Calculating Aperture-Values in relation to FL

On compacts zoon-lenses not only get wide but also speedier.

there are 28/2.0 and even 24/2.0 canons are in the pipe or already available.
I have a590is with 35-140. 35/2.6. its a good speed.
What will happen with my 35/2.6 aperture when i will get a 24/2.0-cam?
How do we calculate aperture -changing when zooming e.g. from 24 to 35mm?
I would rather like a speedy 35/2.0 than a 24/2.0.

If i use 28/2.0 and zoom to 35mm i have no advantage of the speedy 2.0-lens. i will still end at aprox. the same 2.5 or 2.6.
So what is the formular to calculate aperture in relation to FL? Thanks a lot.

Wrong forum for this question - try the canon talk forum http://forums.dpreview.com/forums/forum.asp?forum=1010
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If I knew how to take a good picture I'd do it every time.

Cameras automatically adjust for aperture changes linked to focal length changes (zooming)

The nature of lens design means that every modern lens is different.

In the olden days, the math was simple.

Aperture value is focal length divided by diameter of the opening.

So 50mm lens with a 25mm aperture is an f2 lens (50/25=2)

100mm lens with same diamter opening----- 100/25 = f4

But with a modern zoom, the length of the lens doesn't actually physically change by the amount of millimeters that the formula changes things (modifies the angle of view, gets more or less in the shot, etc.)

BAK

You could find an owner of the camera you want to get and ask him to zoom in a specified amount and then tell you what the maximum aperture is.

From what I can tell by playing with my G10, the change in aperture seems to have a linear relationship wth the change in focal length. This would make sense, given the mathematical relationship between focal length, physical aperture size and f stop.

-John

This is either a Canon talk or maybe a beginner forum issue, not really a pro thing. As BAK elegantly explained, f/ number already factors in the focal length. Now there are two types of zoom lenses (at least in terms of aperture) fixed f/stop and variable f/stop.

A fixed f/stop lenses will open the aperture wider at telephoto... so a 70-200 f/2.8 lens will have a 25mm diameter aperture opening at 70mm, but when zoomed to 200mm it will have a 71mm diameter aperture. The size changes but because the f/number is a ratio of focal length to diameter, the f/number (and the relative amount of light) stays the same. Very few compact cameras (if any) have fixed f/stop lenses, such lenses are usually bigger and more expensive.

Most compact cameras have variable f/stop lenses where the aperture has roughly the same maximum diameter at wide and telephoto settings. So the 5.8-23.2mm lens (using the actual focal lengths and not the 35mm FOV equivalents) is f/2.6 at 5.8mm and f/5.5 at 23.2mm. Doing the math that's 2.2mm at 5.8mm and 4.2mm at 23.2mm. So in this case the diameter is actually able to open a little wider at telephoto, but not as wide as if it was a fixed f/2.6 (where it would need to open 8.9mm... of course that would mean the lens would have to be significantly larger and heavier).

Because the aperture physically opens a little wider at tele than it does at wide, but not enough to keep a fixed f/ value, it's difficult to say, the lens might open a little more and more in a linear fashion or it might jump after it gets past a certain distance... the best bet is to determine by trial and error.

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~K