# How does aperture affect depth of field?



## ktc (Jan 10, 2005)

Forgive my poor understanding of optics, but I vaguely understand that a smaller aperture causes more diffraction, and that somehow leads to more light all converging on the same focal plane (the film/sensor plane). How come that doesn't also make objects less sharp though? Wouldn't multiple focus planes all start blending together?

:dunno:


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## Cliff (Apr 19, 2002)

I don't think I quite understand what you're asking, but this article might shed some light on the topic (pun intended): http://en.wikipedia.org/wiki/Airy_disc


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## Desiboy (Apr 1, 2005)

The best way I can explain it is sort of the way it was explained to me. The simplest way is basically this, imagine anything in a room gives off light particles. Things that are far away give of less particles than things that are closer. The way a photograph looks is determined basically by how many particles hit the film. When the opening is small (a smaller aperture), an equal amount of particles are let in from everything. However, when the opening is larger, more particles are allowed in, and since there are a greater number of particles from the object that's closer, it's more in focus than the object that's far and giving off less particles.

I know this is like the 5 year old explanation but it's the best I can come up with


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## JSpira (Oct 21, 2002)

ktc said:


> Forgive my poor understanding of optics, but I vaguely understand that a smaller aperture causes more diffraction, and that somehow leads to more light all converging on the same focal plane (the film/sensor plane). How come that doesn't also make objects less sharp though? Wouldn't multiple focus planes all start blending together?


Depth-of-field is the zone of sharp focus that starts in front of the point you've focused on and continues to another point behind.

DOF is determined by the aperture of the lens, the focal length of the lens, and the distance you are from your subject. The longer the focal length, the less DOF you will have (although this is somewhat illusory because telephone lenses often are used to "get close" to a subject when proximity is not possible; in practice, wide-angle lenses have far greater DOF than telephoto lenses. The greater the distance between lens and subject, the more DOF you will have available to you (i.e. a macro lens will have a very limited DOF).

To answer the question about the physics of depth-of-field, this has to do with the so-called "circle of confusion" that defines how much a point needs to be blurred so as to appear unsharp. An excellent article in the British Journal of Photography from March 1866 talks about this.


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## EdCT (Mar 14, 2002)

ktc said:


> Forgive my poor understanding of optics, but I vaguely understand that a smaller aperture causes more diffraction, and that somehow leads to more light all converging on the same focal plane (the film/sensor plane). How come that doesn't also make objects less sharp though? Wouldn't multiple focus planes all start blending together?
> 
> :dunno:


If all that technical stuff gets you down, here's all you need to know.

Smaller apertures (higher numbers) let in less light and extend your DOF.

Try it, line up a row of objects, shoot it at f3 or 4, then shoot the same thing at f8 to f11 - you'll get more of the row, if not all of it, in focus at f8 to f11 (or greater).

My Nikor 18/200mm lens is pretty sharp at f8 to f11, but less so as I go to smaller apertures, it gets soft looking at the f22 range.

I shot this at f20 because I needed the long exposure - 2 seconds to blur the water (smaller apps let in less light so longer shutter speeds are possible). I'm at 20mm and far enough away where everything is reasonably sharp.










Ed


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## ·clyde· (Dec 26, 2001)

There's a simpler way...

Why do people squint?


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