Diffusing Speedlites with a Translucent Shoot Thro' Umbrella
Just a quick illustration of one of the commonest and least expensive modifiers, the translucent white or shoot through umbrella in use with a Canon 580 EX II Speedlite.
The above illustrations apply with the majority of hot shoe flash or speedlites which have a zoom head with a fresnel lens in front.
Look carefully at the images and compare the areas marked with the yellow arrows and at the the density of the shadow on the backdrop and the area of transition between dark & light on the shadow edges.
The unmodified flash wight the head zoomed to 105mm the flash is effectively a small point light source or hard edge spot light. By placing the umbrella in the beam the photons are deviated from their direct path creating the softening effect.
The angle of the zoom head also has a direct effect with the beam broadening as the head is zoomed wider. This also has the effect of reducing the intensity of the flash by up to one ƒ/stop.
Any modifier will also reduce the amount of light reaching the subject to a lesser or greater extent. Not all translucent umbrellas and diffusion panels reduce the light intensity by the same amount, so it's worth conducting your own tests to see the effect before serious use.
I use Fred the (polystyrene) Head and a grey plastic scoop for consistency, given me an accurate visual comparison between modifiers.
The white translucent umbrella used for this test was the Elinchrom Small 85cm/33.5in translucent umbrella which has a 7mm shaft so is cross compatible with my Elinchrom flash.
In the last two frames LEE Filters 430 or Rosco 3030 grid cloth was stretched on a DIY frame made from plastic overflow water pipe.