Digital (Aperture) Fstop Query

[AlfaBeta]

Hi,
I recently bought the Canon A520 Powershot. I would like to know if it is possible to make the background in focus, while the subject is blurred?

I am familiar with the Portrait setting on the dial which will do the opposite.I am also familiar with the AV setting which gives you a bit more control over the f stops (It seems the more you zoom in, the higher the Fstops will start from…can someone explain this phenomena??)

I will probably experiment later today, but thought I might try to get some tips.
Thanks heaps…

Regards
Alf….

[Tom LaPrise]

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).

[Curmudgeon]

Tom,

Very nice explanation. If he/she will not thank you.. I will. Great explanation.

Bill

[jamsden]

You explained that better than my Photography professor

[Caboose]

Tom explained this very well. The problem you are going to have is your camera because of the sensor size has a very large depth of field, even with your lens wide open (smallest f/number). I'm not sure what that wide open f/number is on your camera, but lets say it is f/2.8. Because of your sensor size that is the 35mm equivalent of about f/11 which is a pretty large depth of field. You will need quite a bit of seperation between your background and your subject to get one or the other burry and the other sharp.

[Tom LaPrise]

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!)

[saturndude]

So if my lens starts out at 6 millimeters focal length, and it is the equivalent of 36 mm, then my sensor is about one-sixth of the size of a piece of 35mm film, right?

(my model's sensor = 1/2.5)

So the largest aperture (smallest number) possible at 36mm (printed on plastic at the end of the lens) needs to be multiplied by 6 to get the "true" aperture? So for me, 2.7 times 6 equals 16.2, or about f-16 on the old film camera, right?

Is this pretty much the case?

If so, getting a close subject out of focus will be hard and sometimes impossible, depending on the situation. And getting a background out of focus might only be possible in indoor portraits, where I can control things.

Do you think things will get better in time? Will larger, less noisy sensors drop in price, like lasers for compact disc players did?

--
REMEMBER 1. Preserve Nature. 2. Always wear a helmet.
3. Ride safely. 4. Read owners manual carefully before riding.

[geriatric]

Yes we know, we were discussing the main factors . It is derived from the hyperfocal distance. FL^2/(A * C of C)/1000

[JohnG]

geriatric wrote:

Yes we know, we were discussing the main factors . It is derived from the hyperfocal distance. FL^2/(A * C of C)/1000

I'm going to assume you addressedin response to my post(forum is behaving strangely).

The problem I have is your statement:

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

It's a misleading and overly simplistic statement. Take the OPs camera and set to F11 - now take a shot at a subject 5 feet away and then 20 feet away - how much change is there in the blur of the background (what the OP was concerned with)?

The answer - not a heck of a lot. Why? Because aperture, focal length and sensor size are still in play. So suggesting that distance is the main driver is too simplistic of an answer. Only in certain circumstances is it the main driver.

[geriatric]

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).

I have never read such a load of gobbledegook in my life. From the first silly error (a/fl) to the iris mechanically opening up as you zoom. As you look into the lens say at f16 then zoom the lens, it would seem the aperture gets bigger. That is caused by the magnification . Now look thru the other end and repeat the operation, it stays the same.It is the effective diameter of the aperture on the front lens .