aperature

[cypilk]

i was reading the other post earlier and i was wondering if changing the aperature changes the focus ... or depth.. or something..

because i always thought that aperature changes how much the shutter opens..either smaller or bigger depending on the f-stop.. ( oh..and is f-stop a unit for aperature?)

anyway... what else does aperature do..besides let more/less light in?..

 

[Cuervo79]

well when you change the aperture you change the size of the iris in the lens, the shutter is what controls the exposure time to the film, the aperture not only controls how much light comes in but also controls the depth of field, lets say you were taking a portrait and you want that clasic subject in focus and background out of focus, you make the aperature as low as possible say 4.0 and down, now there is another way you can use the depth of field and make the subject and the background in focus you close the aperture 8 and on focusing your subject first...

 

[drlynn]

The aperture of the lens is the opening through which light passes. Making the aperture smaller lets less light in, as you were saying.

Making the aperture smaller also increases the depth of field. This is the distance in front and behind the subject that will be in focus. For f/1.8 on an 85 mm lens, the dof may be 1 foot or less at a focusing distance of, say 5 feet. At f/22, the dof at the same focusing distance may be from 3 feet to 7 feet. Anything within that field will be in reasonably sharp focus. In front of or behind that field, objects become blurry.

A related term is hyperfocal distance. In practical terms, this is the closest focusing distance at which objects out to infinity are in focus. To use the same 85mm lens, at f/22 the hyperfocal distance may be, say 15 feet. So if you set the focus at 15 feet, then everything from, say 8 feet out to infinity will be in reasonable focus. (all measurements are completely random, so don't quote them. )

Hope this helps!

 

[Dweller]

When I was recently shopping for my first camera I stumbled on a site that had a wonderful interactive demo of Depth of Field. You could set the aperature and click a button and the photo would adjust based on the selections.

I have since tried to find that site and have had no luck. I have found on similar to it, but it does not have as many examples as the first page I found. I will keep looking and when I find it again I will post it here. It was a great help in understanding the effects of DoF.

 

[cypilk]

so...when the aperture opens and closes...is it in a circular opening?..like..a ring that expands and contracts?..

how does a bigger "ring" make a larger dof?.. ..yeah..lower aperture is higher dof.. and vice versa.. but how is that possible?.. mechanically?

 

[drlynn]

I'm sure someone here can explain the physics to you.

Not me, tho. It's been WAY too long since my last physics class!

It has to do with the size of the hole in the iris of the lens. You can test it for yourself with a piece of paper and a sharp pin. The smaller the pinhole you put in the paper, the more clearly focused the object you're viewing through it.

It's the same principle as squinting. When your vision starts to deteriorate, sometimes it helps if you squint when you're trying to focus on something.

 

[ksmattfish]

I don't know the physics but I'm sure there are many websites...

It's why folks sometimes need glasses to drive at night, but not in the day. In the day the bright light keeps your pupil small (f/11+) and you have good DOF, but at night your pupils open up (f/5.6-) and your DOF decreases.

 

[cypilk]

heh..it makes sense... how about the actual opening?.. is it a ring? or a square..or something?

 

[Mr.ReDEyE]

its just like a pupil.....circular

 

[cypilk]

why can't you get in between some numbers.. like..sometimes it jumps from 5.6 - 7?....

 

[Mr.ReDEyE]

the basic apertures are--1.4--2--2.8--4--5.6--8--11--16--22--32.....those are all full stops....moving to the right lets in half as much light and moving to the left lets in twice as much light.....some cameras let you set half stops which are between those numbers but letting any more or less light in than that wouldn't make a difference......

 

[markc]

heh..it makes sense... how about the actual opening?.. is it a ring? or a square..or something?

It's overlapping blades, like this:

More info here.
The more pieces used to make the iris, the rounder it is. Cheaper lenses use six. Better ones seven or nine.

 

[ksmattfish]

heh..it makes sense... how about the actual opening?.. is it a ring? or a square..or something?

It is an iris with many blades so it is usually round, at least in most quality lenses, but I have seen square iris (4 blade) in cheap enlarger lenses.

You can get inbetween numbers, but they are standardized so that you don't have to do math in the field. Of course this was developed long before they could put a computer in your camera. Modern cameras, especially with zooms, often show an "odd" f/stop in the display.

f/# equals focal length divided by aperture diameter

The standard f/stops are 1 stop apart (many lenses can be set halfway between these for a half stop). Each stop is a halving or doubling of the light. Traditional standard f/stops are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 60, 90...

 

[DocFrankenstein]

If you remember a basic physics optics textbook, there are pictures are something like this:

This is what happens in the camera, but of course not all of the light rays are parallel. Light rays are coming off from all directions and what happens is because of the way the lenses are constructed, there are many many light rays. Some of them are not useful and only create distortion and blurred images. It so happens, that the further away the light rays are from the middle axis at the focal point (hope that makes sense) the more distortion they create. Basically the rays that are exactly on the axis on the focal point will create an ideally sharp image, but will also get forever to capture enough light. The light rays that are far away, will get you distortion.

The small iris (or high apeture) will bacially cut off those rays that create distortion and artificially will increase the depth of field of the camera.

Cheers