Understanding the Science of Flash in Cameras

[Celluhh]

Homework Statement

how is flash created?

The Attempt at a Solution

flash makes images caught on a camera brighter and the background dark. why does this happen? what exactly is flash made up of? how do you explain the effects of flash?

 

[PeterO]

Homework Statement

how is flash created?

The Attempt at a Solution

flash makes images caught on a camera brighter and the background dark. why does this happen? what exactly is flash made up of? how do you explain the effects of flash?

Am I to assume you are referring to the flash on a camera?

 

[Celluhh]

yes.

 

[PeterO]

yes.

What does it look like comes out of a flash?

 

[Celluhh]

white light rays?

 

[PeterO]

white light rays?

Good.

Tell me about the intensity of light, and its variation over distance from the source.

 

[PeterO]

white light rays?

Perhaps poor wording last time...
How does light intensity vary with distance?

 

[Celluhh]

the greater the distance, the weaker the light intensity?

 

[PeterO]

the greater the distance, the weaker the light intensity?

Yes it is weaker - and weaker in a definite way.

If you stood in front of a wall in sunlight - and could ignore all the extra light bouncing of the environment - you would be 150000 km from the sun, and the wall might be 150000.01 km fro the sun.

With a flash camera, you might be 3m from the flash, while the same wall would be 13m from the flash.

What would that mean?EDIT: you better throw 3 more zeros in those "distance to the sun" figures! ie 150000000 km

 

[Celluhh]

hi, can you explain it in simpler terms, i get the first part, but not the last part?

 

[NewtonianAlch]

http://en.wikipedia.org/wiki/Flash_(photography)

 

[PeterO]

hi, can you explain it in simpler terms, i get the first part, but not the last part?

If the background is twice as far away as the subject, it will receive only 1/4 the light intensity.
You camera, which tries to automatically adjust for the light, will have the subject showing brightly, while the background is dark.

If you have a digital camera with flash, try it.
Have someone stand 2m from the camera in the following situations.
(a) against a wall.
(b) 2m from the wall.
(c) 4m from the wall.


if the last part was the "add three more zeros" it as because I understated the distance to the sun.

 

[Celluhh]

ah, ok i get it now!thank you!

 

[DaveC426913]

A secondary effect:

Because the subject is so brightly lit, the camera gets plenty of light for a properly exposed shot, and can stop down the exposure (higher f-stop, faster shutter). This mean the background, which might otherwise have been adequately lit, is now going to be very underexposed, i.e. black.

Much to the chagrin of many Moon hoaxers...