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Old Oct 27, 2010, 9:24 AM   #1
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Default Effecive aperture and diffraction

There is a very nice set of tutorials available on-line at cambridge in colour. I have been reading one on macro photography and effective aperture (here: http://www.cambridgeincolour.com/tut...cro-lenses.htm) and see that these generally-knowledgeable people claim that diffraction will be affected by the effective aperture. For example, if your lens has significant diffraction blurring at f/22, and your extension tube renders f/16 an effective aperture of f/22, you will have significant diffraction blurring at the f/16 setting with that extension tube.

Now, this doesn't ring true to me. As I understand it, diffraction is due to the wave property of light as it physically interacts with a tiny hole or slit. That is, it should depend upon the aperture, not the f-stop. What you are really changing with an extension tube is the f-stop, but the dimensions of the hole are unaffected. So there should be no effect on blurring from an extension tube. Is my reasoning wrong? If so, how?
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Old Oct 27, 2010, 9:33 AM   #2
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*I think* that the blurring is caused by the light 'spreading out' on its journey from the aperture to the sensor. If the sensor is further away due to the extension tube then it will have spread out more by the time it gets there and the blurring will be worse. I don't think the diffraction is worse but the effect of the diffraction on your final image will be if that makes sense.
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Old Oct 27, 2010, 9:38 AM   #3
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Actually Cambridge explains it nicely here: http://www.cambridgeincolour.com/tut...hotography.htm

I'll quote the relevant part:
"Technical Note:
Since the physical size of the lens aperture is larger for telephoto lenses (f/22 is a larger aperture at 200 mm than at 50 mm), why doesn't the size of the airy disk vary with focal length? This is because the distance to the focal plane also increases with focal length, and so the airy disk diverges more over this greater distance. As a result, the two effects of physical aperture size and focal length cancel out. Therefore the size of the airy disk only depends on the f-stop, which describes both focal length and aperture size. The term used to universally describe the lens opening is the "numerical aperture" (inverse of twice the f-stop). There is some variation between lenses though, but this is mostly due more to the different design and distance between the focal plane and "entrance pupil.""

Extension tubes displace the aperture further from the focal plane, spreading the airy disk so effective "f-stop" takes this into account.

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Old Oct 27, 2010, 10:18 AM   #4
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Thanks, guys. That was very helpful.
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Old Oct 27, 2010, 10:54 AM   #5
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Unfortunately, this does not take into account that diffraction is a boundary condition. If the physical aperture is large enough for diffraction to be negligible, adding an extension tube is not going to increase it. There are other reasons for blurring besides diffraction, however, and extension tubes, TCs, or simply blowing up the print, will increase the perceived blur from these.
Now, if diffraction is beginning to appear at f/16, then increasing the size of the image will increase the perceived diffraction.
I got a blank page asking for my email when I used your link. Not being able to view any content without providing email address, smells crooked. No, thanks.

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Old Oct 27, 2010, 12:02 PM   #6
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Quote:
Originally Posted by VTphotog View Post
I got a blank page asking for my email when I used your link. Not being able to view any content without providing email address, smells crooked. No, thanks.

brian
You may want to check your settings. I see the content when I click on either his link or the one I provided. In addition to the content there is a small box to enter your email address if you want to be notified about new content. It's strictly optional. The site is legitimate.

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Old Oct 27, 2010, 12:07 PM   #7
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Quote:
Originally Posted by VTphotog View Post
Unfortunately, this does not take into account that diffraction is a boundary condition. If the physical aperture is large enough for diffraction to be negligible, adding an extension tube is not going to increase it.
brian
I disagree with this statement for the reasons in my post but I guess we can agree to disagree.

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