Eric,
Focusing at infinity with that long focal length lens would not produce
good results, especially at the longer end of the zoon.
I suspect that you may have mixed focusing at infinity with focusing at
the hyperfocal distance. The definition of the latter is when you focus to
it, everything from half of it to infinity will be in [reasonable] focus.
I don't know what the focal length you used (that determines the DOF), -
let's assume it was at the maximum of the lens, 250 mm.
At the distance of 20', the DOF is only about 1.5'.
So, it is rather shallow, and would not have covered the entire plot even
if you focused correctly (at 20').
At 50 mm the DOF would be much-much larger, and if you focused at 20',
it would have covered from about 10.5' to infinity. With that focal
length, you could've set the focus to the hyperfocal distance (18.3'), and
then everything from half that to infinity would've been in focus.
To estimate these parameters, you can use a "DOF calculator" - you can
search for that in Google. E.g. this one seems to work well:
http://www.dofmaster.com/dofjs.html
Hope this helps,
Igor
On Apr 11, 2015, at 5:23 AM, Eric Weir wrote:
Mark.s post about stack focusing brought to mind a problem I encountered the
other day. I was shooting wildflowers at Arabia Mountain, just outside
Atlanta. (One of several stone outcrops, with unique ecosystems, that occur
here in Georgia.) I was using my DA 50-200 with the camera on a tripod with
aperture set at f/22. My target was plots of wildflowers 3' to 5' in
diameter, each with a mixture of species, some "normal" sized and upright,
others tiny and growing closed to the ground. I was 20' to 30' from the
target.
I assumed that if I was focused at infinity everything within the target
would be in focus. I was also trusting autofocus to give me good focus. I was
very disappointed. The larger plants and flowers were reasonably sharp,
though nothing like what Mark showed us yesterday. But the tiny close to the
ground wildflowers were fuzzy. In the case I'm thinking of the tiny flowers
were in a strip 4' to 6' wide immediately in front of the larger plants.
Any thoughts about why I got this result?
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