[Tfug] Way OT: optics mumbo jumbo

[Jeremy D Rogers]

I like how Bexley hinted at the presence of "optics people" on the
list.. Here and I have a PhD in optics and I am still trying to figure
out how to get a good picture of my instrument. I think others got the
math right below, but in my experience this kind of problem is less
about the math and more about creativity.
One suggestion that someone in my program made to me once (I wish I
could remember who and give him credit) was to set a really long
exposure like 10 to 30 second and then use a flashlight and walk
around the subject with the flashlight lighting at different angles.
This long exposure means you can use a small aperture and have a long
depth of field and the integration of the moving light makes for
really nice even illumination.
It's worth trying a few times to get it right (try with room lights on
or off, etc), but I have been happy with the result.

JDR

On 3/19/08, erich <erich1 at copper.net> wrote:
> Bexley Hall wrote:
>
>  >Hi,
>  >
>  >This is *way* OT but it seems there are some folks
>  >with more than a casual knowledge of optics (while
>  >I have *less* than a casual knowledge!) so...
>  >
>  >I often take photos of things that I have built
>  >(etc.) to show clients problems that I have
>  >encountered (picture, 1000 words, etc.).
>  >
>  >But, I don't waste a lot of time thinking about
>  >*how* I do this -- I just keep dicking around
>  >with where I am standing, ambient lighting,
>  >camera angle, etc. until I get something that
>  >more-or-less shows what I want/need to show.
>  >(much easier when you can click a dozen frames
>  >"for free" instead of having to use Polariods!!)
>  >
>  >Dealing with the flash is almost always a PITA
>  >for me.  It's always "too hot" and too much
>  >reflected glare, etc.  Granted, I can change the
>  >camera angle to eliminate this to some extent.
>  >But, at other times, it just moves the reflection
>  >to another surface, etc.
>  >
>  >Often, my solution is to back away from the item
>  >(light falls off as the square -- or cube? -- of
>  >the distance) until the reflections are manageable.
>  >
>  >
>
> Inverse 4th power (x^(-4)), because the light makes a round trip
>  from a light-dispersing object.
>
>
>  >Of course, this means the image is smaller  :-/
>  >
>  >
>
> The aparture is constant. So, the light gathering ability doesn't
>  increase unless you physically move closer to the subject.
>
>
>  >So, I crank up the (optical) zoom to make the
>  >image large enough to be useful.
>  >
>  >
>  >Now, my naive question:  by doing this, am I,
>  >to some extent, counteracting the effect of
>  >"moving away" from the target?  I.e., does
>  >the magnification I am bringing in to play
>  >*increase* the amount of light that strikes
>  >the (digital) film?
>  >
>  >
>
> Is a thought experiment so necessary? Why not just test it for
>  real? Choose a subject and chalk off distances from it. Use
>  the same camera. Then view images on the same monitor screen.
>
>
>  ><sheepish grin>  I've tried to construct a
>  >"thought experiment" to convince myself one way
>  >or the other but seem to just be thinking myself
>  >in circles...
>  >
>  ><shrug>  I definitely won't lose any sleep over
>  >this but it *is* amusing (to me) to think about
>  >what's really going on...
>  >
>  >We now return you to your regularly scheduled
>  >program...
>  >
>  >
>  >      ____________________________________________________________________________________
>  >Never miss a thing.  Make Yahoo your home page.
>  >http://www.yahoo.com/r/hs
>  >
>  >_______________________________________________
>  >Tucson Free Unix Group - tfug at tfug.org
>  >Subscription Options:
>  >http://www.tfug.org/mailman/listinfo/tfug_tfug.org
>  >
>  >
>  >
>
>
>  _______________________________________________
>  Tucson Free Unix Group - tfug at tfug.org
>  Subscription Options:
>  http://www.tfug.org/mailman/listinfo/tfug_tfug.org
>


-- 
Jeremy D. Rogers, Ph.D.
Postdoctoral Fellow
Biomedical Engineering
Northwestern University