When photos of the first atomic bomb test were published, its blast yield was still classified. But there is enough information in this picture to determine it.

Start with approximate relationship between blast radius R and blast energy E:

$$\gamma \rho R^5 = E t^2$$

E	blast energy	$\frac{kg \cdot m}{s^2}$
$\gamma$	heat capacity ratio	1.4
t	time	$s$
$\rho$	air density	1.2 $\frac{kg}{m^3}$

A kiloton of TNT is equivalent to approximately $4 \cdot 10^{12} J$, so yield in kilotons is

$$\frac{1.4 \cdot 1.2 \cdot R^5}{t^2 \cdot 4 \cdot 10^{12}}$$

Plugging in the numbers from the released pictures, with radius measured from the thin blast wave line rather than the edge of the fireball:

t	R	E
0.016	107	23
0.025	126	21
0.053	164	18
0.062	181	21
0.090	194	14

The average over five images is 19 kT, which turns out to be pretty close to the now declassified value.

Source: Sanjoy Mahajan, The Art of Insight in Science and Engineering, chapter 5.2.2

Pictures from Atomic Archive