This trick will improve the quality of the image a lot ! I was a bit bored with my Planar 85 1.4. I only like to use it from f4 onwards, the rest below to me is just too soft. If you add a second aperture in front of the lens, it will improve the performance at f2 and others a lot with the sacrifice of 1.4. The principle behind is that image quality in general improve with reducing the aperture size. Light passing through the centre of the lens is less prone to abberation,whereas light passing the periphery of the lens is more prone to have optical errors.
In lenses with large aperture like 85 1.4, the problem is such that no matter how small is the aperture, light passing thro' the periphery of the front lens elements are contributing to the formation of the image. Image formed as a result of this is less sharp and less contrasty. In Sonnar 85 2.8, it has less of this problem because the front lens elements are much smaller. The addition of a second aperture in front will convert the pathway of light travelling such that light is divert from the periphery to the centre of the lens.
This new aperture looks ugly in the photo but it is the prototype. Most important thing is it works. I will properly cut a black card for this later. In this ex&le I used black tape to cover the periphery of the filter. The area covered is about half of the original area where light is entering the lens. The original area has a diameter of 63mm. I made the calculation if the diameter of the new aperture is 42mm,it will allow about half the amount of the original light passing thro',it effectively converts the lens from 1.4 to f2. The new aperture is about 1/root 2 the original diameter,if you know what I mean.
Once you made this aperture,light will only enter the lens at the central part rather than the periphery.
In the first roll of my film after this conversion,there is a dramatic improvment of the image at f2 and up, it is sharper and have a much better contrast of different kinds.
The new f2 of the lens correspond to the aperture ring at 1.4.
Try this yourself and you will notice this dramatic improvement. I have tried it on my 50 1.7 by converting it to 50 2. Other lenses you could try are Leica 80 1.4.
Please give me feedback after trying.