I don't understand...

fadingaway1986 · Jan 7, 2005

Aperture.

It confuses me...

Okay so I know its opposite (smaller number = larger opening)

So if I want a whole photo in focus, why do I put it at the biggest number, therefore making the hole smaller... Wouldnt that just make less light get in around the edge - therefore making everything funny?

I don't really need to know...

... but its been bugging me...

Someone save me from the aperture!

Rob A · Jan 7, 2005

im not 100% sure, but i think its the way the light focuses on a spot on the film. if its a smaller aperture (smaller opening) the light defracts less and therefore makes a much sharper image on the film.

its got something to do with the focal lentgh, or hyperfocul lenght i think. sorry if im wrong!!

ferny · Jan 7, 2005

I'm going to get very technical here, so look away if you're afraid of big complicated words.

It's all to do with the way light reacts and bends when going through a smaller hole. That's all I know and can say. I'm interested to see what the people in the know can tell us so I can go "oh, really? That's nice to know" then forget it.

Oh, you can look back now.

LittleMan · Jan 7, 2005

Ok..... technical answer......

Why?

Suppose the lens focuses on the yellow dot as shown in the figure below. This subject generates a yellow dot on the image plane. Once focused, all subjects that have the same subject-lens distance as that of the yellow dot will appear sharp. Now, consider a white dot that is behind the yellow dot (i.e., with larger subject-lens distance). Since it is out of focus, it will not produce a sharp white dot image. Instead, its image is formed somewhere in front of the image plane. On the image plane, the image of this white dot is a circle as shown below. This circle is usually referred to as a circle of confusion. As the subject-lens distance increases, the size of this circle increases. The same holds true for a subject in front of the yellow dot (e.g., the green dot in the following figure). Since these circles of confusion are actually out of focus images of subjects, if we can reduce the size of circle of confusion, we can increase the sharpness of the resulting image. But, how?

It turns out to be very simple. Since circles of confusion are formed by light rays passing through the lens tube, the size of a circle of confusion is proportional to the amount of light that can pass through the lens tube. This means smaller (resp., larger) circles of confusion will be formed if less (resp., more) light can pass through. Restricting how much light can pass through the lens is the function of the diaphragm in the lens tube that sets the aperture values. Therefore, a smaller aperture means a smaller diaphragm opening, which, in turn, means allowing less light to strike the film/CCD plane. Thus, we have smaller circles of confusion and, as a result, a sharper image!

If I lost you..... just read a science book.....

LittleMan · Jan 7, 2005

Here is another way to explain it.... it's a very simple way.... but it gets the point across....

The closer the two black lines are together.... the more in focus the subject is(red lines)
So, the larger the opening is(the lower the f-stop), the farther apart the "black lines" are, so it can't focus on subjects that are farther away.... it's actually pretty simple once you think about it...

BernieSC · Jan 7, 2005

Its a fraction. The larger the number the smaller the opening and vise verse the smaller number the larger the opening. 1/2 is bigger than 1/4

fadingaway1986 · Jan 8, 2005

Riiiiiiiiiiiiiiiggggggggggggggggggggghhhhhhhhhhhttttttttttttt

I'm still lost. But there is a lot of noise in this house right now. I might try again when it isnt as noisy...

Thanks guys

walter23 · Jan 8, 2005

LittleMan said:
Here is another way to explain it.... it's a very simple way.... but it gets the point across....

..

The closer the two black lines are together.... the more in focus the subject is(red lines)
So, the larger the opening is(the lower the f-stop), the farther apart the "black lines" are, so it can't focus on subjects that are farther away.... it's actually pretty simple once you think about it...

Your diagram doesn't make any sense, in my opinion... If you look at the first diagram posted I think you can understand this a bit better. Larger apertures increase the size of the 'circle of confusion', which means that an object must be "more in focus" to be acceptably sharp. Acceptably sharp basically would mean that with the resolution of your film or sensor you can't tell that it's slightly out of focus.

Because of this, a larger aperture results in smaller depth of field. Only a small range of subject distances will be clear, because if something is too far away from the point you've focused on it will be less resolved than the resolution of your film / CCD, so it will look blurry. With a smaller aperture, the circles of confusion are smaller for objects that are further and closer than the point you've focused on, so the objects can be considered acceptably sharp even though they might not be at a larger aperture.

Also, I've heard that diffraction can reduce image quality at really small apertures like f/22 or higher. Sort of like this:

http://www.physics.montana.edu/demonstrations/video/6_optics/demos/pinholediffraction.html
http://homepage.mac.com/whitby/.cv/whitby/Public/Octahedron/pinholeDiff.pdf-link.pdf

Corry · Jan 8, 2005

I think an easier way of explaining the diagram would be...um..like comparing the beam of a flashlight to the beam of a laser pointer.

LittleMan · Jan 8, 2005

core_17 said:
I think an easier way of explaining the diagram would be...um..like comparing the beam of a flashlight to the beam of a laser pointer.

I think everyone has their own way of understanding..... your way doesn't make ANY sense to me.... lol
But I'm sure it's correct in your mind..

Hertz van Rental · Jan 8, 2005

'Circles of confusion' are more to do with the resolving power of the eye than anything else.
In optics an image is either in focus or it isn't.
The human eye has only a limited resoving power - due to various physiological reasons. In general it can only distinguish betwween two points if they are more than 0.25mm apart when viewed at a distance of 250mm.
This means that we will accept a degree of 'fuzziness' in terms of image sharpness. This amount of fuzziness is affected by such factors as focal length of the lens, aperture size and quality of lens.
The degree of enlargement of the negative is also a factor, which is why some images look sharp on the contact sheet but prove to be 'soft' when enlarged.
The physics (and maths) of all of this can be very complex. If you don't believe me I can post the depth of field calculations - and then I will start talking about depth of focus! It gets even more complicated when you realise that the aperture itself can start acting like a lens - this is the basis of the pinhole camera.
It is best to just take it all on trust. :wink:

fadingaway1986 · Jan 9, 2005

walter23 said:
LittleMan said:

Here is another way to explain it.... it's a very simple way.... but it gets the point across....

..

The closer the two black lines are together.... the more in focus the subject is(red lines)
So, the larger the opening is(the lower the f-stop), the farther apart the "black lines" are, so it can't focus on subjects that are farther away.... it's actually pretty simple once you think about it...

Click to expand...

Your diagram doesn't make any sense, in my opinion... If you look at the first diagram posted I think you can understand this a bit better. Larger apertures increase the size of the 'circle of confusion', which means that an object must be "more in focus" to be acceptably sharp. Acceptably sharp basically would mean that with the resolution of your film or sensor you can't tell that it's slightly out of focus.

Because of this, a larger aperture results in smaller depth of field. Only a small range of subject distances will be clear, because if something is too far away from the point you've focused on it will be less resolved than the resolution of your film / CCD, so it will look blurry. With a smaller aperture, the circles of confusion are smaller for objects that are further and closer than the point you've focused on, so the objects can be considered acceptably sharp even though they might not be at a larger aperture.

Also, I've heard that diffraction can reduce image quality at really small apertures like f/22 or higher. Sort of like this:

http://www.physics.montana.edu/demonstrations/video/6_optics/demos/pinholediffraction.html
http://homepage.mac.com/whitby/.cv/whitby/Public/Octahedron/pinholeDiff.pdf-link.pdf