Imatest Documentation

Imatest Documentation
σ 2 = Σ (ti-E(t)) 2 LSF(ti) / Σ LSF(ti)
The sum in the σ 2 equation has to be taken over a limited region so that noise at large distances from E(t) (i.e., outliers) does not
distort the results. If LSF 2 had been used instead of LSF in this equations, undershoots, which make edges look sharper, would
increase σ 2 .
After we learned Luc Marin's definition of BxU = -1 for one
Photoshop sharpen function, we ran some tests and found that the
Sharpen function is the approximate inverse of the Blur
function, at least when measured by MTF50. It's a little off when
measured by MTF @ 0.25 cycles/pixel. The negative values of σ 2
were caused by undershoots due to sharpening.
Test
MTF50
(C/P)
MTF @
0.25 cy/pxl
σ 2
Pixels
Original ideal edge 0.526 0.862 0.121
with one Blur 0.332 0.665 0.343
with one Sharpen 0.753 1.28 -0.270
with one Blur + Sharpen
(order doesn't matter)
0.531 0.973 -0.113
Pictchallenge.com adds a bit of confusion to the issue. Their extensive description of DxO states, "... une valeur de "2"
correspond par rapport a cet ideal a un cran de "plus flou" de Photoshop." This appears to imply that 2 BxUs equals one Blur
more, but I'm not certain— I'm not a master of French. They also have a DxO image that shows BxUs of around 1 for the
outstanding Macro Nikkor 60mm lens. They translate BxUs into their own rating system, where sharpness (pique) of 3 is poor and 5
is excellent. I haven't been able to figure out the precise relationship between these numbers and BxU.
Since one application of Blur More is equal to 2.5 applications of Blur, Pictchallenge.com's statement implies that one BxU is
equivalent to 1.25 Blur applications— a little confusing; 1 seems more likely.
Luminous-Landscape's erroneous BxU description
When we first became aware of DxO in July 2004, we struggled with an apparent discrepancy between
published test results and Luminous-Landscape.com's definition of BxU as equivalent to one Blur More.
Edge after Blur More has been applied 6 times. Unmagnified.
Reported BxU numbers appeared to be much too large to fit this definition. For example, Luminouslandscape.com,
imaging-resource.com and dpreview.com tested the 8 megapixel Sony DSC-F828. The
Imatest SFR analyses of the ISO-12233 charts in dpreview.com and imaging-resource.com indicates
MTF50 = 0.344 and 0.352 cycles/pixel, respectively, near the image center for focal lengths = 22mm at f/5.0
and 8.2 mm at f/4.0 (both excellent performance). The LL report indicates a Y-channel BxU of about 1.7 for
the best combinations of focal length and aperture— much higher than one would expect from MTF50 around
0.35. The LL report on two Canon 400 mm L lenses shows BxUs over 6 for several apertures— up to
nearly 12 for the 100-400 at f/22. Assuming the good technique (sturdy tripod and mirror-lock: de rigeur for long tele lenses), this
would be horrible performance with the assumption that 1 BxU = 1 Blur More. Horrible even for a Brownie; unimaginable for a
Canon L lens.
No, it's not my calculation. You can observe the problem by visually comparing images of the ISO-12233 chart with an ideal edge
with Blur More applied one or more times. (You can download the unmagnified ideal edge, above.)
The resolution: We have concluded that the Luminous-Landscape.com's definition of BxU is erroneous. We'll stick with 1 BxU = 1
Photoshop Blur (not Blur More). Case closed.
*DxO, DxO Analyzer, and BxU are registered trademarks of DxO Labs. Photoshop is a registered trademark of Adobe Systems.
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