze 2x at the plane of focus, but the anamorphic element squeezes less than 2x in front of the focal plane and greater than 2x past the focal plane.  This is why you get stretching and squeezing when you rack focus, anamorphics are essentially controlled astigmatisms.  If you rack focus through a point source on an anamorphic lens, it will be a circle when it’s in focus, start to stretch vertically when it’s behind the plane of focus, and stretch horizontally when it’s in front of the plane of focus.  We’re just used to looking at out of focus backgrounds a lot more than out of focus foregrounds.

 

all anamorphics have it, front or rear.  I haven’t personally done extensive tests on the optical differences between the two, it’s possible rear anamorphics have less dramatic astigmatism (and I would venture to guess more extreme petzval field curvature?)

[Awcine]

Actually, I am incorrect on this, on older anamorphics the astigmatism is the most prominent feature and you get fatter out of focus points in front of the focal plane and very tall focus points behind, but I believe the correct answer is that the vertical focal length of a 2x anamorphic lens is twice that of the horizontal focal length and that the aperture reflects this if the anamorphic element is in front of the entrance pupil.  

 

Say you have a 50mm anamorphic at t2.8, the vertical axis of the aperture is 17.8mm, the horizontal is 8.9.  Subsequently the vertical axis will defocus to a greater extent than the horizontal axis.  So this then is why you get the vertical stretch and squash when you rack in and out of focus on front anamorphics and not rear (since the entrance pupil is what matters when you are talking about aperture transmission).  I will have to do more thinking on rear anamorphics, I was under the impression they still had this stretch, but perhaps that’s just the result of my previous answer, the natural astigmatism that comes with anamorphic photography.  I stand corrected on my previous answer, old anamorphics you do get very strong astigmatism which is often more noticeable than the difference stated above, but if you look at modern anamorphics like the master anamorphics that have largely eliminated the effects of astigmatism, you do see them stretch vertically in the foreground as well, it’s just usually not as far out of focus as the background.  

[thephoenix]

for sure is has to do with the shape of the front element

[Awcine]

to put it another way, a front anamorphic 100mm lens has a cone of light in image space that is vertically a 100mm image, and horizontally a 50mm image.  Since a 100mm lens has 4x less depth of field as a 50mm, it will defocus at 4x the rate of the horizontal axis, while the image will be desqueezed at a constant 2x, meaning the further you throw something out of focus, the taller it will get.

Conversely, a rear anamorphic 100mm lens has a cone of light in image space that is 100mm vertically and horizontally, and is then squeezed at a constant 2x, so you wouldn’t have the same asymmetric depth of field.

[Caleb Genheimer]

@Awcine I’ve always understood this, but yours is the most clear explanation I’ve ever come across. 

[thephoenix]