APOD: Venus and the Sisters (2012 Apr 06)

[DavidLeodis]

What does the "multiple leaves in the aperture of the telephoto lens" mean I know I should try to find out but I'm surely not the only one not to know, so I thought I would ask.

iris.jpg

A standard camera iris, for adjusting the size of the aperture, is made up of multiple leaves. This is a five-leaf iris, which would produce ten diffraction spikes around a bright point. Today's image was made using a six-leaf aperture, which produces just six spikes.

Thanks Chris for your help, which is appreciated.

I did not realise that cameras still used that method to adjust the size of the aperture, though to be honest I'd never given it any thought with my compact digital camera being always on an automatic setting so I don't know what is actually going on when I take a photograph.

[Beyond]

Now wait a minute! If a five leaf aperture produces ten spikes, why does a six leaf aperture only produce six spikes, instead of twelve??

[bystander]

Symmetry!

[Chris Peterson]

Now wait a minute! If a five leaf aperture produces ten spikes, why does a six leaf aperture only produce six spikes, instead of twelve??

Each edge produces a pair. A six-leaf iris does produce 12 spikes, but half of them overlap the other half, so you only see six.

[bystander]

Symmetry!

[Beyond]

Ah, ok. Thanks for the explanation chris. Bystander comes in a loose second. Only because there's no third place.

[Ann]

As a language teacher, I'd say Chris' reply contains the relevant information. As for bystander's answer, it helps if you know the answer that Chris gave you to understand his answer!

Ann

[Beyond]

That's why bystander came in a loose second. He used a catch-22 answer--Symmetry.

[ta152h0]

Would a pinhole camera with a perfect hole produce spikes ? and don't answer the way my father would. Prof Kotenberg would have me build one and find out for myself.

[Chris Peterson]

Would a pinhole camera with a perfect hole produce spikes?

Diffraction spikes are linear structures, caused by linear elements in the light path (the vanes supporting a central mirror, a polygonal aperture, a leaf iris, etc). Circular structures (the central mirror itself, the aperture, etc) produce circular diffraction artifacts... rings. A pinhole lens does not produce any spikes, but it certainly produces an Airy pattern when imaging a point source, and its circular diffraction artifacts affect the PSF of the system just as those from the circular aperture of a reflective or refractive system do.

[neufer]

MOscOW photo by Shuttle Expedition 30 crew on March 28, 2012.

Now wait a minute! If a five leaf aperture produces ten spikes, why does a six leaf aperture only produce six spikes, instead of twelve??

Each edge produces a pair. A six-leaf iris does produce 12 spikes, but half of them overlap the other half, so you only see six.

Symmetry!

[bystander]

MOscOW photo by Shuttle Expedition 30 crew on March 28, 2012.

Asterisk*