>>Mark Lyon discussed the inherent distance light drop off of increased off
>>axis flash illumination of walls.<<
Mark,
I believe there are three parts of the light falloff onto a linear
object like a wall.
1) The flash design. (everbody blames!)
2) the increased distance (which you identified) as the light angle increases.
(varies as cos^2 of half angle.)
3) the increasingly acute angle at which the light strikes the flat surface.
(ie it spreads the light over a larger area as the light strikes at an acute
angle, instead of straight on)
As far as I remember without doing the sketch, this all leads to a cos^3 fallof
of light for an ideal equal light output per unit angle flash . There used to
be really great website discussing this in extreme detail and comparing the
light uniformity of various flash reflectors and diffusers for studio lights.
Unfortunately it is no longer online and the companies products are not
available.
These power laws guarantee you get uneven wall illumination for wide angle
lenses, unless you use multiple flashes, or decrease the angle by bouncing
flash off ceiling moving them back etc. You could in theory design a flash
especially for linear objects, but it would not work well for equal distance
objects!
Super wide angle lenses also have aparent "vigneting" due to light drop off as
they map the light from a flat surface at an acute angle. This is on top of any
actual vigneting due to aperture effects. So the combination of
flash,aperture,wall and lens mapping all conspire against you!
Regards,
Tim Hughes
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