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01-09-2010, 06:04 AM   #1
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What's wrong with my A 28/2.8?

Last weeks P52 challenge made me aware of a strange effect in my A 28/2.8. Instead of a regular sun star, this lens makes uneven stars with one or two exxagerated beams, among some small ones. Anybody any idea what is causing this???

And yes, I know what is causing those terrible specs on the pics, sorry for those!!

Some examples from yesterday









01-09-2010, 08:16 AM   #2
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You probably have a bit of wear on the back side of the aperture blades.
01-09-2010, 08:23 AM   #3
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it looks like the apperature does not close in a circle but in an ellipse.

If you look at the star pattern you get a line for each point in the aperature "circle" due to the slight irregularities in the theoretical circle as two blades meet.

If all blades meet equally the lines are all the same strength, because your lens is not having all blades meet in a uniform way, some of the lines of the star are more significant than others

if you take the lens off and photograph the aperature opening at each F stop you should be able to see this
01-09-2010, 08:23 AM   #4
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QuoteOriginally posted by Wheatfield Quote
You probably have a bit of wear on the back side of the aperture blades.
I second the opinion, some loose/wear on the aperture blades.

01-10-2010, 03:54 AM   #5
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Hi Rense,

The orientation of the main spike changes, so I think you have a rotating (pola) filter in front of the lens which causes this.

George
01-10-2010, 06:16 AM   #6
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Original Poster
Thank you all! George, there is no polarization filter on the lens, removed it for these pics... But I noticed what you have noticed indeed...

Checking the aperture blades and I can't see any irregularities in closing . But I noticed some (ahum) fat and dust on the back side of the lens. Removed it, but I can't test how the lens is working right now, because there is no sun (and I've given away my camera to a friend, who's is considering buying a Pentax). But I guess that is what caused the irregular star shapes....
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