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Aperture shape

 
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st_m



Joined: 08 Jan 2006
Posts: 28
Location: Germany

PostPosted: Wed Jun 21, 2006 1:47 pm    Post subject: Aperture shape Reply with quote

I have a thechnical question again:
Does anyone know, how the shape of the aperture diaphragm affects the image?
Well, of course, if it's close to a circle, it shouldn't have too much influence, and just changes the "bookeh" of a photograph a bit.
But I wanted to know, what happens physically to the image.
I thought about a model with two circular apertures, one inscribing the "real" one, the other touching it from outside.
Then I could imagine, how the larger and smaller openings affect depth of field and brightness at a given setting.
But now I have this in a 5-to-9-blade aperture. Could I roughly say, the image has radial sectors with higher / lower aperture values? I know, what happens due to the non-infinite depth of field, if I take a photo of a small, bright spot, e.g. the sun, which gets its "star-shape".
Thanks,

Steffen
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wilash



Joined: 13 Apr 2005
Posts: 104
Location: Japan

PostPosted: Thu Jun 22, 2006 1:30 am    Post subject: Reply with quote

It has no affect on brightness, depth of field, or the in focus image. It can affect the out of focus image and usually that is refered to bokeh. The more blades, the better the bokeh.
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Carl_Constantine



Joined: 21 May 2006
Posts: 55
Location: Victoria, BC, Canada

PostPosted: Thu Jun 22, 2006 7:42 am    Post subject: Reply with quote

Well, if you do any night photography and have some lights in the image, the more blades you have in your lens, the more of a star pattern you see around the lights. For example, this image from a friend's photo-blog:

http://turingmachine.org/silvernegative/index.php?/archives/94-Oak-Bay-Marina-at-Night.html

HTH
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Carl B. Constantine
http://photo-op.ca/
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rjlittlefield



Joined: 06 May 2005
Posts: 727
Location: Richland, WA, USA

PostPosted: Thu Jun 22, 2006 9:13 am    Post subject: Reply with quote

Steffen,

Study http://www.luminous-landscape.com/essays/bokeh.shtml .

It has quite a bit of discussion of aperture shape, including some 'sink-strainer' apertures consisting of many small holes.

It also discusses and illustrates that there is a lot more to bokeh than the shape of the aperture.

You are correct that aperture shape does affect depth-of-field, at least as a thought experiment. As an extreme example, consider circular and slit-shaped apertures that let through the same amount of light. The slit will produce much more depth of field for subject detail in the narrow direction of the slit, and much less in the wide direction. But this should be only a minor effect for ordinary symmetric apertures.

Your radial sector model sounds correct. I'm not sure offhand what problems it's useful for solving.

You might also be interested in this article about the effect of aperture in stitched panorama photography. It hammers on the concept that what an aperture really does is to clip the ray fans, and explores some of the less obvious effects of this simple concept.

The classic "starburst" is caused by reflections off the edges of the diaphragm blades. It is not explained by any of the models in which apertures just block light. The picture linked by Carl is a beautiful example, in all senses of the word "beautiful".

--Rik
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