Asymmetric bokeh

[gaius1]

What causes asymmetric bokeh of this kind? The lens is the Nikkor 80-

200/4.<div></div>

[gaius1]

Here's a closeup. Thanks!<div></div>

[Ed_Ingold]

You are seeing an image of the aperture circle in OOF highlights, which is distorted at the edge of the image. You would see this by looking through the lens at different angles.

[Robert_Lai]

Hi Guy,<p>This is optical vignetting due to the lens barrel. You'll see it at the edges of the frame, especially if you shoot wide open. The reason is that the light beams from the subject that are close to the optical axis have the entire aperture to pass through. However, the rays coming from the edges basically have only a smaller sliver of aperture to go through, as a lot of them are cut off by the lens barrel. Hence, the "cat's eyes" effect at the edges. This is also why the corners of images with the lens shot wide open tend to vignette, even more than what you would expect from cos exp4 light falloff. The way to correct for this, and to even up the corner exposures is to stop down the lens.<p>Paul van Walree has a great explaination for this in his wonderfully understandable Optics pages. Check this <a href="http://www.vanwalree.com/optics/vignetting.html" >link from Paul van Walree's Vignetting page.</a> <p>Another lens that does this wide open is the fabled 105mm f/2.5 Nikkor. Here's mine wide open, using my portrait distance bokeh tester (focus is 1.5m from the film, on the bird statue's eyes). The bokeh is provided by bushes in the background.<div></div>

[gaius1]

Thanks everyone!

[lex_jenkins]

That page is very handy for cogently explaining the basics. However I disagree with his use of the term "vignetting" for both true vignetting (what he calls "mechanical vignetting"), which is simply physically blocking the light; and what he calls "optical vignetting".

 

This seems to unnecessarily complicate matters. What he calls "optical vignetting" is virtually always referred to elsewhere as light fall-off. His explanation for the phenomenon is excellent. I only disagree with the terminology.

 

Vignetting is better used to describe interference with the light path due to an ill-fitting lens hood or other devices, such as the romantic heart-shaped vignetting cut-outs once beloved of wedding photographers.

 

But I nitpick...

[jbq]

Yes, it's vignetting. And I agree with Lex, I prefer to keep the word "vignetting" for cases where the light path is mechanically obstructed (partially or entirely).

 

Vignetting is one of the causes of falloff, just like cos^4 "optical vignetting", or microlenses on digital sensors.

[hugh_crawford1]

"prefer to keep the word "vignetting" for cases where the light path is mechanically obstructed (partially or entirely)."

 

But it _IS_ mechanically obstructed, it is just that the obstruction is inside the lens, probably because the front elements aren't big enough for the aperture.

[alex_lofquist]

This are typical off-axis images of the aperture diaphragm. The only way to minimize it is to either stop down a couple more stops or avoid the highlights. You can see this effect if you place a bright point source of light near the viewing eyepiece and observe the aperture from the front of the lens at various angles.

[lex_jenkins]

Nope, Hugh, light fall-off is an optical phenomenon, not a "mechanical" thing.

 

Think about it: If something physical inside the lens could cause true vignetting, it would be the diaphragm. As it is stopped down, shouldn't that make vignetting worse? But it doesn't. In fact, stopping down *decreases* light fall-off.

[gaius1]

I don't mind the effect particularly, was just curious as to how it happened :-)