Spots on Sensor ... f/ stop dependent? .... or????

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GeorgianBay1939
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Spots on Sensor ... f/ stop dependent? .... or????
6 months ago

I followed up on Bobby J's observation here:

http://www.dpreview.com/forums/post/53326749

All of those were shot with the 14-140 mm Mk II on the GX7. The image he noticed was shot at 140mm.

So today:

I shot a f/ range of defocused frames of a snowbank (what else??) at 140 mm.

All were identically processed in LR to bring out the spots.

Results from three:

f/22 lots of dots, notice the largest one at centre, bottom third.

f/11 All dots have enlarged!

f/6.3 dots almost disappear.

I wasn't smart enough to try different focal lengths other than the 140mm.

Obviously, since the character of the spots changes with changes in f/, the problem is in the lens. Right?

After realizing the above I removed the lens, cleaned the objective and rear lens, shone a light through the lens from either end and observed the results:

Lens fully extended (140mm):

Little bright spots were visible within the lens, one quite large. Visible from either end. They looked like miniature spheres reflecting the light.

Lens collapsed (14mm):

The little bright spots were barely visible within the lens. One (the largest one) was visible.

Conclusions:

The effect is worst (and serious) when fully stopped down (f/22) and when fully extended (FL 140mm).

The effect is noticeable at f/11 when fully extended.

The effect is barely noticeable when the lens is opened up when fully extended.

The effect seems to decrease as the FL is decreased (judging by visibility of the bright spots when looking through the lens) .

Questions for folks who are more knowledgeable than I am about these matters:

1  What could those "spheres" be?  Oil?

2  What can be done about this?

3  Is the effects of these "spheres" as a function of f/ and FL typical?

4  When one buys a new lens is it wise to inspect it by shining a light through it?

5  Has anyone else observed these little "spheres" by shining a light through a lens?

6  Has anyone seen the above effect in the 14-140 Mk II?

Although I will (continue to) use the lens normally stopped down to less than f/11 I am not thrilled if I have a "defective" lens and would appreciate any advice from you experienced photogs.

Many thanks.

Tom

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