Camera Flash

[apeman2001]

When I point a camera at the sky and use the flash, it brightens up all the stars I'm looking at (not just the ones on the screen). Does anybody know why this is?

 

[russ_watters]

Welcome to PF....

No, it doesn't.

 

[apeman2001]

Welcome to PF....

No, it doesn't.

Have you tried it?

 

[russ_watters]

Have you tried it?

No. Can you post sample pics?

 

[apeman2001]

No. Can you post sample pics?

If I post a picture, it will just look like a picture of the night sky. I'm trying to say that when I take a picture with flash, I can see several stars that the camera is pointed at become brighter for a second. It doesn't make any sense to me because all of those stars are many light-years away.

 

[russ_watters]

It must be an illusion.

 

[Phrak]

Black and white photo emulsions consists of grains of silver iodide in an organic matrix. To develop a photograph each grain of silver iodide must absorb a multiple number of light quanta before it becomes unstable and decomposes the translucent grain into silver. (The larger the grain, the more quanta are required.) When this happens the grain explodes leaving a small black spot of finely dispersed silver.

The larger the "grain"-- a single crystal of silver iodide actually--the more electromagnetic quanta are required to make it disassociate. But some clever fool discovered that he could sensitize film by pre-exposing it in a bath of low intensity light. After this is done the number of photons required to hit the explosion threshold for a crystal of SI is lowered; it requires fewer photons to cause the crystal to explode and put a spot on the film.

[btw, this is called pre exposed film.]

Your eyes works in the same fashion. It takes multiple photons to trigger a rod or cone to send a neural impulse to your brain.

It is possible that the back scatter from the flash, trips the photon count threshold on the retinal image where the stars are projected onto your retina, but is sufficiently weak that the critical threshold, where there are no stars, is not reached. So for a brief fraction of a second you could perceive better contrast of a star field.

Or are you looking at an enhanced image on a monitor?