Photodo ratings really need to be taken with a grain of salt. For example,
the highest rated lens on their site is the Canon 200/1.8. But the rating is
based on MTF as measured at f4 and f8. If I'm spending megabucks on a
200/1.8, it's performance at f4 and f8 probably aren't of utmost importance
to me. I want to know how it does at f/1.8 and maybe f/2.8. I expect a good
performance from a lens like this when stopped down.
If you look deeper than the simple ratings, things get more interesting. For
example, I took a look at a few 90mm macros. Our beloved 90/2 Zuiko, of
course, the Tamron 90/2.5, Tokina 90/2.5 ATX, and Sigma 90/2.8. Here's a
summary of the numbers:
Zuiko Tamron Tokina Sigma
rating 4.2 4.2 4.6 4.2
MTF f/2 0.76
MTF f/2.8(2.5) 0.78 0.68 0.77 0.72
MTF f/4 0.82 0.81 0.85 0.80
MTF f/8 0.84 0.85 0.87 0.86
MTF 10 lp/mm 0.90 0.91 0.92 0.90
MTF 20 lp/mm 0.80 0.81 0.84 0.80
MTF 40/lp/mm 0.63 0.60 0.68 0.61
OK, what does that all mean? Well, first off, the Zuiko definitely is the
best performer at f/2 ;-) In fact, it measures better at f/2 than the Tamron
and Sigma do at f/2.8, The Zuiko is the best performer of them all at f/2.8
("performer" of course meaning performer on the optical bench, not
necessarily in making photos).
Stopped down to f/4 or f/8 the Tokina really shines, which explains it's
higher rating (which is based on the f/4 and f/8 numbers). The Zuiko is much
more consistent across the aperture range (a characteristic that other Zuikos
tested at photodo seem to exhibit as well).
The numbers showing MTF at specific lp/mm demonstrate (at least the way I
understand it - I am by no means and expert here, so all you experts please
correct me if I'm wrong) how well the lens will perform when images are
enlarged, higher lp/mm numbers relating to bigger enlargements. If you liek
to make big enlargements look at the 40 lp/mm numbers. Again, the Tokina
shines, but the Zuiko is better than the other three.
In this case, it would seem to me that the Zuiko measured noticeably better
than the Tamron or Sigma, yet received the same 4.2 rating. The Tokina does
measure teh best here, however.
Now how these numbers relate to real worl photography is another matter. Can
you spell "bokeh"? I can't - maybe it;s boke?
Paul Schings
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