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jchenault
31st October 2009 - 10:50 PM
My understanding of lenses is that if you make the aperture smaller, the depth of field increases and the lens produces a sharper image.

I'm trying to understand the physics of why this is so. Both for depth of field and sharpness.

If you made a mask with a hole it it to represent the smaller aperture, would it matter if it were centered on the axis of the lens, or would it work just as well off axis.


If there is a text that explains this I will be happy to follow a link.

Thanks in advance.

John


flyingbuttressman
1st November 2009 - 12:47 AM
QUOTE (jchenault+Oct 31 2009, 06:50 PM)
My understanding of lenses is that if you make the aperture smaller, the depth of field increases and the lens produces a sharper image.
I'm trying to understand the physics of why this is so. Both for depth of field and sharpness.

Pinhole cameras use this principle to create images without a glass lens.
Pinhole Camera
It's hard to explain the concept without a diagram, but that page has a good one near the top.
As the aperture shrinks, the depth of field approaches infinite.
Blurring occurs when photons from two different objects strike the same place on the film. By shrinking the aperture, you ensure that the photons will accurately reproduce the image on the film.
QUOTE
If you made a mask with a hole it it to represent the smaller aperture, would it matter if it were centered on the axis of the lens, or would it work just as well off axis.

It would distort the perspective of the photograph.
El_Machinae
12th November 2009 - 02:55 PM
Microscopy University might be what you need. In general, it's a pretty good website regarding microscopy technologies.

This page is a good start.
http://www.microscopyu.com/articles/formul...fielddepth.html
Enthalpy
12th November 2009 - 10:51 PM
As far as I believe to understand, reducing the aperture of an existing lens may improve image quality only because it limits the effect of the lens' imperfections.

On a nearly perfect telescope (that is, with many corrections!), reducing the aperture only increases diffraction effects, making resolution worse. Though you may try to use a fuzzy mask, nearly as big as the initial aperture, in an attempt to reduce the intensity of the strongest diffraction rings - this is known in signal theory and Fourier transformation as windowing, look for Hamming Windows, Hanning, Raised Cosine and many more. Say, to see planets around a distant star.

If your put your mask off-axis, it makes some imperfections worse, especially coma aberration.
sovtek
14th November 2009 - 02:49 AM
In 35mm and APS-C photography formats you generally don't want to stop down below f/16 to prevent introducing diffraction effects. My EF 50mm is perfect at 1.4 but horrible at f/2. Lenses are finnicky. What can you do?
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