Tuesday, June 17, 2014

Stopping down a fast lens to flash sync speed with a neutral density filter

Getting ready for the summer, how can you use flash with a wide aperture under full sun power? You can either use small speedlights at hypersync, which will give very low power, or go down to flash sync speed using a Neutral Density (ND) filter, which is my preferred option.

On location you normally do not have time to do math, so some preparation is useful. Find below a table based on exposure settings at ISO 100, Sunny 16 rule, which you will encounter at the beach, and the corresponding shutter speeds when using ND filters for 3 and 6 stops.

f/stop Sunny 16 ND 3 ND 6
1 1/25600 1/3200 1/400
1.4 1/12800 1/1600 1/200
2 1/6400 1/800 1/100
2.8 1/3200 1/400 1/50
4 1/1600 1/200 1/25
5.6 1/800 1/100 1/13
8 1/400 1/50 1/6
11 1/200 1/25 1/3
16 1/100 1/13 1/2

As you can see, if you have a 2.8 lens, the most reasonable option is to close down to f/4, and use a ND3 filter. If you set the speed to 1/250, you will get a slightly underexposed ambient light.

If you have an f/1.4 lens, you need to use a ND6, to achieve the same result. The ND6 filter is also good if you have an f/2 lens, giving a comfortable 1/100 shutter speed.

So, the settings become quite simple. Measure your flash to give f/16 on your subject. Your camera at 1/250, and f/4 or f/1.4, with ND3 or ND6, respectively, and done.

Another setting I also use quite often is f/11 1/250 on the lens, with a circular polarizer on the lens, that additionally substracts one stop and a half, depending on the filter maker, and gives you those intense skies and pristine waters.

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