1

**Lighting / Re: Flash Zoom - Difference in Stops?**

« **on:**July 29, 2014, 03:53:20 PM »

2^F = (GN2/GN1)^2

Then from 24mm to 70mm, F=1.67 stops (+1 2/3)

70mm to 200mm, F=0.52 stop (+1/2)

So of course 24mm to 200mm, F=2.19 stops.

OK, so you're taking the log base 2 of both sides of that formula to solve for F.

But can you tell me where that formula comes from? All I know is the GN = f * d formula and the Inverse Square Law formula (Intensity = 1 / distance^2). Would like to know how you derived it.

First, to clear things off, I advise to just stick to the basic GN = F*d which gives the right results and is much more tractable on the field.

As for my formula, I just went on from basic physics.

Intensity I is Power over Area --> P/A

A, the area lit, increases to the square of the distance from the source -->A proportional to d^2

Therefore, I is proportional to P/d^2 (not equal to)

Then, given that power remains the same --> P1 = P2 entails that I1*d1^2 = I2*d2^2 or, I1/I2 = (d2/d1)^2

Now, that is for the flash in a fixed zoom setting.

Now considering that the subject distance is fixed and posing that Case1 is for 24mm and GN28m and Case2 is for 70mm and GN50m. Zooming from 24mm to 70mm is like moving the subject from a distance of 50m to a distance of 28m. Then d2/d1 = GN2/GN1 and :

I1/I2 = (GN2/GN1)^2

Finally, in photography the aperture F goes up one stop every time the light intensity doubles so I is proportional to 2^F. It ensues that:

2^F = (GN2/GN1)^2

I hope this helps.