On Mon, 20 Jan 2003 17:25:43 GMT, Robin Burns
<r_f_burns@hotmail.com> wrote:
>I got my first autofocus camera recently (Maxxum/Dynax
9) and have noted
>some interesting observations on its autofocus
performance. First off, let
>me say that I don't care too much about autofocus speed.
I'm mainly a
>landscape photographer, so speed isn't important to me,
but accuracy is.
>
>I tested by mounting an Air Force 1951 resolution chart
on a wall and
>photographing it with the camera mounted on a tripod. I
lit the chart with
>a 500w halogen spotlight (EV 10) to make sure the AF
system had plenty of
>light to work with.
>
>With a 50mm 1.4 lens at its closest focus distance, I
found that the
>camera's AF system focused probably 99% as well as I
could do focusing
>manually. It was fairly repeatable too.
>
>When I moved back to 15 feet, I started getting results
that surprised me.
>At f/1.4, most of my shots were not in focus. This is
doubly surprising,
>since at 15 feet the calculated DOF is 2.16 feet (14.0'
to 16.2'). Why the
>AF system couldn't focus to +/- 1 foot at 15 feet on a
well-lighted, high
>contrast target is beyond me. I tried various other
targets, but saw no
>difference in the results.
>
>Switching to MF mode, I found that the focus
confirmation LED in the finder
>goes on well before critical focus is reached and stays
on well past it.
>This is also interesting and tells me that the AF system
has plently of
>slop.
>
>Comparing AF to MF results, I found that the AF system
consistently focuses
>at the far end of its range, e.g. the point right before
the focus
>confirmation LED would go out when focusing manually and
moving towards
>infinity.
>
>I also noticed that the lens distance markings are not
calibrated too
>accurately. When focusing on the target at a measured 15
feet from the
>camera's focal plane indicator, the lens distance scale
reads just slightly
>closer than the 20' marking, and not exactly in between
the 10' and 20'
>markings as I would expect.
>
>Anyone else have the same experience with AF? How and
when do you use AF?
>All of the time? Most of the time? Rarely?
Welcome to the wonderful world of AF (and AE...;-).
Manual still works better, and MF is still generally
faster and more accurate *if* your camera has sharp
VF optics (many do not), and *if* you can sharply see
at the VF screen effective focus distance (see
www.David-Ruether-Photography.com/articles.html#glasses for
a good glasses solution that can work for many for
both general seeing, and for 35mm camera focusing - wait
until the long page fully loads, and it will then
"pop"
to the article...). A few years ago I developed a problem
with large, soft eye "floaters" which made me
(kicking
and screaming) buy AF bodies and start acquiring those
icky, floppy plastic AF "lenses"... - but fortunately,
Nikon's first really good AF bodies had just appeared
(the F100 and F5). Previously, I had watched with
amusement as every new body from the 2020 forward was
introduced, and people announced that "finally, this
time Nikon has it right" only to find that none did
much better with AF than the lowly original 2020.
All could focus fairly well about 50% of the time,
missing slightly the rest of the time - and the
"electronic focus" was a bad joke, showing
"correct
focus" over an amazingly wide range, with focus
"clearly" incorrect over most of the range... Now
I scale-focus (beyond 3' or so) the 20mm and shorter
lenses, and use AF with 24mm and longer lenses, and
this works well enough 95% of the time with the F100...
(BTW, AF accuracy depends on both the quality of the
camera design, and also the quality of the AF adjustment
on the particular body you use - testing for
"centering"
of the AF within the DOF range is a useful thing to
do when buying an AF body...)