I've been thinking, for low light work, about the Sigma 18-35/1.8 versus
the Pentax/Tamron 24-70/2.8.
With the crop factor of 1.5x, in their native modes, the two lenses have
approximately the same angle of view.
My understanding of depth of field is that you get approximately the
same depth of field at 27 mm at f/2.8 as you do at 18mm at f/2.0.
In other words, to a first order approximation, with a "one stop larger
sensor", and stopped down by a stop the 24-70 will have *close* to the
same angle of view and depth of field as the 18-35.
In order to shoot at the same shutter speed, stopped down by a stop, you
will need to increase the ISO sensitivity by a stop. When you do this,
your per pixel noise increases by about a stop (3dB) and your per pixel
dynamic range decreases by about a stop. However, you have twice the
number of pixels (area of silicon) to use, so that when you process your
photo to the final size, your effective noise and dynamic range will be
approximately the same.
In short, when the dust settles, and you look at the final images, my
guess is that the results with the two lenses will be very close,
despite the Sigma being 1.3 stops faster.
Note, that in about a month, someone could actually put this to the test.
If my hunches are reasonably accurate, then the differences between the
two come down to second order differences, that the Sigma is 1.3 stops
(4dB) faster, the Tamron has a 2.5 (about 1.3dB) greater range, size,
cost and weather sealing.
--
Larry Colen [email protected] (postbox on min4est) http://red4est.com/lrc
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