The Sony A9 mark III with global shutter and flash

Friday 8 March 2024, 13:26 by Nando Harmsen | 2,735 views | 0 comments

This article was translated from Dutch with the help of AI. Read the original.

The Sony A9 mark III is naturally in the news because of its global shutter. It is rightly called revolutionary. It promises no rolling shutter effects and flash at any shutter speed. That last part is not as simple as it seems.

A global shutter means that the sensor is read out completely in a single moment instead of line by line. This means you no longer have to deal with distortion of lines that are at right angles to the direction of movement. That is perfect for action photographers.

The Sony A9 mark III, the first camera with a global shutter.
The other advantage is the absence of a flash sync speed. This means you can use flash at any shutter speed without relying on high-speed sync. The Sony A9 mark III can therefore use flash at the fastest available shutter speed: 1/80,000 second.

This is only in theory, however. In practice, it is virtually impossible. This is not down to the camera or the global shutter, but to the way a flash works.

The flash intensity is determined by the duration of the flash. This time is indicated by t0.1. The exposure is made during this time, and the rest of the flash duration has virtually no further influence on the exposure. The less light a flash has to produce, the shorter the t0.1.

In a studio, you do not need a fast shutter speed when using flash, but outdoors it can sometimes be very useful not to have a flash sync speed. (Sony A9 mark III with Zeiss Batis 85mm, ISO 250, f/5.6, 1/500 sec, Profoto B10, edited to taste)
If you want to take a flash photo at 1/80,000 second, the flash duration t0.1 must be 1/80,000 second or shorter. There are hardly any flashes that can do this, and if you have one, it must be set to the lowest flash intensity.

I managed to find one: the Profoto Pro 11 in combination with the Prohead Plus UV 500W. This 2,400 Ws generator can achieve a flash duration of 1/80,000 when set to 2.4 Ws. This generator costs 18,000 euros, while the matching flash costs more than 3,000 euros.

I used a Profoto B10 of 250 Ws for a test with the Sony A9 mark III. This flash has a t0.1 of 1/400 second at full power. By setting the flash intensity to 0.5 Ws, the lowest setting, the t0.1 is as short as 1/4,900 second.

In freeze mode, this flash reaches a maximum of 1/14,000 second at 0.5 Ws. This is according to the manufacturer's specifications. I set up the flash to get a good exposure at 0.5 Ws. This was at a distance of 10 cm. The exposure was ISO 250 and f/8.

The Profoto in freeze mode and at its lowest setting to achieve the shortest possible flash duration makes it possible to use flash at up to 1/1,600 second. But that is with 0.5 Ws instead of the 250 Ws the flash can deliver.
The result of this test shows that the flash can be used at shutter speeds of up to 1/1,600 second. At faster shutter speeds, the flash duration (t0.1) is so much longer that the required amount of light is not reached within the set shutter speed.

This is a shutter speed that is faster than the one specified by Profoto. This is because, according to Sony, the flash must also meet special requirements. A fully compatible flash must be used, and remote flash is also different from a flash connected directly. The exact details are difficult to determine.

The possibilities offered by the global shutter are of course fantastic. Although you could use a flash at any shutter speed you want, no flash is fast enough to make this possible. In many cases, around 1/500 second will be the maximum you can use.

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