Optimisation of the cheap Sigma 135mm/2.8 YS to get more overcorrection of spherical abberation

After I found out that this Sigma 135mm f/2.8 YS lens (not Pantel) has a kind of up to now unkown bokeh control lens, I tried to optimize this.

In the direction of spherical under-correction this lens has a good range. But in the direction to spherical over-correction it is only a small range.
This direction is the one of the well known and loved Meyer Goerlitz Trioplan 100mm f/2.8.

This modification is easy - as long one has a lathe. Without this it is hard to get the quality, and not to damage the helicoid for the macro focusing. Normal focusing is made with a second helicoid.

This is the front helicoid where the two front lenses are.

I took 2 mm of the helicoid away, and 2 mm of a block. Both are on the outline of the barrel of the two front lenses, those wo where moved for the macro focussing.

Relative simple needs probably clarification: These helicoids are not as easy as a normal screw and a nut. The helicoids have not one start for the thread like normal screws, there are several starts, and this is much harder to screw together! And you most likely need to take the two lenses out of the barrel, and clean them, take it in the barrel again. For this you need a spanner wrench or such a device.

Now sample images with the extended blur control!

Here an example how different bokeh may look like:
First distracting bokeh

Now the bokeh blur is a bit smoother, but far away from really smooth bokeh.

And now in this image the blur is smooth!

Another sample about the differences between spherical abberation: Left pleasing undercorrection, and right accented blur structure
because spherical abberation is over-corrected:

Here in sample images you see the effect of the spherical abberation on fore and background blur:
Right image with over-correction of spherical abberation the background blur stressed the structures there - but the foreground is smooth.
On the left image you see the contrary, the background is blurred away, and the foreground is accentuated.
Lenses with an apodizsation element like the Sony STF 135 or my DIY clone could smooth booth foreground and background bokeh in one image!

Further images on how one can use the bokeh control. But This tool is not as simple as the Nikon Nikkor DC lenses - one need to use
extension tubes to accomodate the focus to the different focal lengths the lens has with different bokeh settings.

Please visit my other tinker work, or my DIY directory with ~1700 DIY links to other photo tinkering sites.

This lens modification was inspired by the Nikkor DC lenses 105mm and 135mm f/2.0.
A good informations source about the Defocus Control of these lenses:
NikonJin Nikon AF 135mm f/2D DC review
Max Zomborszki´s site about the Defocus Control on Nikon DC lenses
Jakub Trávník's bokeh page.