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Old Dec 15, 2006, 2:32 PM   #1
radkol's Avatar
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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?
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Old Dec 15, 2006, 8:16 PM   #2
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12x zoom is a meaningless marketing number that shows the ratio between the longest and shortest focal lengths of the lens. In your case 72mm/6mm = 12. It has no other meaning and should be ignored.

Zoom lenses are very complex optical systems and certain performance limits are designed in for image quality purposes and economy. To allow the lens to open more than f/2.7 would introduce distortion and colour fringing problems. Correcting these problems would raise the cost of the lens substantially. Just look at the price of professional grade DSLR lenses such as the Nikon AF-S VR Zoom-Nikkor 70-200mm f/2.8G IF-ED compared to the Nikon AF Zoom-Nikkor 70-300mm f/4-5.6D ED to see what I mean.
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