weird background style?

[purpleroan]

Hey everyone! The past couple months I have been primarily shooting with Canon's 55-250mm lens. In some of my pictures, the background gets blurry, but then seems to get almost spherical, like it curves up around the subject? I'm curious what I'm doing to cause this, as it isn't all the time...it isn't unappealing either, just strange. I can post a couple examples below...thank you for any explanations!! (I'm not too fond of the example pictures, but I do think they show the effect I'm talking about well! There is one picture where it is very pronounced, but I do not have the owner's permission to post the subject and so shall post only a portion of the shot.)

 

[Derrel]

Often referred to as 'swirly bokeh" or "circular bokeh" or "circle bokeh"...

What lens type causes the circular blur bokeh Archive - Rangefinderforum.com

One optical expert there says its caused by spherical aberration that is ALSO accompanied by another optical aberration, called coma. There are people who actually like this swirly bokeh, or blur, and who go to great lengths to seek out the many old communist-era manual focus lenses, some of which have exceptionally strong swirly bokeh rendering characteristics.

There have been some lenses made in the distant past that had VERY pronounced circular bokeh, and it can have its appeal! it's part of that imperfect, "lensy" look that some people are now seeking out;this optical defect disappeared for the most part when computer designed lenses came on the scene.

I'm not sure if it would be under-corrected or over-corrected spherical aberration, but the geeky fellow there insists its spherical aberration that MUST also be accompanied by the presence of a pretty fair amount of coma. Coma makes small points of light render not as dots or points, but as sort of elongated orbs, sometimes looking like footballs in profile, and I've even seen it look almost like the profile of seagulls or other birds in flight.(seriously!)

A LOT of normal, regular high-speed lenses like 50/1.4 lenses, and even f/2.8 professional grade zoom lenses have strong coma problems at wide-open, and it typically gets better one stop closed down, and by two stops closed down it's better, and then at three stops down, the coma is usually minor enough that points of light render as "dots"; this is a huuuuuuuuge deal for star photography, and other types of night-time shooting, where there are street lights or Christmas lights, or whatnot; correcting a lens for coma at its widest aperture costs a LOT of money, more than many people are willing to pay on 'regular' lenses, but for lenses optimized for ultra-dark environments, coma correction becomes one of those Rolls-Royce amenities.

 

[TCampbell]

^^ Yep!

Search for "Lomography Petzval" at images.google.com and you'll find many examples of this.

I don't think I like enough to buy the lens (I think of it as an "effect" that looks good if used sparingly, but I'd probably quickly grow tired of it if used excessively) but I had considered renting the lens for a weekend to have some fun with it.

 

[Derrel]

I used the term communist-era lenses mainly as sort of a nod to the Helios lenses produced after WW II and into the 1960's under totalitarian government era rule... a Google search on this string will lead to some lenses that various users consider possessing "swirly bokeh", at prices far,far,far below the new Petzval lens...man, that thing is priced high!

manual focus forum swirly bokeh lenses - Google Search