What causes swirly background rendering of some lenses?

[baachitraka]

I am wondering what specific optical flaw that leads to swirly background rendering when used in wide-open apertures?

 

[Ian Grant]

It's part of the design, I assume you're taliking about the swirliness of Petzval lenses which is due to uncorrected field curvature.

Ian

 

[frank]

Caused by coma (and a bit of spherical aberration) a type of optical aberration in a lens design, I believe.

 

[baachitraka]

Actually I have a Helios 44-2 which at wide-open apertures produces nice swirliness which I thought was because of optical flaw introduced by the people who copied it from Zeiss Biotar.

But then Biotar too exhibit swirliness at wide-open which I am not aware till now. So, I try find what optical flaw(s) they have introduced during copying from original lens.

 

[Nodda Duma]

Caused by coma (and a bit of spherical aberration) a type of optical aberration in a lens design, I believe.

Not quite.

Spherical aberration is what's called an on-axis aberration similar to defocus and doesn't increase with field angle. It therefore doesn't contribute necessarily to the swirlyness, although its effect is visible in bokeh.

Coma elongates objects away from the center of the image and so doesn't create a swirly effect...it creates something more like a "warp drive" effect. Imagine comets flying in towards the center of the image.

Uncorrected sagittal field curvature can elongate off-axis objects radially around the center of the image, and when grossly uncorrected is the prime contributor to the swirly effect. The surface of best focus is called the Petzval surface (not a coincidence), and is usually not flat. When astigmatism is not corrected, the tangential and sagittal image surfaces deviate away from the Petzval surface at different rates. The specifics depend on the design. Petzval calculated how to flatten the Petzval surface with the side effect being to increase the speed of the lens. Faster f/#'s increases the impact of higher order aberrations which are used to balance third order aberrations and in this case flatten the field curvature within the Petzval lens' corrected field angles.

Out past a certain field angle (dictated by the design), the Petzval surface rapidly goes non-flat as the balancing act breaks down. This means increased astigmatism and resultant sagittal or tangential defocus. In the Petzval portrait lens, sagittal defocus increases more rapidly than tangential, directly causing the swirly effect. Since astigmatism is coupled to focus, defocused background objects enhance the effect.

 

[EdSawyer]

as a side note, the 'petzval' type swirl you see in many recent users of old vintage glass on large format comes from using the lenses on a larger format than "originally designed". Given that, the swirls on the edges were not visible in the "old days" when the lenses were used where the coverage didn't exhibit those aberations as much. Nowadays, it's all the rage though...

 

[Nodda Duma]

Yes, EdSawyer, thanks for clarifying.

 

[MattKing]

Thank you for making absolutely no reference to the number of blades in the aperture!

 

[BrianShaw]

And thank you for making absolutely no reference to bokeh!

 

[Nodda Duma]

I'll leave those references to those who consider lens design to be magical

 

[Old-N-Feeble]

Tonight, I ate a TV dinner. It sucked.

 

[Huss]

Tonight, I ate a TV dinner. It sucked.

Why did u buy one? was it for the way it makes the back ground swirl after u were done eating it?

 

[Old-N-Feeble]

Why did u buy one? was it for the way it makes the back ground swirl after u were done eating it?

Yes, I prefer my distorted vision. In fact, sometimes I wear damaged eyeglasses just for the effect.

 

[Hatchetman]

Thank you for making absolutely no reference to the number of blades in the aperture!

Here's a question for you: why does the Summitar with 6 blades have a swirly background but the one with 10 blades does not?

 

[frank]

Here's another question: why is the swirliness the strongest at wide open aperture where the shape of the aperture created by the blades does not factor in?

 

[Dan Fromm]

Here's another question: why is the swirliness the strongest at wide open aperture where the shape of the aperture created by the blades does not factor in?

Because the aberrations that cause the swirls are better controlled at smaller apertures. Stopping down, um, tames them.

 

[frank]

My point was that shape of aperture does not contibute to swirliness.

 

[Hatchetman]

My point was that shape of aperture does not contibute to swirliness.

so what is causing it? different lens design? I thought they were identical.

 

[frank]

so what is causing it? different lens design? I thought they were identical.

Don't know, but if the swirlies are strongest wide open then it isn't the the aperture shape. Also, there are lots of other lenses with hexagonal apertures that don't exhibit swirls. I'm just trying to apply logic. I'm not a lens design expert.

 

[Hatchetman]

I don't know, in the f2.8 to f4 range it could be the cause. I just assumed it was the aperture shape. Not that I really care, just curious.

 

[Nodda Duma]

Sagittal field curvature varies with pupil diameter, so a wider aperture setting will increase swirlyness.

It is also a function of other design aspects such as stop position, Ray height and angles, etc. So yes, it does vary from design to design. As we all know, it is pronounced in Petzval designs outside of the corrected field angles. This isn't necessarily the case for, say, a double gauss design.

Regards,
Jason