I don't know from your post the focal length but, at f22 refraction
becomes an issue with almost any "small" format camera, (in the digital
age almost everything not using film is a small format for these
purposes), rule of thumb is it starts taking a toll at about f8~f11. on
35mm format.
It's a case where it's actually dependent on the physical size of he
aperture not not the f stop per. se. So the lack of sharpness in the
center of the frame is probably exacerbated by detraction.
When focusing at infinity, DOF still is limited, at 50mm and f22
assuming a COC of .016mm, (the assumption for an 8x10 print from APS-C),
at a focus distance 1,000,000,000 ft, (the closest to infinity that
f/Calc* will let me enter to infinity**), your near focus will be 22.08
ft at 200mm your near focus will be at 353.286 ft. Anything close than
that will appear to be not sharp.
* f/Calc is a pretty neat tool, there's a free online version, it used
to be freeware for the PC but now it's shareware for every platform, to
use it long term you need to buy it. I'd buy the latest version for
Android if I had a smartphone or tablet, as it is I just continue to use
the old freeware version on my PC.
** Yes I could get close to infinity if I did the calculation myself,
but why bother, this is good enough.
On 4/11/2015 8:23 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?
------------------------------------------------------------------------------------------
Eric Weir
Decatur, GA USA
[email protected]
"Imagining the other is a powerful antidote to fanaticism and hatred."
- Amos Oz
------------------------------------------------------------------------------------------
Eric Weir
Decatur, GA USA
[email protected]
"What does it mean...that the world is so beautiful?"
- Mary Oliver
--
I don't want to achieve immortality through my work; I want to achieve
immortality through not dying.
-- Woody Allen
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