Aperture Question

[mikeyjm26]

I have a bit of a techinical question and hope someone can answer it.

How does an image become sharper when you make the aperture smaller? If less light is getting through how does the dof become clearer?

Many thanks

Mikey

[Lowner]

Mikey,

It does not as far as I'm aware. Now if you are thinking of DEPTH OF FIELD thats a different story. The aperture has a direct bearing on that without question.

Lots of other things will affect how sharp an image appears. Things like camera movement (hand-shake), too slow a shutter speed if hand-held, quality of lens, light quality, image contrast (or lack of) etc, etc.

What exactly are you trying to shoot? and whats the problem?

Richard

[shutterfiend]

1. The image doesn't get any sharper, only more of it appears to be in focus.

2. By limiting angle of incidence you're limiting the circle of confusion over a greater depth. Hence objects at greater depths appear to be in focus.

2A. You need the same amount of light to expose the same scene regardless of aperture. Of course, the smaller the aperture the longer the duration of your exposure (shutter speed) needs to be.

[mikeyjm26]

Richard,

there isn't a problem, I am just trying to understand how when you shoot at F4 you get dof and when you shoot at F22 you don't. How does changing the size of the aperture affect the dof?

Thanks

Mikey

[René Damkot]

The technicalities: Click (http://www.vanwalree.com/optics/dofderivation.html).

[shutterfiend]

dof= depth of field.

f4 (larger aperture) gives you a shallower depth of field than f22 (smaller aperture).

There are other factors that come into play at f22 that may reduce sharpness.

[Wilt]

It does not as far as I'm aware.
The image doesn't get any sharper, only more of it appears to be in focus.

Lenses DO GET SHARPER when you begin to stop down the aperture from max...typically highest resolving power of the lens is about 2-3 f/stops from wide open! While a lens' aerial resolution is max at max aperture, resolution on film in an optical system will typically show better MTF 2-3 f/stops from max aperture

[carianoff]

The technicalities: Click (http://www.vanwalree.com/optics/dofderivation.html).


...Mmmmath -_-

[Jon]

You want a graphic example, try:

http://photography-on-the.net/forum/attachment.php?attachmentid=51954&d=1138043675
The cyan lines show the paths of light rays through a lens from a single point in your subject at large apertures; the dark magenta ones are at a smaller aperture. And any point that's not at your subject's plane of focus will leave a blur proportional to the angle of the incoming light rays. So at smaller apertures, the cone of light coming through the lens is much narrower; the picture looks sharper.

[shutterfiend]

I have a bit of a techinical question and hope someone can answer it.

How does an image become sharper when you make the aperture smaller? If less light is getting through how does the dof become clearer?

Many thanks

Mikey

Lenses DO GET SHARPER when you begin to stop down the aperture from max...typically highest resolving power of the lens is about 2-3 f/stops from wide open! While a lens' aerial resolution is max at max aperture, resolution on film in an optical system will typically show better MTF 2-3 f/stops from max aperture

Wilt, I don't believe the OPs question had much to do with maximum aperture.

[Wilt]

Wilt, I don't believe the OPs question had much to do with maximum aperture.

He did state, "How does an image become sharper when you make the aperture smaller?" And the image qulaity does become higher in delivered resolution as you go from max aperture to around f/5.6 typically, then after peaking the resolution does start to decrease. So what other circumstance should we be trying to answer?!

[Jon]

Well, he did go on to ask If less light is getting through how does the dof become clearer?

[Wilt]

Well, he did go on to ask


So, OP, were there two questions posed about single topic sharpness?

Or were they two difference questions...one topic about improvement on sharpness, and a second topic about improvement in DOF?

[bomberman]

At a small aperature (f2) the lense opening is very big. This allows a light from the subject to enter the lense at many different angles which are focused onto the sensor. Light from objects that are farther than the subject will come in to the lense at shallower angles, and will be focused in front of the sensor. By the time the light from these objects reaches the sensor it is out of focus. Light from objects that are closer than the subject will enter the lense at steeper angles, and would be focused behind the sensor. So when the hit the sensor, they are still out of focus.

With a larger aperature (f22) the lense opening is very small... This means that light entering the lense from the forground or background will be more or less parallel to light from the subject, and will be focused at nearly the same distance from the lense (very close to the sensor) so the image on the sensor appears in focus.

A good diagram would go a long way in explaining this.

[shutterfiend]

He did state, "How does an image become sharper when you make the aperture smaller?" And the image qulaity does become higher in delivered resolution as you go from max aperture to around f/5.6 typically, then after peaking the resolution does start to decrease. So what other circumstance should we be trying to answer?!

dof= depth of field.

f4 (larger aperture) gives you a shallower depth of field than f22 (smaller aperture).

There are other factors that come into play at f22 that may reduce sharpness.

Lens construction and diffraction do play their parts in image sharpness, however, such information may clutter understanding of basic concepts, IMO.

[mikeyjm26]

I think basically the way it works is that if you have an aperture of f2 is is allowing the light from the object to be in focus where as if you have an aperture of f22 you are allowing light from the whole to be in focus. I understand it in my head but can not write it in words.

Thanks for all your help guys.

Mikey