[Guest]

I'm testing a lens (the HD Pentax DA 40 Limited) and for the purpose have a Zeiss star test chart. On the sample below, I deliberately focused beyond the chart, and I noticed two donut-shaped areas in focus in the middle of the chart. What exactly is going on here?

 

Can it help explain the strange bokeh sometimes seen by the lens? (See Klaus' Bokeh section here: http://www.opticallimits.com/pentax/630-...28?start=1

 

Note on my photo: Contrast is deliberately bumped to exaggerate the effect. Taken from 4.8m away at 40mm (of course) and f/2.8. Chart is aligned dead center to the sensor, and is flat. The effect is *not* a wave in the chart!

 



 

Thanks for the help!

Sean

[popo]

Heard of constructive/destructive interference? The defocus characteristic with a possible bright centre, and bright outline means that along lines of contrast, you can get brighter and darker areas next to it. In the Siemens star, the lines are not parallel so the spacing distance is changing. At times, the spacing will be just right that it boosts contrast, which gives you the what looks like in focus areas. Other times, they cancel each other out and you get grey, like the region between the two contrast rings. I'd consider it a side effect of the defocus characteristic.

[Scythels]

Popo may be correct and this may be spurious resolution. 

 

What happens if you stop the lens down or change the focus slightly?  If it disappears -> spurious resolution/interference pattern.  If not -> probably an instance of total internal reflection inside the lens.

[Guest]

Many thanks popo and Scythels!

 

It does continue when you change focus slightly; I will report back if it happens at other apertures too.

[Scythels]

It's an optical characteristic of the lens if changing focus a bit doesn't alleviate it.  No idea what it is specifically in that case.

[Guest]

It's an optical characteristic of the lens if changing focus a bit doesn't alleviate it.  No idea what it is specifically in that case.

Changing focus a bit does not alleviate it - even stopping down a little still shows the effect.

 

So a mystery then?

[Guest]

Second the vote for constructive/destructive interference.

 

What happens if you move the star chart away from the centre of the lens?

[Scythels]

Interference would only be correct if small changes to focus/other blur factors changed the pattern. 

[BasilG]

Heard of constructive/destructive interference? The defocus characteristic with a possible bright centre, and bright outline means that along lines of contrast, you can get brighter and darker areas next to it. In the Siemens star, the lines are not parallel so the spacing distance is changing. At times, the spacing will be just right that it boosts contrast, which gives you the what looks like in focus areas. Other times, they cancel each other out and you get grey, like the region between the two contrast rings. I'd consider it a side effect of the defocus characteristic.

 

Actually, what you see here is the contrast transfer function. The grey ring between 1/3 and 1/2 way out along the radius of the siemens star is where the contrast transfer function crosses zero (no contrast transfer). After crossing zero, it causes contrast reversal (or phase inversion). You can clearly see that - if you follow a black line, and extrapolate it through the grey ring, it continues as white at the other side of the grey ring. This effect is caused be the defocus. If you go closer to focus, the grey ring/zero crossing will move and finally disappear when focus is achieved.

[Scythels]

Withdrawn