The A520 has a range of: F2.6 - F5.5 whilethe S3 onlyhas a range of: F2.7 - 3.5. What is the reason for this? Does this mean that the A520 is more versatile in this aspect? I would guess that the A520 would be better at producing a more noticable DoF, correct?

Those numbers refer to the 'widest' aperture value available. Many zoom lenses will have a wider aperture value available at the wide end of the zoom but as you zoom out, the widest aperture value decreases.

So, for the A520, when zoom is set to widest value, the widest aperture value you can use is 2.6 (pretty good). But if you zoom out, the widest aperture value you can use is 5.5 (about 2 stops worse). At either end of the zoom you would be able to use a value of say f8 though.

So, the S3 at the long end provides you with a 3.5 (about 1.3 stops better than what the 520 gives you).

Typically you won't ever see the narrowest aperture available because it isn't as important.

I don't recall what the lowest aperture value these cameras allow you to shoot at, but I'm sure it's not that important as depth of field will be pretty large even at 5.6 on these cameras.



note: the best lenses maintain a constant max aperture value as you zoom. For instance,so whether you're at the min zoom or the max zoom you can still get, say, f2.8

Quite the opposite. These are the maximum apertures of the lens at each end of the zoom.
The A520 has f2.6 at it's wide angle but only f5.5 at full zoom. The S3 still has f3.5 at full zoom. The aperture can be made smaller (higher f-number) at any zoom so the S3 has a much greater aperture range.
f3.5 is a much wider aperture than f5.5 so the S3 would be capable of producing a much more blurry background than the A520 assuming (I haven't checked) that the two cameras have similar focal lengths.

Edit: lol, beat me to it.

jacks wrote: :-) I started to read your reply and my initial reaction was "what do you mean, quite the opposite - you're saying the same darn thing I was" then, of course my brain got the necessary coffee induced charge and figured it out :-)

Thats pretty funny Well, now it makes a lot more sense to me! Slowly but surely I am cracking the cryptic code of digital photography Thanks for the explaination!

For any kind of camera (digital, etc.) with zoom there are four significant aperture numbers:

Maximum aperture at minimum zoom,

Maximum aperture at maximum zoom,

Minimum aperture at minimum zoom,

Minimum aperture at maximum zoom.

Generally, the greater the zoom range, the greater theratio between the first two numbers.

The second aperture number (for f/stop)is alwayslarger than the first and the fourth aperture numberis generallylarger than the third. This is because, when the iris opening stays the same diameter-wise, the ratio of it to the focal length (f/stop)gets smaller as zoom is increased. Usually only the first two numbers are advertised but depth of field also depends on the last two (and also some manual influence over the aperture). Some digital cameras don't have an iris, relying on just shutter speed to control exposure, in which case the last two numbers don't apply.