Sticking my neck out - again

After the grumph about the Law of Inverse Squares, I am not sure I should
stick my neck out again.  (The main point of that particular subject was to
show how important it is to put your lights as close to the water surface as
practical. :-) No, I did NOT make that point very well!  :-(  )

Do you have a 35mm camera with a built-in light meter? One of the "older"
kind, in which you manually set the shutter speed, ASA or DIN film speed, and
hand-adjust the f-stops?  

If so, you can use it as a reasonably accurate light intensity measuring
device. IT WILL NOT MEASURE LUMENS!  However, it _will_ give sufficiently
accurate comparative light intensities to be somewhat helpful.  You use the
closest available focus (macro lenses or telephoto lenses are best, but the
one that comes with the camera will do.) to measure relative intensities.
(Macro lenses allow you to get really really CLOSE to the tank, telephoto
lenses allow you to be much further away, but focus on or aim at a very small
area. )

(a) with the camera on a tripod, (so it will not move while you are fooling
around with your lighting system), aim at a small area on the tank bottom, on
a plant leaf, or whatever. (b) set the f-stop in the middle of its' range.
(c) set the ASA speed and shutter speed to ANY COMBINATION that will give you
a reading of "OK" at the f-stop originally chosen. This gives you a
"reference" light intensity.  You can aim at a particular area, you can have
the camera in or out of focus, whatever. Whatever you decide, DO NOT change
the camera position or the focus while you make changes to your lights.

Now, don't change anything on the camera but the f-stop setting.  Make
whatever change in lighting you want.  Adjust the f-stop reading so things
are again "OK." Each f-stop, either up or down, exactly doubles or halves the
light. Half a stop is similar - it measures an approximate  "one-half"
reading.  Thus, if (for example) your light change results in changing the
f-stop from f-8 to f-16, your light intensity has changed two f-stops, from
f-8 to f-11, then to f-16.  It has doubled twice.  It is four times as bright
as before.  Accurate? No. But, more accurate than your eye, and I think
helpful. At least, it gives a "number." The camera "retains"a reading of what
the light intensity was BEFORE you made the change. Your eye does not
"retain" a reading, so it can be fooled pretty easily.

If you had to OPEN the f-stop from f-8 to f-5.6, that again is ONE f-stop,
which indicates you just cut the light intensity in half. 

Does this work?  Yes.  Is it reasonably accurate? Yes.  Is it useful?
 Possibly. Is it a truly precise measurement? Heck, no. Does it measure the
amount of light coming from the source? No.  It only measures the amount of
light REFLECTED from the area of the tank you are checking.  This could be
the bottom of the tank.  It could be an individual plant leaf. 

It will show you that the "formula" for watts per gallon is not bad at all!
 It will show you how dramatically light intensities change as you raise or
lower your light source relative to the water surface. It will also show you
how light intensities vary as you vary light sources, e.g. adding a single
fluorescent bulb, adding two bulbs, changing the reflector, changing to an
Electronic Ballast, etc. 

The good things about trying this are: It doesn't cost anything. (assuming
you already have the camera!!  :-)  ) It gives you "ammunition" with which
you may freely argue light intensities with your friends.  It will very
probably show you that measuring the light intensities does not do much for
the plants.  It will almost certainly show you that the "formula" which Karen
Randall keeps bringing up is a very simple, practical aid to lighting plant
tanks!  (Thanks, Karen. You are very good at bringing us back to the real
world. ) 

The bad things are: It isn't very precise.  It won't make your plants grow
better, more healthily, or faster.  It may prove to be a complete waste of
time, because it really won't change anything. It will just tell you that
your lights got brighter or dimmer, which you could probably see without a
camera. And, you may be quite annoyed with me for bringing this subject up

SUBJECT CHANGE:  Food for thought.  If white paint is really a better
reflector than aluminum, why isn't the Hubble Space Telescope Mirror coated
with white paint instead of aluminum?  

Answer: Dumb question. Totally different situation! Aluminum DOES reflect
light significantly better than white paint, but the paint "scatters" the
light, which works a lot better for our purposes.  We are not trying to
sharply focus light rays on a point, so as to get an image of the light
source, we are trying to illuminate the leaves on our plants. The whole leaf,
the whole tank, not just a single spot. 

Don't get too grouchy.  It is the weekend, and I get goofy then. (!!! :-D  )