Digital (Aperture) Fstop Query

[geriatric]

Tom LaPrise wrote:

Yep. Depth of field is a function of the actual size of the aperture. That's why smaller digital cameras usually stop at f/11 instead of f/22 (or even f/32)--the opening would be so small that diffraction would degrade the image, and you would probably be better off with a pinhole instead of a lens.

(I studied to become a teacher, but my multi-tasking "abilities" couldn't keep up with second-graders!)

Yes the aperture does contribute to the DOF, but the main contributer is the distance.

[Tom LaPrise]

I hadn't thought of that. I should have said, "For a given distance..." Really close-up/macro shots, even at f/16 on my camera, can have less than 1/8" DOF, and I have a shot of a butterfly taken from about 4 feet away with decent blurring of the background just inches behind. (I don't have a copy with the EXIF data with me right now, so I can't tell what the aperture was.)

[JohnG]

DOF is controlled byseveral factors:

sensor size, focal length, aperture and distance to subject. They all play a part in the equation.

Also, while DOF is controlled by those factors, background blur can also be aided by distance between your subject and the background. So, while DOF may be 2 feet - how blurred the background looks is also dependent upon how close that background is to your subject.

But, back to DOF. Here's a handy on-line calculator that allows you to compute DOF:

http://www.dofmaster.com/dofjs.html

Want to see how they all play a part, simply change some of the parameters. When doing macro work, distance plays a huge part. But if you're doing other types of work - say sports, then focal length, sensor size and aperture becaome greater factors because you're not as close (i.e. going from 6 inches to 24 inches makes a huge difference but going from 30 feet to 35 feet does not).

Here, also, is a good write-up on depth of field and the circle of confusion:

http://www.luminous-landscape.com/tu...ries/dof.shtml

[radkol]

Tom LaPrise wrote:

OK--first, the aperture vs. zoom thing:

Aperture setting is effectively the lens diameter divided by the focal length. (You're not changing the actual diameter of the lens, though, when you change the aperture setting; you're changing the diameter of an opening of an iris inside the lens assembly.)

You're probably aware that an aperture setting is written as f/some number, with f being the lens' focal length setting. For f/3.5, the iris is set so that the effective diameter of the lens is your focal length divided by 3.5. At f/8, the opening will make your effective lens diameter focal length divided by 8. As you can see, since it's a fraction, if you make the denominator smaller, the iris opening gets bigger.

There is a physical limit to how big that iris opening can get because the parts that make up the iris (often, metal or plastic "leaves" that pivot at one end and overlap) have to fit inside the lens barrel. The smallest number shown for your available aperture settings represents that limit.

Since the aperture setting is a ratio of effective lens diameter (controlled by the iris diameter) to focal length, as you zoom in and increase the focal length, to keep the same aperture setting, the iris has to open up to compensate. When the iris is open as far as it can be, a longer focal length will result in a smaller aperture setting (larger number, i.e. f/5.6 instead of f/3.5) because the iris can't widen any further as the focal length gets longer.

If you're at a medium focal length and set the aperture to a medium setting (let's say f/5.6), as you zoom out to wide angle (shorter focal length), the camera will automatically make that iris opening smaller to keep the aperture at f/5.6; as you zoom in (longer focal length), it will make the opening larger to compensate. Once you reach wide-open or fully closed, though, the camera will not maintain the aperture setting at f/5.6.


With that out of the way, to blur the subject while keeping the background focused, you could aim at the background, press the shutter halfway to focus, keep the button pressed halfway to keep that focus setting, then aim at the subject and press the shutter the rest of the way. If your subject is a lot closer to you than the background is, the subject should be pretty blurry, especially if you use a large aperture (low f/ number, like 3.5).

Hi, thank you for explanation but may be i miss someting.
FOr example if I understood right it sould be the following:
I have Canon S3IS;
Lens: 6.0-72mm f/2.7-f/3.5

So in shorther focal lenght and f/2.7 I'll have diameter = f/2.7 = 6/2.7 =2.22
But on largest focal lenght 72mm i have f/3.5, so diameter = f/3.5 = 72/3.5 = 20.2

I'd like to if the diamater can be so opend 20, why we have only 2.22 in short focal lenght?
If the formula is so simple then with x12 zoom the AV should be 12 times bigger in shortetst FL.
What about the lens that keeps AV fro whole FL range?

Please explain me what I have missed?

[ac.smith]

I suspect that if we measured the diameter of the front element of the example f2.8-3.5, 6mm-72mm lens it would be 20mm in diameter and that is the ultimate physical limit for the "speed" of this lens. While the diameter of the front element would theoreticaly permit an f0.3 lens at the 6mm setting it's not a physically realizable piece of glass. That is we can't practically produce the curvatures required to produce a sharp image across the entire picture area for all colors at 6mm and still have it work at 72mm. The fastest general purpose 35mm film lens ever produced was the f0.95 50mm that Canon produced for both their rangefinder and SLR 35s. In our example the iris is located at a point in the optical path that produces a near constant aperature (optical diameter) regardless of the focal length.