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Exercise: Hotspots In a swimming pool, with a digital camera (for instant feedback) using the widest angle of lens you have available together with a flash gun on a long flash arm, switch the flash on and position it so it’s visible to your eye just inside the viewfinder on your camera. Select f11 on your camera, and a fast shutter speed that will synchronise with the flash. Set the flash to manual full power and take a shot. Now review the LCD.You will see the appearance of snow/backscatter immediately in front of the flash gun. Whilst still leaving your flash gun visible in your viewfinder, position it at an angle at which, in theory, you would not expect to see backscatter.Take another shot and review the LCD.The flash gun must be present in the LCD for you to see the effect clearly. I suspect this hotspot is still visible! Try once again, but change the aperture on your camera to f22 and see how the hotspot diminishes. Now set it to f5.6 and notice the increase. Figure 41.15 This was taken at f22 at 1/90 s using a Nikon 12-mm–24-mm zoom lens on the 12-mm end and one Inon Z220 on full power. I have placed my flash just inside the viewfinder to illuminate a collection of silk flowers in a swimming pool. In the following examples, notice how the hot spot continues to increase and becomes wider as the aperture is opened.
Figure 41.16
Figure 41.17
Figure 41.18
Figure 41.19
At f 5.6.
I nve r s e s q u a r e l aw This hotspot is related to the inverse square law, which states that: The intensity of light falls off or diminishes at a rate inversely proportional to the square of the distance it travels from its source.