Vesa Perala wrote:
>
> Referring to earlier discussion if Canon f3.5 and f4.5 are half stops
> or not (even though as *values* they are not):
>
> It seems they *are* half stops, when EOS-3 is set to display half-stops.
> Actually based on my measurements I'm sure of it.
>
> If EOS-3 is set to show 1/3-stops then 3.5 and 4.5 naturally are
> one-third steps.
>
Yes, it's the same with the 1N.
[...]
>
> This showed quite accurately that the aperture was jumping in 1/3
> stops.
>
> Then I repeated the same for half stops (CF5=2 I think) 2.8, 3.5,
> 4, 4.5 and 5.6 and again it was quite apparent that the aperture
> jumped with half stops.
[...]
>
> Any comments in method or results?
I think it was a well thought out test, probably as good as
you can get without sophisticated equipment.
> In my opinion everything was as expected.
Yes, just as expected. I have no doubt, that INTERNALLY the values
of f/3.5 and f/4.5 are handled as real half stops when the camera
is set to use half stops. So, with a lens that is faster, like the
35/2, there are no more mysteries ;-)
But what about the 28-135 f/3.5-5.6 at 28mm wide open? What real
speed did your meter readings indicate? Was it slower than
"f/2.8 + 1/2 stop" or slower than f/3.5 (real, numerical value)?
Anyway, 1/4 stop slower than marked is nothing special. A lot of
lenses are speced that way. I've seen tests where the lens was 2/3
of a stop slower than the specs wide open. Typically superzooms at
the long end ... And that's not light falloff in the corners,
that's the result for the central part of the image.
Back to the f/4.5-xx lenses. While its easy to imagine what will
happen if you tell a f/3.5 lens to open up to f/2.8 +1/2 (it will
be wide open), what will happen if you tell a f/4.5 lens to stop
down to f/4 +1/2 (which means an f/number > 4.5)? Will it be
wide open or closed down a little? Hmm, where's that
100-300 f/4.5-5.6? ;-)
Thomas Bantel
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