[Tfug] Way OT: optics mumbo jumbo

[jblais]

Don -

Hello -

There used to "guide numbers" for flash units (before they were integrated
in the body of the camera).  You'd take the guide number and divide by feet,
and for a given ASA (ISO) number, that would give you the f: number.  The
flash usually is much faster than a shutter, so 35mm film camers had a
"sync" speed for the vaarious types of flash (electronic / bulb).  Newer
flash devices had a sensor that would turn off the flash when it sensed
enough light.  Through-the lens sensor and CCD imaging - I don't know where
the ontrol is.

The distance from the source to the subject is the critical thing for
lighting.  The camera can be anywhere.

If you can separate the flash from the camera, put it off to a side, perhaps
put a handkerchief over it, or even aim it at the ceiling, or backwards onto
a white card, and let that reflection illuminate the subject.  You may want
to play with exposure until you get it good, but  a new CCD camera may
figure it out.  No flash, and good exposure, with a fairly bright, steady,
even, light will be better than any flash.  If you can, take the thing you
want to photograph and put it by a windoe facing north, or go on the north
side of a building (plenty of light, no harsh shadows)-- reflected light is
a good thing.

Some cameras have a built in flash that can be aimed at the ceiling.  Or
just tape a piece of paper over the flash when you do closeups.

There are copy stands and such that hold light at 45 deg. to the line of
sight.  Nikon also used to make a ring flash and a macro lens with a built
in ring flash for very good close ups.

Joe



> -----Original Message-----
> From: tfug-bounces at tfug.org [mailto:tfug-bounces at tfug.org]On Behalf Of
> Bexley Hall
> Sent: Tuesday, March 18, 2008 8:11 PM
> To: tfug at tfug.org
> Subject: [Tfug] Way OT: optics mumbo jumbo
>
>
> 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.
>
> Of course, this means the image is smaller  :-/
>
> 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?
>
> <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...
>
>
>
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