It is easy to reproduce in almost any situation when you are using a lens with a large entrance pupil.  The entrance pupil is the focal length divided by the f-number.  So your 300mm f/4 has an entrance pupil of 300/4 = 75mm diameter (about 3").I don't have that lens, but here are some very quick test shots with my 75mm f/1.8 lens (entrance pupil of 42mm).No obstructions in front of the lensPen held vertically in front of lensPen held horizontally in front of lensPen held diagonally in front of lensThe pen was dark in colour and approximately 12mm diameter.  With a 42mm entrance pupil, the lens easily sees around the pen, but the shape of the bokeh is changed.Shaped bokehis a well-known phenomenon.  It is sometimes done deliberately, but often happens accidentally with nature photographers using long lenses when twigs get in the way.

Tom Axford wrote:It is easy to reproduce in almost any situation when you are using a lens with a large entrance pupil. The entrance pupil is the focal length divided by the f-number. So your 300mm f/4 has an entrance pupil of 300/4 = 75mm diameter (about 3").I don't have that lens, but here are some very quick test shots with my 75mm f/1.8 lens (entrance pupil of 42mm).No obstructions in front of the lensPen held vertically in front of lensPen held horizontally in front of lensPen held diagonally in front of lensThe pen was dark in colour and approximately 12mm diameter. With a 42mm entrance pupil, the lens easily sees around the pen, but the shape of the bokeh is changed.Shaped bokehis a well-known phenomenon. It is sometimes done deliberately, but often happens accidentally with nature photographers using long lenses when twigs get in the way.Wow Tom. Fantastic examples. Explains everything so clearly.

A very eloquent way to explain the issue.....Thanks, Tom......... now I wonder what was in front of Stevevp's lens ?

Tom Axford wrote:It is easy to reproduce in almost any situation when you are using a lens with a large entrance pupil. The entrance pupil is the focal length divided by the f-number. So your 300mm f/4 has an entrance pupil of 300/4 = 75mm diameter (about 3").I don't have that lens, but here are some very quick test shots with my 75mm f/1.8 lens (entrance pupil of 42mm).No obstructions in front of the lensPen held vertically in front of lensPen held horizontally in front of lensPen held diagonally in front of lensThe pen was dark in colour and approximately 12mm diameter. With a 42mm entrance pupil, the lens easily sees around the pen, but the shape of the bokeh is changed.Shaped bokehis a well-known phenomenon. It is sometimes done deliberately, but often happens accidentally with nature photographers using long lenses when twigs get in the way.Many thanks Tom for taking the time to produce these examples. There's no arguing with that!

J4Hug wrote:A very eloquent way to explain the issue.....Thanks, Tom......... now I wonder what was in front of Stevevp's lens ?Unfortunately I can't go back to Panama and check but I'm guessing a bit of tree!

What a brilliant demonstration.Stephen

stevevp wrote:Grateful any thoughts on what is causing the vertical wavy bokeh in the top right-hand corner of this photo (and others in a batch) please. Electonic shutter?The lens.

embe71 wrote:stevevp wrote:Grateful any thoughts on what is causing the vertical wavy bokeh in the top right-hand corner of this photo (and others in a batch) please. Electonic shutter?The lens.