who's good at math?
 

Putting aside for the moment the argument of whether slim DOF can be achieved with small chip cams, consider the following

1 DOF gets slimmer with increase in focal length
2 DOF gets slimmer with increase in aperture size


Most lo-end cams reduce maximum aperture with increase in focal length. eg 1.6 wide to 2.4 tele

Therefore there must be a sweet spot on the zoom scale or focal length where the combination of aperture size and zoom combine to give the optimal point for minimum DOF.

Anyone know how to calculate this?

You would need to know the other factors in the DOF formula. But if subject size stays the same, the DOF will stay the same. You may want to read this article or post in this thread.

from your replies it would seem that the answer to my question is the last point on the zoom where the aperture is still maximum - is it not?

Yes, but the aperture continuously changes as the focal length of the lens changes. This is a result of the zoom and the optics used, not the lens opening (diaphragm) actually getting larger or smaller.

yes Jeff that was my understanding too, until I noticed my cam actually stops down at full telephoto, hence my question

thanks

DOF depends on these 3 things: the lens aperture, the lens focal length, and the distance in which the lens is focused. A wide angle lens creates a much larger depth of field. A tele-photo lens has a much smaller depth of field---hence, smaller DOF with zooming in. :)

John, what do you mean it actually stops down? Your looking in the lens and can see the aperture actually get smaller?

yes Jeff,

aperture physically begins to shut down at about 60% zoom

I guess I have found the sweet spot at 60%

The manufacture may do that to help with sharpness or optical defects.

What camera is this? When zooming there are a lot of lens elements moving around. In my XL1 16x there is no trace of this effect.

this is the case with my sony cams

to see if your lens behaves the same do the following

switch to manual

set speed to 1/10000 to protect the CCD

shine a maglite down the lens when at full telephoto - full zoom

you may see the aperture closed down from its wide open position