Tests for Backfocus with the EF
50mm f/1.2L
After I had completed my usual series of focus
and resolutions tests, several contributors to Fred Miranda Forums
indicated that copies of the EF 50mm f/1.2L were backfocusing when tested
using a Jackson 2004 FocusTestChart. I went back and ran additional tests
to be sure I wasn't missing something that would impact photographic
performance under conditions I use. Tests with the FocusTestChart were
initially run with my lens at a working distance near 18 inches (46cm). I
was able to duplicate backfocus reported by others using an EOS-1D Mark II
and an EOS-1Ds Mark II. Results with the FocusTestChart at 46cm are shown
below with autofocus and manual focus showing that the lens backfocuses at
f/2 and f/4 when autofocused, but not when manual focused. I ran
additional tests at 46cm using a USAF 1951 resolution pattern in a similar
manner to other resolution tests in the review. I found that higher
resolution was obtained by manually focusing the lens at a 46 cm working
distance than by using autofocus. I then repeated the autofocus and manual
focus tests at 129 cm (51 inches) and at 258 cm (104 inches). No effect of
backfocus was identified at these greater working distances (see following
graph using an EOS-1D Mark II at 129 and 258 cm working distance.)
 |
| Lens resolution tests were run at the three working distances using
a USAF 1951 target using a variation of the method specified in the
methods section of this review. A Canon EOS-1D Mark II set at ISO 100,
high jpg setting on tripod with cable release was used. For autofocus
tests, the central focus point was centered on the target. Three
exposures were taken at each aperture setting with defocus of the lens
between each autofocus and shutter release. For manual focus, the lens
was set to manual focus, and focus was adjusted while looking through
the viewfinder with the aid of a Canon 2.5x magnifying angle finder.
Three series were taked at each aperture. The sharpest image out of
three for each test point was taken to calculate resolution. At 129 mm,
there was more variability (greater numbers of slightly out-of-focus
images) using manual focus than autofocus. Only data at f/2 is shown in
the graph. |
I then checked out two other lenses with an EOS-1D Mark II and the
Jackson 2004 FocusTestChart. My EF 50mm f/1.4 and EF 85mm f/1.8 lenses
gave front focus and back focus results in the tests, respectively, when
tested at the closest possible working distance. No backfocus was detected
at longer working distances. I have never encountered autofocus accuracy
problems with my EF 50mm f/1.4 or my EF 85mm f/1.8 lenses that I have used
to make thousands of images.
I concluded that the Jackson 2004 FocusTestChart detected real focus
problems at the very short working distance (46cm). These misfocus
problems are of questionable significance because I almost never try to
produce wide aperture images at the shortest possible working distance to
produce tack-sharp images. With the EF 50mm f/1.2L focus function was
normal at longer working distances (129 and 258 cm). The backfocus
detected by the Jackson 2004 FocusTestChart is not relevant to most of the
photographic applications I use lenses for in the real world.
Test with Jackson 2004 FocusTestChart at 45 Degrees to Target - 46cm
working distance shows backfocus
|
Autofocus |
Manual Focus |
| f/1.2 |
 |
 |
| f/2.0 |
 |
 |
| f/4.0 |
 |
 |
Test with USAF 1951 Test Chart at 90 Degrees to Target - 46 cm
working distance shows decreased resolution with autofocus due to
backfocusing
|
Autofocus |
Manual Focus |
| f/1.2 |
 |
 |
| f/2.0 |
 |
 |
| f/4.0 |
 |
 |
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