Here are two lenses I got off Ebay – they look pretty nice on images. The Canon 85mm 1.8 does have marks visible to the naked eye on the front element but they are very small. The Helios looked perfect on front and back but on an internal glass there is some dust particles.
Canon 85mm 1.8:
Helios 44-2 58mm F2:
[image examples from two lenses I got from ebay]
Here are two shots with them, with wide apertures:
Confirms that “dust inside” or “scratches in front” doesn’t affect IQ.
But then I noticed spots when stopped down:
I used Helios when taking a macro of a brie and I needed to stop with to F16 to get just a little bit DOF – the black speckles are dust particles:
But this turned out to be on the sensor, because the spots were the exact some place as the 85mm stopped down. I cleaned the sensor and it is fine now! Yeah :]