Bokeh with different lenses and optical design

[Michael Notar]

other than the obvious of rendering out of focus subjects/points of light in the same shape of a the aperture, in what other ways does a lens with 6 blade aperture (RPP) vs 14 (CP2) render a different image. when would you choose the lower blade count over the higher..?

 

[Mitch Gross]

The iris is only one aspect of Bokeh, and in fact I would consider it the most minor one. The shape of the image itself, not just the out of focus highlights, is the rendering of the Bokeh. In my class "Secrets of Optics" I show three different designs of a 50mm lens, all with their irises set wide open so tha they do not come into play, and each lens showing very different Bokeh.

 

[Michael Lindsay]

... In my class "Secrets of Optics" I show three different designs of a 50mm lens, all with their irises set wide open so tha they do not come into play, and each lens showing very different Bokeh.

sounds interesting.. online?

Michael

 

[Patrick Tresch]

sounds interesting.. online?

+1.

 

[PatrickFaith]

The iris is only one aspect of Bokeh, and in fact I would consider it the most minor one. The shape of the image itself, not just the out of focus highlights, is the rendering of the Bokeh. In my class "Secrets of Optics" I show three different designs of a 50mm lens, all with their irises set wide open so tha they do not come into play, and each lens showing very different Bokeh.

Wish i can make it ... but i doubt it on that day ( http://training.abelcine.com/event/how-to-choose-your-lens-new-york-2/ ). Mitch ... what's the theory for the class for us California/SF guys? You going to have part of it on Abelcine site ?

From my experience ... which is minimal with RPP ... I like the shape/feel of the bokeh of my rpp 18 and 35 mm at t1.8 a lot more then t5.6. That is the major reason why i primarily "change" my T stop with ND filters rather then with the iris, but it seems like the bokeh change is more then i'd expect by changing the iris by 4 stops (i.e. i'm assuming things are "bouncing" around or something more with the lens wide open - it's a ton of exposed glass with the rpp wide open, aside from the pure optical dof change)?

 

[Mitch Gross]

We will be offering these classes in other locations in time, and we will eventually have them available online as well.

There are various reasons why Bokeh renders as it does and changes in various ways as one stops down. There is a whole lot of geometry and other math going on in optical design. You would be amazed how different designs of lenses will render an image even when at the same focal length and stop. Start changing the stop and a series of other differences come into play. One aspect of the design difference is where the iris is physically located within the lens design. Highly telecentric lenses often have the iris up front in the housing, which can have a very different effect from a design with the iris further back.