Apodization

[D Cox]

hjulenissen wrote:

Interesting.

With the current popularity of adapters for fitting older lenses to (short register distance) mirror-less cameras, might it be possible to build an adapter that also contained some interesting light-modifiers (at the right distance from the sensor)?

If you are using a multi-element lens (which 99% of camera lenses are), then the diaphragm has to be inside the lens.

However, you can experiment with a simple achromatic doublet (two elements cemented together). In this case the aperture just has to be close to the lens.

I'd be interested in a mechanically rotated wheel (ala the colour wheel used in certain image applications) that contained e.g. perfectly circular (top hat) apertures, gradually faded apertures (Apodization) and apertures incorporating ND-filtering (less light). Perhaps odd shapes (stars, encoded apertures) might be interesting as well...

Star-shaped and other non-circular apertures can give fun results.

Another use of non-circular apertures was in process cameras for making blocks for half-tone printing, in the days before Macs and RIPs. The shape of the aperture affects the shape of the half-tone dot, which in turn affects the tonal rendering. The half-tone dot is a slightly out-of-focus image of the holes in the screen.

 

[fvdbergh2501]

Roland Karlsson wrote:

http://en.wikipedia.org/wiki/Apodization ‎

There exist one apodization lens I know about - the Minolta/Sony 135 mm SLT lens.

I assume apodization is the holy grail to image quality. All high quality optical systems, being it laser systems or microscopy needs apodization. Doing FFT or other signal treatment you always do apodization.

In a photographic lens it assures that OOF parts are unsharp, i.e. smooth. OOF parts if you do not do apodization consists of overlayed sharp circles, which of course adds noisy cr@p.

From the Wikipedia description, it sounds like the "overlayed sharp circles" you are referring to are the secondary maxima of the Airy pattern (http://en.wikipedia.org/wiki/Airy_disc).

Pulling the figures from the Wikipedia page: about 83% of the energy will be inside the primary (central) peak of the Airy pattern. In other words, the energy contained in the rest of the rings is at most 17%. Keeping in mind that the support of the Airy pattern is infinite, 17% is not a lot.

More to the point: how bright is the first of the ring (outside the main disc)? The peak intensity of the secondary ring is about 1.7% of the central peak. The total energy in the secondary ring is about 7% (it is fairly wide).

Keeping in mind that an MTF of 0.1 is considered the minimum contrast level that can readily be detected by our eyes, you can assume that a 7% (0.07) of the contrast is not going to make a day-and-night difference to your image quality (speaking purely from the maths perspective).

You also have to keep in mind that the apodization will have to be wavelength-dependent, i.e., it will actually require multiple layers of filters, if you want to completely suppress the secondary Airy rings.

So once you have a lens that deals with all the other aberrations perfectly, then you can chase down that 7%

 

[pauljames1983]

Roland Karlsson wrote:

DSPographer wrote:

Thinking about this some more, I think you would probably want to open the aperture most of the way quickly, then slowly cycle it between say the last half-stop and back during the exposure, followed by quickly closing it all the way. With an electronic servo-motor driving the leafs I think this would be doable. This would also make the Apodization function adjustable.

Yepp - there was a Minolta Maxxum camera that had an apodization mode. It used the aperture, not a leaf shutter. Leaf shutters are not all that common in system cameras.

But - leaf shutters might actually do it unintentionally - at least at high speeds.

Maxxum/Dynax 7 camera used multiple exposures at different apertures to simulate the effect of the STF lens. Can't say I used this mode much. The STF 135mm is a gem of a lens.

 

[Roland Karlsson]

Interesting.

With the current popularity of adapters for fitting older lenses to (short register distance) mirror-less cameras, might it be possible to build an adapter that also contained some interesting light-modifiers (at the right distance from the sensor)?

The light modifier haS to be in the aperture position. That is more often than not somewhere in the middle of the lens. but there are lenses with the aperture positions near to the front or rear lens, even outside of the lens packet, but that is rare.

I'd be interested in a mechanically rotated wheel (ala the colour wheel used in certain image applications) that contained e.g. perfectly circular (top hat) apertures, gradually faded apertures (Apodization) and apertures incorporating ND-filtering (less light). Perhaps odd shapes (stars, encoded apertures) might be interesting as well...

Yes!!! Me to.

In older lenses you had something called waterhouse stops. That was metal plates with a hole that you could stick into the lens as aperture. I would really like to be able to do that. Unfortunately nearly all modern lenses have the focussing handle around the optical packet. So, it is not possible to make a slit to insert waterhouse stops.