> fraction of focal length, "constant" aperture zooms are really
> "constant ratio" zooms, aren't they?
Yes.
> Could that in any way
> explain the difference in viewfinder brightness asked about between a
> prime and a zoom?
In a constant-aperture zoom, the amount of light comming in should be the same. In a
variable aperture zoom, the light would dim a bit (I didn't notice it with my 3.5-5.6
zoom though).
> In a variable aperture zoom, if you dial in max aperture, and then
> zoom out, the aperture increases in steps, i.e 4, 4.5 5.6.
Because the aperture area is constant...
> and each
> step remains constant for a certain percentage of the zoom range.
No, as I said above, the aperture area is constant...
> This obviously means that you aren't getting precise f-stops
> throughout the range, which is probably part of the lesser cost of
> such lenses.
No, th fact that the aperture changes over the zoom range "masks" optical
short-commings if a 3.5-5.6 lens had been 3.5 all the way, the lense properties would
most likely be utter crap at the long end (the lens wasn't designed to be good at say
80/3.5, but "stopped" down to 5.6, it performs acceptable).
> In a "constant aperture" zoom how is this avoided? Or
> is it? Is there actually a continuous range of aperture available?
If the aperture area had been constant in a constant-aperture lens, then the lens
would be utter crap in the short end (because the resulting aperture would be
something like f/0.x, at wich only extremely few lenses are good).
> I'm sure someone will straighten me out if this is really
> misconceived!
There might be some errors in what I wrote (swapped big for small, long for short
etc.), but I think it's correct. I'm no expert on the subject, but I recal seeing this
explanation on rec.photo.equipment.35mm some time ago...
Corrections are welcome...
Snorre
*
****
*******
***********************************************************
* For list instructions, including unsubscribe, see:
* http://www.a1.nl/phomepag/markerink/eos_list.htm
***********************************************************
