Thanks for clarification, further STF has two apertures.
Physically, yes; functionally, no. They are right next to each other and only one aperture actually closes when the image is taken. As far as the optical design is concerned, there is a single aperture.
Originally Posted by Steve Smith
If you want sharp focus and minimal depth of field at all aperture settings then you need to fit a variable aperture in the same shape as a cat's eye. i.e (). No matter how closed up it is, the image is formed from the full diameter of the lens.
That's going to look really weird but probably cool. You'll have elliptical blur aligned with one particular axis of the image, which means that you'll have anisotropic defocusing - detail at a given position in the image will be subject to a different level of blur depending on its orientation. You could take a photo of a chainlink fence and make half the wires disappear and the other half remain! It can be a cool effect though: google for "heart bokeh" sometime, the use of a cardboard mask on the front of a lens with some point-lights (e.g. coloured christmas lights) in the background can produce some very cutesy effects.
Originally Posted by baachitraka
Something analogous to elliptic horn(Antennas theory).
Well now you're just making shit up. Elliptical horns are used because they have different gain in each of the two planes, i.e. to form an elliptical gain pattern, i.e. a fan-beam instead of a pencil-beam. Antennae are not imaging devices, they do not have planes of focus and therefore they do not have defocused areas. The effect is also in the opposite direction: a wider horn causes a narrower beam whereas a wider aperture produces less DOF in an imaging sensor; the only thing in common is that they both obey the Rayleigh limit (diffraction) for their resolution/beamwidth limit. You can think of a horn as a lens with a single pixel (corresponding to its feed point) that's been stopped down well into diffraction - if you stop it down any further (make the horn narrower), the beam representing that one pixel will get even wider/fuzzier.
Last edited by polyglot; 06-23-2011 at 03:10 AM. Click to view previous post history.
No need to delete anything and you should feel free to disregard me sounding grumpy
You are right that an elliptical aperture on a lens is like an elliptical aperture on a horn and they both have differing resolution in each dimension. It's just that the mechanism and physics is so very different and in fact causes variation in the opposite direction as each dimension varies.