Add aperture blades to old manual lenses

petrochemist wrote:

neil bayin wrote:

I wonder if anyone has tried this before, some old Minolta prime lenses are really sharp and nice, for example 50/1.4, problem is bokeh is kind of bad. But if we are more blades to it, would that improve the bokeh?

Most fast 50s are Double Gaussian formulas for which you can place a new aperture in front of the lens -- a Waterhouse stop. To work as a stop, the opening must be smaller than the focal_length/f_number for the f_number the lens is set at -- so less than 35mm for your 50mm f/1.4 wide open. Depending on details of your lens design, it might need to be even a bit smaller than that.

I've used paper discs up against the front element (with both circular & shaped apertures) and have tried an iris in front of the lens. In a few cases I've ended up with noticeable vignetting (usually when the lens has a long built in hood) but many have been OK.

Vignetting happens if your opening is too large. I know that's counter-intuitive, but it's how optics work. You're then vignetting rays from the lens aperture, which is placed differently, and that explains why the front element is bigger than focal_length/f_number.

Here's a quick example of shaped bokeh via a paper disc https://flic.kr/p/27KRhGp

Yup; that works. Shape of the bokeh will mirror the shape of the aperture. BTW, you can even use that to shoot stereo anaglyphs with a single shot and single lens. My Instructable explains the anaglyph trick and shows how to make 'em from paper, but I also have a Thingiverse Customizer program that designs ones to 3D-print ... all free.

The cooler thing is to make an apodizing filter, but it's really difficult to make one that is of very high quality. Here's one made using film , which produces really good quality, but for front mounting, you'd want one bigger than 135 film. I've played with 6x6 film, laser-printing on overhead transparency material, and even home-made sputtering... they all sort of work for giving really smooth bokeh, but I have a Sony 100mm STF with is perfectly apodized -- all my attempts fall a bit short of that standard. The Sony is also a nearly perfect optic to begin with -- no vignetting at all, which is needed to keep the bokeh smooth near the corners of the frame; most fast 50s, including yours, will need to be stopped down a couple of stops to remove all vignetting, and that means a slower aperture and smaller opening in the apodizing filter.  BTW, you can also do apodization in the time domain using multiple exposures.

For lenses with more space between the rear element & the camera I'm planning on experiments with my iris there, and then might end up buying some of the adapters that have an iris built in.

I don't recommend this. Stay up front if you can -- it's a lot less touchy. That said, behind the lens can sometimes work with a smaller filter.

I personally wouldn't attempt to modify the aperture mechanism in a lens my DIY skills are definitely not up to it. Cutting a slot to allow a waterhouse style aperture to be inserted is a possibility, but I'd only consider that if I cant arrange something outside the lens.

Agreed. Keep in mind, it's hard to know where to cut a slot, and there can be lots of stuff in the way. I've never been desperate enough to cut a slot -- it's so much easier to just find a different lens with similar rendering.