Are you happy with your Canon digi cam?

Agreed, you're right. I didn't read your post carefully enough and never (until now, that is...) really bothered about how passive and active autofocussing work.

Regards,
Tomaz

By all means, F-values are F-values. But I remember a question on dpreview when somebody was asking why F-values on digital compacts are limited to low values only (up to around 8) while on SLRs they can go much higher. One of the answers was explaining the limitations imposed by a much smaller aperture (the diameter in absolute units) on small-sensor compacts in comparison to larger format cameras at the same F-value. I do not remember the details and I cannot find that thread, sorry...

Tomaz

PhoThomas said:

About F-values. I thought that F-values were F-values, but now I'm
not so sure. Somebody who can clear this up?

Click to expand...

tomjar said:

By all means, F-values are F-values. But I remember a question on
dpreview when somebody was asking why F-values on digital compacts
are limited to low values only (up to around 8) while on SLRs they
can go much higher. One of the answers was explaining the
limitations imposed by a much smaller aperture (the diameter in
absolute units) on small-sensor compacts in comparison to larger
format cameras at the same F-value. I do not remember the details
and I cannot find that thread, sorry...

Click to expand...

It's all down to optics and physics.

The f-number is simply the focal length divided by the diameter
of the aperture, and is a way of gauging the amount of light
which will fall on the sensitive medium (film, CCD etc...)

The actual focal length of the lens will depend on the film
or sensor size and the angular coverage desired- a small sensor
or a wide angle of view will need a shorter focal length.

Note that gigicam focal lengths are often translated to 35mm
equivalents, as that's what people are familiar with, when
relating focal length to angular coverage. The real focal length
depends on the sensor size.

Minimum aperture diameter is a physical dimension related to
the acceptable level of image degradation caused by the
effect of diffraction, which varies with the wavelength of
light (IIRC), this will set a limit on how small it can be.

So a lens with a longer focal length (as used for film, or an
extreme telephoto for example) might go down to f32,
whereas a digicam with a small sensor may be limited to f8,
but both with the same actual aperture diameter.

Phew!
I hope I've got the above right, but please reply with
corrections if not...

I do like my Ixus 40, despite its known limitations ;+)

--
Chris Eley (Oxfordshire, UK)

Yes, something like this. Thanks for clearing it out for us.

Regards,
Tomaz