Dirty negatives

[zehner21]

I suppose that this topic has been debated already, but...
Today I printed some photos whose negatives have been developed by a lab.
In general, I'm satisfied, but there is something that bugs me. All photos whit large portions of sky have these damned white spots.
I have examined the negatives but I haven't noticed any dust on them, so what are they?
BTW, printing from Portra is pleasantly easy.

Thanks

 

[Les Sarile]

Are you optically printing from the negatives and how large are these prints?

 

[zehner21]

Are you optically printing from the negatives and how large are these prints?

1) Yes, I am

2) 13cmx18cm

 

[Les Sarile]

If you cannot see these spots at the enlarger but after the photo is made then my guess is these are being introduced on the photo development process.

 

[jeffreyg]

If the white spots on the prints are from the negatives the the negatives would have corresponding opaque spots. Have you checked them with a loupe on a light box? Are you using variable contrast filters below the negatives? At least the spots are white and you can spot them out.

http://www.jeffreyglasser.com/