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Question: A question about diffraction in very narrow apertures!?
I read recently in a photography article that very wide apertures are sometimes not desirable because the depth of field is too short (as we all know), but at the other extreme, very small apertures, despite giving us a comfortably deep fields, introduce the physical phenomena of diffraction!.
For those who don't know, diffraction occurs when light passes through an aperture that is comparable in size to its wavelength!. Now, although the aperture of a camera at f22 may be very large compared to the wavelength of visible light, it might be small enough to get a very week diffraction effect; sufficient to engender a blurring effect in the image!.
The article then compared 4 images at varying f numbers and, indeed, the image did seem to blur towards f22!. I tried the experiment at home with my tripod photographing a map (so I could zoom in to the detail!. The image I took at f 22 was considerably clearer than the one I took at f8!. even though I had focused to the best of my ability!Www@QuestionHome@Com
Best Answer - Chosen by Asker:
Diffraction is most noticeable (as is when an image is out of focus and there is any camera movement) when rather large enlargements are made of images!.
For instance enlarging a typical 36 x 24 mm frame (film or sensor) to a 18 x 12 inch print is a 10x enlargement and most professionally hand held, well focused images shot at the lenses sharpest aperture (typically three stops down from wide open) will not show any problems!.
Increase the enlargement to a 40 x 30 inch image and unless the image is tack sharp, it will fall apart, whether it is being enlarged from film or an image file !.!.!. hence the use of 4x5 or 8x10 film for images destained for use as posters or large print adverts!.
More on Diffraction:
http://www!.kenrockwell!.com/tech/diffract!.!.!.
For those who need to read the nitty, gritty:
http://www!.kenrockwell!.com/tech/focus!.ht!.!.!.Www@QuestionHome@Com
Screwdriver explained it perfectly!. I would add, the reason your map images had considerable less evidence of diffraction is due to them being of a two dimensional object at full frame with even light reflected back into the lens!. In the 3D world you can have a lot of stray light at various intensities and angles!.Www@QuestionHome@Com
I think it's B!.S!., Ansel Adams has some photos taken at f32 with 4 X 5 film which are Crystal Clear!.Www@QuestionHome@Com
Most lenses nowadays are designed to work at the set f numbers!.One of my cheap Nikon point and shoot cameras only stops down to f8 ,while another a top quality optic stops down to f32!. Some of my old large format lenses stop down to f128!.The only figures I have to hand show a slight drop in resolution below f 11 for most lenses,while Hasselblad's figures show a consistent performance throughout the range!.The results from most cameras are adquate for purpose and most of the testing is very rigourous and mostly of academic interest for normal photographers!.Www@QuestionHome@Com
perception of a clear or correctly focused image is just that!. perception!. everyone sees differently!. you said that you had focused to the best of your ability!. did you manually focus each image!? how do you know that your focus was consistent from one image to the next!? the constantly changing pupil size (aperture) of your eye changes your perception of depth and focus!.
use the auto focus on your camera for this type of experiment!. small amounts of human error make a huge difference in perception from f22 to f8!.
at worst you have learned something!. setting up a tripod and shooting the same pic with different camera settings is a basic exercise that is done by almost every phtog student at some point!.
there probably is some diffraction at different apertures!. different lenses may also have an effect on image quality, especially when they are near the extremes of their engineered limits!. perhaps this is something you can use to your advantage when selecting equipment or creating artistic effects!.Www@QuestionHome@Com
The wavelength of light is measured in Angstroms which is a lot smaller than the smallest aperture (100,000's of times smaller in the case of f64) it's not the aperture per say that causes diffraction!.
Diffraction is caused by the light hitting the edge of the aperture leaves AND IS ALWAYS THERE!. It's just that at wide apertures the effected parts of the image are only a very small percentage overall so the effect is unnoticeable, think of it as a slight 'halo' around the aperture edges, at very small apertures the effect is occurring over a greater part of of the image and the effect (a slight softening) gets more noticeable!.
This effect is more noticeable at high resolutions so a high resolution sensor (10Mp +) the effect will be more noticeable than at a lower resolution sensor (say 6Mp)!.
On my Pentax K10D (10Mp) the effect is only noticeable at f30 or smaller on my Pentax lenses, but occurs at f22 or smaller on my Sigma lenses, but that could be the focal length of the lenses could be exacerbating the effect (the Sigma is a telephoto lens)!. It's also supposed to be more prevalent on super wide angle lenses too, but, as yet, I haven't tested them!.
Hope that helps
ChrisWww@QuestionHome@Com