The Tasman 135mm f/2.8 is a fine lens. You've probably never heard of this lens or its distributor, Tasman, either. Tasman was an Australian exclusive photographic equipment distributor from the 1980s (maybe earlier/later). Because it serviced such a small market, little information is preserved about the company. As a distributor they didn't make camera lenses and instead contracted out lenses to other companies. Distributors like this have a mixed reputation with some like Vivitar have a have a very good reputation whilst Hanimex/Hanimar are known for making some of the worst lenses of all time. As far as I can tell the Tasman 135mm f/2.8 is a re-brand of the Vivitar Komine 135mm f/2.8. It is hard to tell this without having one to tear down. That is to say that the Tasman is a good lens. Not great wide open like most vintage telephoto lenses, but good enough stopped down. Indeed I have used it extensively for landscape photography without issue.
But this isn't a lens review. This is about modifying the lens. As a matter of fortune I got a second copy of this lens. I could have simply sold the lens, however given the low profile of the Tasman brand, it would be hard to attract buyers. Instead I decided to give the lens an aperture mod. I won't give a detailed account of how to modify the lens like I did for the triangle aperture mod for the Pentacon 50mm f/1.8. This lens technically has six aperture blades but it isn't possible to cut the blades like the Pentacon to produce the triangle aperture effect. Instead there is an easier way to modify the aperture. This involves cut a piece of black cardboard with a square in the centre and placing it over the aperture. This alters the aperture of the lens when shot wide open. Importantly because this new aperture sits over the old aperture the lens can still be stopped down and used as normal. Since the square is larger enough (16mm x 16mm) the lens performs as normal from f/4 onward.
This worked out much better than I expected. Part of the reason is that this is a telephoto lens. The Pentacon 50mm produced some great effects, but it required me to get fairly close to the subject in order to create noticeable bokeh. The Tasman with it's thin depth of field had no trouble blurring out the background and producing a distinctive square shaped bokeh.
You should note that the squares are aligned so that the edges are at a 45 degree angle to the edge of the photo frame. This can be changed by simply aligned the aperture differently when you install it.
I am fairly impressed by the results, given it only took a few cents worth of cardboard and some small amount of work to produce. This technique can be reproduced in most vintage lenses. It would be preferable to modify the actually aperture blades like with the Pentacon, but in lieu of that this is a great way to produce some creative effects.