What is this optical phenomenon called (pix)?

[Rodluvan]

I've seen in many of my photos (especially those with shallow DOF) that the circles of confusion in them-self stretches as to form a circular shape with the centre in the middle of the frame as can be seen here:

Not very apparent here, but a little:

And here, especially at the bottom of the frame (could be fooled by vignetting though?):

I've only seen this in film from what I can recall. Maybe because it's not that outspoken on a cropped sensor.
Any suggestions?

Thank you/
Daniel

 

[Freakscene]

That's coma, where the outer edges of the lens focus light at different points to light closer to the centre. It's correctly called abaxial spherical aberration.

Marty

 

[maddoc]

"Swirly bokeh" as in the first frame ? Can be seen often when using older lenses, especially three element lenses like the Tessar design and having a inhomogeneous background (trees, leaves). Have a look in this thread here at RFF.

 

[Rodluvan]

Excellent, thank you!

Btw, the first two are captured with Summicron 50mm and the third with 50mm/1.2 Nikkor.

 

[btgc]

btw just a minute later I realized in first picture at medium distance on bush is so nice effect. Maybe even nicer than light donuts in further...upper part. They make me talking in terms closer/further instead of upper/lower part of image.

 

[pvdhaar]

That's coma, where the outer edges of the lens focus light at different points to light closer to the centre. It's correctly called abaxial spherical aberration.

Marty

And here was I thinking the stretching was due to the aperture forming an ever thinner ellipse as seen further and further away from the lens axis..

 

[cjm]

Rodluvan, is this the phenomena you are describing? I've always wondered what caused that. 🙂

This was taken with a Leica Summar at f2 or f2.2

 

[oscroft]

especially three element lenses like the Tessar

Tessars have four elements (though in three groups)

 

[kermaier]

I've also seen that "swirly bokeh" on digital using a Canon 50/1.2 LTM wide open, with the background elements not too far away (5-10 meters).

 

[Fuchs]

Tessars have four elements (though in three groups)

Tessars are modified triplets (as also Leitz Elmars are), in which one of the elements is composed by two lenses.

 

[Rodluvan]

Rodluvan, is this the phenomena you are describing? I've always wondered what caused that. 🙂

[.cut.]

This was taken with a Leica Summar at f2 or f2.2

Yes, that's it. Now we know! I really like it.

 

[gdi]

And here was I thinking the stretching was due to the aperture forming an ever thinner ellipse as seen further and further away from the lens axis..

I think that is correct, this would be coma - creating a "tail" on round highlights. The highlights are become more V shaped with distance from the center of the frame...

 

[oscroft]

Tessars are modified triplets (as also Leitz Elmars are), in which one of the elements is composed by two lenses.

It's based on a triplet design, yes, but the modification (which consists of replacing one element with a two-element group) then makes it a four element lens in a 3-group configuration - "element" referring to each individual piece of glass, and "group" referring to each cemented set of pieces of glass - "lens" refers to the entire assembly. You don't have an "element" made of two "lenses", you have a group made of two elements, and thus a lens made of three groups (and four elements).

 

[maddoc]

Tessars have four elements (though in three groups)

I stand corrected. Four elements in three groups was what I meant.

 

[charjohncarter]

I've always wondered about that, but not being a lens nut never bothered to research it. Thanks for asking and thanks for answering.