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Overrated? Trioplan 100mm 1:2.8
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PostPosted: Thu Nov 07, 2013 7:53 am    Post subject: Reply with quote

iangreenhalgh1 wrote:
Interesting. Doesn't the Tamron, like other lenses with a softness control, add more and more spherical aberration by moving an element? If so, that would point to spherical aberration being the cause of the outline on the bokeh highlights and would suggest that the Trioplan 2.8/100 has those outlines because it has a lot of spherical aberration?


You are partly correct, Ian. if You want to understand better, about how it works, I am re-posting the link again: http://www.luminous-landscape.com/essays/bokeh.shtml. It clearly shows how the outline of bokeh highlights appear with soft focus lenses and how it can be present or absent in different parts of images.

It is not only the presence of aberrations, but also the way they were corrected, that matters here. Because in most lens design correction for aberrations are not symmetrical for the areas in front and behind the best focus point. Remember people saying how lenses have smoother out-of-focus areas either in front or behind the focused part?

I do not have time right now to test my other lenses, but here is a simple test, and please ignore the dust here! It is not relevant to the discussion. Same lens here, same Trioplan. Same "point" light source. The shape of OOF highlight when the light source is placed behind and in front of the best focus point. It is obvious, that the Trioplan is corrected for spherical aberrations differently for different parts of the image.



I will do the same test for Tamron later on.


PostPosted: Thu Nov 07, 2013 8:28 am    Post subject: Reply with quote

Very well done Alex!


PostPosted: Thu Nov 07, 2013 9:56 am    Post subject: Reply with quote

Overcorrected spherical abberation gices bubbles in background, and soft bokeh in foreground.
Undercorrected spherical abberation gives soft bokeh in background, and bubble bokeh in foreground.

The Trioplan 100mm f/2.8 seem to have a very strong over corrected spherical abberation. The Diaplan / Pentacon AV 100 the same. The Pentacon AV 80mm/2.8 seem in first tests less strong over corrected spherical abberation.

Spherical abberation in normal helicoid focussed lenses (not IF, no floating elements, not front or backlens focussing) is corrected for one distance, and so front and back bokeh behavior changes with distance.
Knowing this I took a lens with extra macro focussing ring to get the possibility to have a lens with spherical abberation control - like Nikon Defocus Control!
I found the Sigma YS 135mm lens as a good lens for such bokeh control.

In both fore- and background only apodization lenses gives very smooth bokeh.