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Author Topic: Diffraction and Shutter Speed  (Read 2449 times)
saiine
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« on: January 05, 2006, 03:43:26 PM »
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I read an article stating a majority of cameras have a "Sweet Spot" for aperture, being around 8.0, anything higher could produce diffraction (Image Quality Loss), and recommended 8.0 was a good aperture for detail.

My question is, if you're in great sunlight, would you notice a difference between say..

F4 @ 1/1000TH and F8 @ 1/500

I'm just curious what the rule of thumb here is, or if it even matters. I do a lot of daytime shooting with a Canon 17-40 L and wanted to run that by this enourmous forum of knowledgable people

Thanks
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boku
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« Reply #1 on: January 05, 2006, 07:58:00 PM »
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To answer your intended question, probably not. Diffraction should not be an issue at f/4 or f/8. Depth of field will be different. Lens sharpness/eveness characteristicts may be a factor.

Now, you need to understand that: "F4 @ 1/1000TH and F8 @ 1/500 " are not equivelent exposures. The latter is one stop darker. So based on that, you would notice a difference. (But, I know that you made a mistake and really meant equivelents, no?)

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I read an article stating a majority of cameras have a "Sweet Spot" for aperture, being around 8.0, anything higher could produce diffraction (Image Quality Loss), and recommended 8.0 was a good aperture for detail.

My question is, if you're in great sunlight, would you notice a difference between say..

F4 @ 1/1000TH and F8 @ 1/500

I'm just curious what the rule of thumb here is, or if it even matters. I do a lot of daytime shooting with a Canon 17-40 L and wanted to run that by this enourmous forum of knowledgable people

Thanks
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Bob Kulon

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pfigen
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« Reply #2 on: January 05, 2006, 10:21:35 PM »
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There is the traditional diffraction from too small an aperture and there is an additional type of diffraction that applies to how the aperture interacts with a single sensor site on a digital camera chip. Here a a link that describes the latter in very interesting detail. http://www.cambridgeincolour.com/tutorials...photography.htm

On most DSLR sensors, you won't start seeing  negative effects until around f/11-16, depending on your sensor, assuming you're shooting digital. It's easy enough to test your particular outfit to see where the sweet spots are for your individual pieces of equipment.

Most lenses are optimum 2-3 stops down from wide open and then fall apart at 22, but it varies with each design and the format for which the lens was designed.
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61Dynamic
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« Reply #3 on: January 05, 2006, 11:46:31 PM »
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To answer the original question (as I understood it), shutter speed alone does not effect diffraction.
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Brian Gilkes
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« Reply #4 on: January 06, 2006, 05:26:31 AM »
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Diffraction. in this case , is the bending of light rays at the aperture (iris) edge, It happens at all apertures, but it affects the image more at small apertures as a greater % of the rays are affected.In other words with little holes relatively more light rays don't focus where the lens says they should, as they hit the edge of the iris and bend away from their intended path.
Lens aberrations are usually worse at large lens openings, so best results are usually a compromise and a couple of stops down. Some lenses though, especially some Leitz glass for Leicas, are designed to have minimum aberrations when use wide open ie they are designed for low light photography of moving subjects.
Diffraction effects seem to be more visually acceptable when the iris is composed of a large number of blades, which results in a hole that is more spherical.This is more common in large format cameras where small apertures are required for depth of field purposes. See work by f64 . Note that even if the image is a little soft, thin bladed circular apertures give a pleasant softness and bokeh.
Shutter speed is not an issue.
Brian
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saiine
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« Reply #5 on: January 06, 2006, 12:48:45 PM »
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Thanks to everyone for the great replies, once again this board has impressed me with the knowledge shared on here.
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