'Dirt' on the side

I sometimes come across such a phenomenon when the circles of confusion in the pictures have strange artifacts - dots, specks, lines, grooves, concentric rings, etc. Sometimes this mud in the bokeh is very annoying. But, in fact, why is it happening? In this article I will give an example and describe my findings.

Dirt on the bokeh

For the experiment, I used the lens Industar-50-2 3,5 / 50that I’m not sorry to torment. The shooting took place using a camera Nikon D80, I shot the Christmas garland in a defocused state (the lens was focused on MDF).

The first picture already has a characteristic canvas in the form of a strip and small spots on the circles of blur. I want to note that dirt on the side has nothing to do with sensor cleanliness. By the way, my camera Nikon D80 recently was in the service of cleaning the matrix. If the dirt were on the sensor, then it would appear not only in circles of blur, but also in any other parts of the pictures and always in the same place. In the case of mud on the side, one can observe the repeatability of the mud pattern in different parts of the frame.

Canvas. Best seen on the green circle on the right. Canvas is repeated in all circles of confusion.

The picture below was taken after carefully wiping the front and rear lenses of the lens with a special cloth with microfiber to care for the optics. The circles in the blur zone became almost completely clean.

After wiping the lenses

To see how real dirt affects the lenses, I sprayed Industar-50-2 3,5 / 50 rear lens with monitor cleaning fluid:

Monitor back lens splashed with monitor cleaner

After that, the nature of the dirt on the bokeh has changed:

With splattered back lens

Please note that when the lens is very dirty, a repeating pattern of dirt is clearly visible on the circles of blur. And another shot with stronger splashes:

Splashing bokeh

For the sake of interest, I completely filled the back lens with cleaning fluid, it turned out something very funny:

With fully flooded rear lens

On the rear lens of Industar-50-2 3,5 / 50, I glued the letter 'a', imitating a large speck of dust :).

The letter 'a' from the kit for children is glued to the rear lens of the lens

Bokeh has become like this:

Bokeh with a letter on the back of the lens

By the way, the letter 'a' is actually a kind of diaphragm located behind the rear lens. If this letter is placed on the front lens, then we get the effect 'homemade bokeh'.

The letter 'a' from the kit for children is glued to the front lens of the objective

Here's what happened:

The letter on the front lens

Then I wanted to scratch the lenses of the lens, but my conscience did not allow me, because I just covered the rear lens with kitchen salt, simulating a lot of dust :)

The back lens is covered with kitchen salt

With such a strong pollution, the following dirt turned out on the side:

Heavy mud

After that, I seemed to wipe the rear lens away from the salt qualitatively, but after the test shot it turned out that my efforts were not crowned with complete success, I left dirt at the edges of the lens, which then became very visible in the pictures:

A speck of poor rubbing of the rear lens from salt

Results

1. Dirt on the bokeh (on circles of blur) in the first place connected with real dirtlocated on the lenses of the lens. This can be dust, scatter from the cleaning fluid, scratches, air bubbles in the optical glass, peeling off enlightenment, anything. Dirt can be not only on the front or rear lens, a similar effect is obtained from dirt on the inner lenses of the lens.
2. The dirt that is visible on the circles of blur is not displayed on objects in the field of sharpness. Similarly, the diaphragm blades, their appearance and shape, not displayed on objects in the field of sharpness during everyday shooting.
3. The dirt on the lenses works like aperture device, and just like the diaphragm blades are displayed on circles of blur, this dirt is also displayed on them.

Questions for which I do not have an exact answer:

1. Do special elements (aspherics, low dispersion and other types of glasses) affect bokeh artifacts?
2. Concentric circles on circles of blur come from poorly polished lenses? If not, then why? Example of such circles.
3. Can a service center be required to have lens prophylaxis if severe 'bokeh contamination' is detected?

Leave your answers in the comments.

↓↓↓ Like it :) ↓↓↓ Thank you for your attention. Arkady Shapoval.