How Does f-Number Influence Depth of Field in Photography?

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SUMMARY

The depth of field in photography increases and image sharpness improves when a camera lens is stopped down to a larger f-number due to a smaller aperture diameter. This smaller diameter reduces the amount of light entering the lens, which minimizes aberrations and astigmatism, leading to a more focused image. Even with high-quality lenses, the convergence of light rays is affected by the aperture size, causing varying degrees of sharpness across the image. Understanding these principles is essential for photographers aiming to control depth of field effectively.

PREREQUISITES
  • Understanding of f-number and aperture mechanics
  • Basic knowledge of light behavior and ray diagrams
  • Familiarity with lens aberrations and their impact on image quality
  • Experience with photography concepts, particularly depth of field
NEXT STEPS
  • Research the relationship between f-number and exposure in photography
  • Learn about lens aberrations and how to minimize them
  • Explore advanced depth of field techniques in portrait and landscape photography
  • Study the effects of aperture on bokeh and background blur
USEFUL FOR

Photographers, both amateur and professional, who wish to enhance their understanding of depth of field and improve their image sharpness through effective aperture control.

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Homework Statement


Why is the depth of field greater, and the image sharper, when a camera lens is "stopped down" to a larger f-number? Ignore diffraction.


Homework Equations


f-number=f/D where D is the diameter of the hole.


The Attempt at a Solution


At a larger f-number, the diameter of the hole is smaller so there is less exposure, although I'm not sure what that has to do with the depth of field. Can someone help me?
 
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If you drew a diagram of light rays from subject to film plane, where would you draw light rays passing through the aperture? More specifically, how many different light rays could you draw? What's bad about different light rays when it comes to sharpness of an image?
 
Ahh, with small aperature comes less aberration and astigmatism right?
 
JSGandora said:
Ahh, with small aperature comes less aberration and astigmatism right?

Well yes but it's more than that. Even with a theoretically ideal lens (no aberration or astigmatism) the effect is still there. It is not an effect of cheap lenses (otherwise the most expensive lenses would laugh at depth of field issues, would they not?)
 
Oh, because the diameter is smaller, the light rays don't really converge on a point and so some of the rays that come from the same point but pass through the hole at different points don't converge on the same point on the film which makes it blurry. Is that correct?
 

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