As J. del Col points out, when you are doing macro work, the depth of field
(DOF) is small. This is because the DOF scales with the image distance. When
you take a picture of something very close, the DOF is very small. The only
way to get around this is large f/ratio. As J.del Col points out, you
will either have to use a very steady arrangement (I do fairly well in the
field with a tripod and cable release) or flashes. Taking pictures of plants
from the greenhouse I have more control, and so I use a heavy tripod or clamp,
ring-light, cable release, and a bit of my father-in-law's expensivo
equipment (extra flashes, macro-lenses and the like). I'm surprised
JdC has ventured into exposures long enough to give reciprocity failure
but it can certainly modify colours as he pointed out.
Michael: I am astonished your ring light caused damage to your plants.
I am at a loss to explain why. I regularly use my ring light, not at
your 25--30 cm, but at 5--10 cm. Certainly the light is very bright
but only for an instant, so the total integrated amount of energy
is not too high. Either you have a ring light that pumps out an
enormous amount of infrared, or something else must have happened.
I know that when I take my Nepenthes from their 100% humidity terraria
they start to wilt within a few tens of minutes---did you maybe take the
plants out of their coddled environments and sit them in an arid room
for a while as you photographed them? Maybe that shocked them...
Don, thanks for the info on the metal halide lights. I don't think I want
any---who needs the DEA knocking on your door and confiscating plants!
Barry
P.S. Sorry if this photography discussion has gotten technical or
uninteresting fast. I can tell that JdC is very experienced and I
snap a few pictures too (in fact, the cloudy Arizona skies have
cleared after weeks of rain long enough to let me and a colleague
take some images from the 90" telescope on Kitt peak). Fortunately
CCDs don't suffer from reciprocity even with 2 hour exposures!
(and waiting for them to finish is why I have time for email in the
middle of the night!)