Aperture setting and depth of focus

Silver Simon

Silver Simon

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I understand that the aperture setting, simply put is the size of the hole that allows light onto the camera's sensor, so a wide aperture f2.8 allows alot of light in and f22 allows alot less light in.

Where I am struggling is how can this effect the depth of focus? f2.8 will give you a sharp subject with a blurred background, whereas f22 will give you a sharp subject and a sharp background.

Help please. :D
 
The effect is depth of field, not depth of focus, that refers to variations at the film / sensor not the subject

In extreme examples, you don't actually need a lens to form an image. (Think about a pinhole camera)

As you stop down to a small aperture, you're approaching the pinhole effect, and getting a greater depth of field
 
The way I think of it is the smaller the hole, the more concentrated the light so more is in focus.
 
mortimerhill said:
The effect is depth of field, not depth of focus, that refers to variations at the film / sensor not the subject

In extreme examples, you don't actually need a lens to form an image. (Think about a pinhole camera)

As you stop down to a small aperture, you're approaching the pinhole effect, and getting a greater depth of field
Click to expand...

Sorry Mr confused.com here but I don't understand any of that apart from I got depth of focus wrong, I have no idea what a pinhole camera is. :( thanks.

AESamuel said:
The way I think of it is the smaller the hole, the more concentrated the light so more is in focus.
Click to expand...
Nearly getting that, thanks also, need to try and work it out.
 
My reply to an earlier post may, or may not, help :)

Neglecting subject movement, camera shake etc whatever you have focused on should be in focus and sharp.
Things nearer than the subject will be focused to a point slighly behind the sensor/film but as the light rays reach the sensor/film before they reach this focal point they will form little circles at the sensor/film.
Things further than the subject will be focused to a point slighly in front of the sensor/film so by the time the light rays reach the sensor/film they will have spread out again into little circles.
These little circles (known as Circles of Confusion) cause those things nearer and further than the subject to be out of focus.

However the human eye cannot see the little circles if they are very small so when we look at a picture there is an acceptable range of things in front of, and behind, the subject which we see as being in focus. This range of acceptable focus is known as Depth of Field (DoF).
If you have a large aperture the circles are larger than if you have a small aperture which blocks rays of light going through the edges of the lens and therefore creates smaller circles of confusion so a small aperture gives a greater DoF.

I did add a diagram to my earlier post but it gets very messy trying to show rays of light from different distances and at different apertures.

Have a look here instead http://www.dbp-consulting.com/Photography/CircleOfConfusion.html and have a play
 
Last edited:
DayDreamer said:
Simon

Take a look at this link. I found it very useful to help me understand how it all works.

Good luck

http://camerasim.com/camera-simulator/
Click to expand...

Suvv said:
My reply to an earlier post may, or may not, help :)

Neglecting subject movement, camera shake etc whatever you have focused on should be in focus and sharp.
Things nearer than the subject will be focused to a point slighly behind the sensor/film but as the light rays reach the sensor/film before they reach this focal point they will form little circles at the sensor/film.
Things further than the subject will be focused to a point slighly in front of the sensor/film so by the time the light rays reach the sensor/film they will have spread out again into little circles.
These little circles (known as Circles of Confusion) cause those things nearer and further than the subject to be out of focus.

However the human eye cannot see the little circles if they are very small so when we look at a picture there is an acceptable range of things in front of, and behind, the subject which we see as being in focus. This range of acceptable focus is known as Depth of Field (DoF).
If you have a large aperture the circles are larger than if you have a small aperture which blocks rays of light going through the edges of the lens and therefore creates smaller circles of confusion so a small aperture gives a greater DoF.

I did add a diagram to my earlier post but it gets very messy trying to show rays of light from different distances and at different apertures.

Have a look here instead http://www.dbp-consulting.com/Photography/CircleOfConfusion.html and have a play
Click to expand...


Thanks Kev and John for your help and links, I am, I think getting a better understanding of it.

Cheers. :thumbs:
 
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