Old post, but I have a strange update over this topic.
I'm facing a strange behaviour. Half of my prints has visible layers, the other half has an almost perfectly smooth surface.

It shows that is possible to have not visible layers, but I can't understand how!
Moreover, I noticed if I raise the temperature layers are more visible If I reduce the temperature surfaces seems smoother, but If I reduce too much the temperature surfaces become much smoother but also surfaces become deformed and bent.

How is it possible?
How can I have 100% of the piece perfectly smooth?

Has someone faced the same?

Your machine has mechanical problems. That is called z banding and unless the lower section is supposed to look that way which I guess it is not your z axis is moving from side to side as the print ,moves up till it gets to a point half way where the mechanical issue is no longer present and it smooths out. I assume the model is supposed to have the same surface through out as the left side. The right side bands are the issues and should not be there. If it is the other way around then there is still a mechanical issue of some sort.

It shows that is possible to have not visible layers, but I can't understand how!
Moreover, I noticed if I raise the temperature layers are more visible If I reduce the temperature surfaces seems smoother, but If I reduce too much the temperature surfaces become much smoother but also surfaces become deformed and bent.

How is it possible?
How can I have 100% of the piece perfectly smooth?

Has someone faced the same?

Refer to Blue coloured Yoda on the left in this pic:

Thats just old fashioned .25mm with PLA and a slight hotter nozzle to make it a tad runnier... The transparent yoda is in ABS, and consequently, highlights the occasional deposition lines as its pretty static once laid down...

Now - knowing I printed that at .25mm as a test piece... lets have a look at the time comparisons between the different layer heights for the same printer using that STL : Yoda-Lite.stl (mesh reduced Yoda bust https://www.thingiverse.com/thing:10752 from faberdasher )

1h 37m .40mm
1h 55m .30mm
2h 09m .25mm
2h 28m .20mm
4h 32m .10mm
8h 43m .05mm

Massive difference there in time to print exactly the same piece ultimately. And there is no quality benefit in switching below .25mm .. it literally just takes 4 times as long! (I say this for this model, as it has no fine surface texture or detail, so there is no quality improvement. If you were printing a lifelike textured sandstone wall - then sure, .1mm etc might bring you much greater detail than .25mm would. For the models in the video, and for old Yoda here - it don't do nuthin' but stretch the print time.... yes theres a tiny bit of extra face craginess you can eek out - but it doesn't really effect the final product....)

As for leaving the machines running and 11 hours for a tiny print 'not so bad'..... the bigger issue is it wears very very thin the 4th time at around hour 6 there is a slight fault in the print...

Ultimately if you are concerned about layer lines, move right along to SLA and forget worrying about FFF/FDM. Especially when you consider the time taken to print the (relatively) small model at 0.05mm to avoid any layer lines, you  could have printed 4 other models in the mean time... Or put another way - The pic in this post would have taken 22+ hours to produce, vs the 4 hours it actually took

Worrying about layer lines on FFF/FDM printed objects is like fretting over the most appropriate way to use a teaspoon as a screwdriver (i.e, you've already started with the wrong implement ..... it can be done, but its often not worth the effort of just using a screwdriver in the first place ) 

At .05mm layers, I doubt the lines would be easily distinguishable, but they are still there.  There is an article somewhere out there examining high-res prints with a microscope in order to show that even high resolution prints can get bits of food stuck in them, even if the eye cant see them.

Doesnt seem like a problem to me. i think most people who want higher resolution want it to not be seen by the human eye. I guess you could put the print in acetone for very short time to also get rid of some of those tiny invisible lines and not actually deform the shape too much like when puting a lower res print in acetone for a little longer.

Doesnt sound so bad either, if the machine can run when youre not around.

Looks like they do to others tho.

what is that, a replacement part?

Here's a HD video with a black print, though printed at 0.05 mm youtube.com/watch?v=LfKCLc-VkeA
Thoughts on this? Again the camera, or 0.05 makes the difference?

(BTW can i make Solidoodle do 0.05 mm?)

that's how this was accomplished

I watched some more videos and I see no such ugly lines in these videos:
youtube.com/watch?v=5BtL9scagn8

That's a 480p video, very low resolution. Think of it as a 640x480 still image.

You can see the visual effect of the banding when the print is rotating so we know the slices are there.

One other thing to note, cameras have a very tough time finding focus with all white objects. They are much better with color objects. In my print pictures, the red one has more pronounced filament slices than the natural white ones.

Not sure about bad camera focus for the first two videos. Seems fine by looking at the detail on the disc the print is on. But video is only 480p though...

Anybody got some macro photos of 0.1mm prints with good tuning? These videos are honestly all i could find that look good.

I watched some more videos and I see no such ugly lines in these videos:

And yes im aware of acetone or sandpaper finishing which I personally think aren't very good solutions. How do I know these havent been finished? Because I looked closely at the prints while there where still being printed.

So what do you think? How come those, especially the ones in the first two videos, look so ideal? They are said to have been printed at 0.1mm which is what ive seen before.