Bob Talbot wrote:
> Arrghhhhhhhhhhhh
>
> Not again!!!!!
>
> <RANT>
> An extension tube DOES NOT change the aperture.
> If it really was f4 before; it remains f4.
> </RANT>
Ok Bob, calm down. :-)
I wasn't writing in a technical sense (indeed, I made a half-hearted effort not
to get caught up in this issue). Since aperture is a function of focal length,
and an extension tube simply changes the focusing range of a lens, then
technically the aperture doesn't change. Right? Nonetheless, use of an extension
tube reduces the amount of light reaching the meter and the AF sensors by "X"
amount, typically 1/2-stop for the EF tubes (so in my example I should have said
f/6.7 rather than f/8). Therefore, while technically the aperture does not
change, for metering and AF purposes the "effective aperture" does change, since
less light reaches the sensors. Since with AF what matters is the amount of
light that reaches the sensors (no?), an extension tube could have an affect on
AF performance. Correct? No? I ask because in the 1V Technical Overview there's
a diagram that purports to show why vertical-line sensors at f/5.6 are superior
to sensors at f/2.8, and in it the AF sensor pairs are shown as closer together
in the f/5.6 example than they are in the f/2.8 example, so it would seem that
the physical spacing of the sensors as well as the amount of light reaching the
sensors has an affect. But since I didn't really understand what either the
diagram or the accompanying text (which I quoted last night) were attempting to
explain, I don't know whether this is the case or not.
Cheers,
fcc
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