Optimal aperture position?

Started 2 months ago | Discussions thread
fferreres Veteran Member • Posts: 3,761
Re: Apodizing filters

ProfHankD wrote:

fferreres wrote:

If only it was easy to add an aftermarket apodizing filter instead of changing the aperture (which would also be nice).

Actually, it isn't hard.

There are just two problems:

First, the smoothness of the shading of the apodizing filter has a strong impact on the bokeh quality. Basically, unless it's smooth at the level of the wavelength of the light, an apodizing filter adds a texture by diffraction. This is why the Minolta/Sony/Laowa lenses all use smoked+clear glass in a glued optical flat to make their apodizers. Then again, some people like the textures -- think about the old Fuji "sink strainer" apodizing filters. For example, laser printing the following appropriately-sized on transparency material works, but the bokeh will look a bit grainy and the imperfect optical properties of the transparency material also degrade the image a bit:

Second, any internal vignetting will clip the OOF PSF shape exposing a sharp edge. This is the really hard part to get right, and it's why the Sony STF 100mm is so impressive: it has virtually zero vignetting. The only way to deal with this on a typical lens is to keep reducing the size of the apodized aperture until the base lens doesn't vignette. A typical 50mm f/1.4 might need to be stopped down to f/2.8 or slower before vignetting isn't a problem, so you'd leave the iris wide open but size the apodizing mask for an outer diameter corresponding to f/2.8.

Anyway, it really does work this way, and the nature of the apodizing mask actually tends to make the aberrations induced by suboptimal aperture placement less severe... so the front placement usually works quite well.

I am only now realizing how smart it was to provide the image mask with such black margin, which makes it so easy to create a variety of sizes jut by cropping, as opposed to having to create each. Was this intended? I was wondering if I had to install Gimp and learn to create a larger canvas, etc. and then downloaded the image and it had all this margin.

I am exploring slide transparency as opposed to film negative. There are these film `10 options:

- Fuji RDPIII Provia Transparency

- Kodak Ektar 100 Color Negative( I guess I;d invert thee image before asking for the negative)

- Kodak T-Max 100 B&W Negative (same)

Slide a transparency be the better choice here? Not sure which material scatter light less.

I probably won't measure each possible Iris, and exactly how much it vignettes, but likely make a series of sizes, considering 50mm 1.4 to 135mm 2.8 FL max, with varieties from iris maybe 15mm to 35mm (size of cut diameter) and bright area steps of maybe almost full to 1/2 of diameter (which I guess is 1/4 the light or two stop down), ie, bright diameter from 7mm to 30mm, with more varieties.

A bit sad that glass would be so much better an option.

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