> Since the 35-80 is my favorite lens on the A7, I'd welcome some
> illustrations of your point. Is outperforming about more then better
> corners (I understand why smaller sensors have a benefit, not why they
> would 'outperform')? And at which F stops?
I suppose it is all relative. Last night, we attended the high school
Christmas concert and I took pictures with the 35-80 on the 6D. The
resulting images are as "pixel sharp" as they come. Corner to corner.
I chose to shoot at F4 and F5.6 to optimize that corner sharpness as
well as DoF control. As high-ISOs are a non-issue with the 6D, having
it shoot at 6400 is child's play. I did do in-camera JPEG
(largest-best quality) and the noise-reduction took care of the
cooties.
I'm also filming our church services with the 35-80 on the 6D every
Sunday. But have been starting to shift over to the 100mm lenses as
the reach is a little better. What I'm finding is that the 35-80
doesn't snap into focus (even on live-view) like other lenses. With
the 100/2.8, if I have edge-detect turned on, you can sweep the
marching ants across the entire scene if you want, but with the 35-80,
not so much. But when you zoom in, it's still sharp, but there is a
loss of contrast which the eye (and edge-detect) is sensitive to. The
100/2 is better at pixel-level contrast than the 35-80, and the
100/2.8 is better than the 100/2.
The 35-80 is an unusual beast of a lens because the exit-pupil is so
massive. While it isn't an F2 lens, it seems to interact with the
sensor like an F2 lens. With most lenses, as you stop it down, the
apparent exit angles reduce from all the glass to just a pinpoint view
of the glass. (not sure this makes any kind of sense), but the 35-80
is odd in that when you stop it down, it acts like the amount of light
is reduced coming through the lens, but the projection from the rear
element remains the same.
A comparative would be the standard Double-Gauss 50/1.4 lens.
Wide-open, the lens is scattering light every which way into the
individual sensels and you get a lot of smearing, as well as
vignetting. But when you stop it down, the smearing goes away as it
behaves at F5.6 like an F5.6 lens. The 35-80, at 50mm and F5.6 behaves
like a 50mm at F2.8.
I do believe this has everything to do with the microlens and
filter-stack over the sensor itself. Traditionally, the microlenses
were used to direct/redirect the light path to make it perpendicular
to the sensor surface. This is why they are offset as they get farther
away from the center. But Canon (and others too, but Canon
specifically so) has designed the microlenses to effectively increase
the QE of the sensor optically. Instead of only, say, 40% of the
sensor surface dedicated to light-gathering, with the microlens, they
are able to get closer to the ideal 100% of the light going towards
the sensel to be captured by the light gathering portion of the sensel
itself. If you can't increase the capturing area of the sensor to be
100%, you can at least optically make it so. The microlenses are just
like the magnifying glass you used as a kid to set fire to ants on the
sidewalk.
At issue is that with the magnifying glass, the exact alignment of the
sun, the glass and the ant was important, as well as the focusing
distance. Chasing the ant meant changing the glass position. Going off
access meant no roasted ant. The position of the capture portion of
the sensel is just like that ant. The light source, lens and sensel
have to be precisely aligned to be perfectly effectively. When it is
out of alignment you get vignetting towards the corners of the image
area. This is why they use offset microlenses.
HOWEVER, with a large-diameter exit pupil, you have light going
towards the sensor at angles other than just the ideal center of the
exit pupil. At some point, this contributes to obvious vignetting.
With the 6D, that obvious vignetting is there with apertures wider
than F2.8. F2.8 is right at the threshold. Yet, while the vignetting
(or even corner smearing) may not be visible at F2.8, these
super-bright lenses have less local contrast than the pedestrian
versions.
While I still haven't gotten a complete handle on the E-3's sensor in
regard to these issues (the 14-45 and 40-150 live on it most of the
time), the E-1 and L1's sensors don't have the same issues (even at
the same physical distance from sensor center as the 6D, not comparing
corners, which is impossible when the sensors are so vastly different
in physical size). The local contrast with the E-1 and L1 remains
higher than the 6D for bright lenses, the same or less so with the
lower grade lenses.
But, as mentioned before, the 28/2 works on film and the 6D, but not
the 4/3 sensors because of center hot-spotting.
There is one other thing I've encountered and that's physical
obstruction with the exit pupils by the mirror-box design itself. That
contributes to some vignetting, as well as significant
light-scattering.
AG
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