Relative to my flash coverage measurments, Andrew asked:
What were you using for a target and how far away was it?
- If it's too shiny instead of matte, light will bounce away
from the lens instead of scattering in all directions
- If it's too close even a matte surface will have falloff
as it won't be perfectly "lambertian".
Earl also responded with:
Since there is no universally accpeted standard for the term "coverage"
or "covers", in this context, it really depends on the specific photo's
requirements.
------------------------------------------------
There was no target. What I listed are direct, incident light
measurements using a Sekonic L-358 flash/light meter at a distance of 10
feet from the flash head. The 10 foot distance is to an imaginary plane
(wall) directly in front of the camera. Obviously, some (if not all) of
the falloff is due to the fact that all of the measurements except the
center are actually somewhat further away than 10 feet. How much is
reflector/flash tube design and how much is the inverse square law I
don't know. I haven't done the trig and am not so inclined.
This is probably a good illustration of Earl's point. Do we really
expect "coverage" to provide even illumination over the imaginary wall
or do we accept that this is a very tough requirement given the inverse
square law. Should I have measured falloff from the center of an
imaginary sphere 20 feet in diamter such that all measurement points are
precisely the same 10 feet from the flash?
While there are certainly no standards, I opt for measurement across the
flat plane. Using that as a "standard", what I have pointed out is that
the two competitive flashes I measured provided equal or better evenness
of illumination at a coverage angle wider than their stated
specification than the T-32 did at its specification. Thefore my
statment that Oly is optimistic.
I reiterate that no actual film was exposed to injury by high speed
photons during any of these experments. YMMV.
Chuck Norcutt
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|