ISO Changes Flash Range??

Can somebody explain this concept to me because I've never understood it. I read all the time that increasing the ISO also increases the range/length that the light from the flash can reach? How is that possible?

at certain distance, the flash at full power don't have enough power to proper light a subject. but if you rise the ISO, the camera became more "sensitive" to light, so now the flash can "reach" a longer distance. (as the light became more "weak" with the increase distance from flash > subject)

i mean, its not a "flash become more powerful", but "the camera can take good exposures with the flash farther away" with ISO increase.

that helps?

RandMan wrote in post #17596515
Can somebody explain this concept to me because I've never understood it. I read all the time that increasing the ISO also increases the range/length that the light from the flash can reach? How is that possible?

What makes you think that photons from flash you used ten years ago aren't 10 light years away in space, heading away from planet earth?

Light does not run out at a certain distance; it just spreads out and occasionally gets converted to heat or some other form of energy.

The further away from your flash, the weaker the light, but it never has a limit to how far it goes at all.

John Sheehy wrote in post #17596543
What makes you think that photons from flash you used ten years ago aren't 10 light years away in space, heading away from planet earth?

Light does not run out at a certain distance; it just spreads out and occasionally gets converted to heat or some other form of energy.

The further away from your flash, the weaker the light, but it never has a limit to how far it goes at all.

I understand physics (to a certain extent), but this does not help explain the relationship between ISO and flash range that I was inquiring about.

RandMan wrote in post #17596589
I understand physics (to a certain extent), but this does not help explain the relationship between ISO and flash range that I was inquiring about.

There is a minimum amount and a maximum amount of light that a flash can give out. For ISO 100, this means a minimum and maximum distance for a "normal" ISO 100 exposure. For ISO 400, the minimum and maximum distances double. For ISO 1600, the distances are 4x what they are at ISO 100. Every doubling of distance causes a quartering of light.

The light you see is direct light if you stand in front of the flash or in direct light if you stand behind of the flash (reflected).

If just one foton is reflected back and just fly just past your eye, you wont see it. Now we multiply you by ten and you all stand side by side (higher ISO) then the chance that foton will hit an eye is increased.

Now you see the light?

Your subject is two feet from your flash and it's properly exposed. Move your subject four feet away and it will receive one-quarter of the flash's light; it will be two stops underexposed. Change your ISO from 100 to 400 (two stops) and, with the same flash intensity, your subject will be properly exposed.

Light from a point source spreads based on the inverse square law. i.e. if light is quantity X at 1 unit it is 1/4 the quantity at 2 units (2x2=4)

There are three ways for the camera to control the amount of light that reaches its light collecting sensor. Shutter speed (time), Aperture (Volume), ISO sensitivity to light.

A strobe (speedlite) has the ability to alter its power. This fourth element replaces the shutter since the duration of the flash is shorter than the time the shutter is open at the x sync speed. The other way to control the volume of light from the sensor is the distance of the light from the subject because of the inverse square law.

So, if proper exposure, with flash is X-sync shutter speed, F8 at ISO 200 with the light, set to 1/4 power, 5.6 feet from the subject the following options exist;

To maintain proper exposure while reducing the aperture to F11;
One either can
A) move the light in to 4 feet
B) increase the ISO to 400
C) increase the light power setting to 1/2 while still at 5.6ft.

And the reciprocal is also true, if one want to increase the aperture to F5.6
One either;

A) moves the light in to 8 feet
B) decreases the ISO to 100
C) decreases the light power setting to 1/8 while still at 5.6ft.

Learning the exposure triangle and its artificial light corollary is essential when one wants to use artificial light efficiently.

It's pretty simple.

The farther the subject is from the light source, the less light falls on the subject.
The higher the ISO, the less light needed to capture the subject.