John H. writes:
<< If the information at
http://www.photodo.com/nav/prodindex.html
is to be believed, it does appear that Leica M lenses as a whole are top
dog. The 90/f2 Zuiko does not really stand out when compared to the
Leica M ratings as a whole. >>
Ah, but look under "Understanding the MTF graphs, numbers and grades"
at:
http://www.photodo.com/nav/artindex.html
and you will find the following quotes:
"The grade is based on the average weighted MTF for the lens. No other
variables, such as distortion, flare, or ghosting are taken into
account."
"When we report an average MTF number for all f-stops at one focal
length we use f4 (or wide open, if f4 is not attainable) and f8. Most
lenses have their highest MTF at f8. High-speed lenses almost always
have a very low MTF at full aperture. That is why we decided to use f4
and f8 as standard apertures. Since MTF for f8 is slightly more
important than for f4, we assign weights of 60 0.000000or f8 and 40 0.000000or
f4."
"Higher is better. MTF is measured 0, 3, 6, 9, 12, 15, 18, and 21mm from
the image center (on axis), at 10, 20 and 40 lp/mm, in two different
directions (sagital and tangential). That is all together 48
measurements at each focal length and each f-stop. Rather than listing
all these numbers we calculate a weighted average number for each f-stop
of all lenses and each focal length of zoom lenses. The center of the
image is more important than the edges, and 10 lp/mm is more important
than 20 and 40."
So, what they are saying is that:
* only f/4 and f/8 data was used, with the center given higher weight.
* they have a weighting system they aren't telling us about which favors
the center data
* they have a weighing system that favors results at 4x6" enlargment
sizes (vs. 8x12" and 16x24")
Which takes us full circle to:
"If the information at is to be believed . . ." Personally, I don't
agree with their weighting system. They say: "10 lp/mm is important for
evaluating the overall contrast and resolution when making small
enlargments (10 x 15cm or 4 x 6 inches)." Well, give me a break. Most
any lens looks good to our eyes at 4x6" print size! Don't Popular
Photography SQF tests most always show A+ at that size? And I'd argue
that a zone 1/3rd out from center is the most important in an image
because it represents the most picture area in a 35mm frame - i.e., its
the only one of three arcs representing 1/3rd out (7mm), 2/3rds out
(14mm) and corner (21mm) that is completely within the picture area.
Well, it turns out that 1/3 out is where I've been evaluating my
"center."
Conclusion: If a photographer only makes 4x6" prints and only shoots at
f/4 and f/8 and fails to fill the frame with their subject matter, then
I guess Photodo.com ratings weights reflect that photographer's reality.
Gary Reese
Las Vegas, NV
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