Camera flash sync speed

Question

I’m thinking of buying a Sony A7c, but I’m concerned about the flash sync speed and max shutter speed . The flash sync speed is 1/160 and max shutter speed is 1/4000. From what I’ve read and understood, 1/160, that’s the fastest shutter that I can use with a flash . But then I watched this video https://youtu.be/vEnAhkL0i38 . The guy uses a D90 which has a max sync of 1/200 but he is taking shots at 1/1600 using the flash . I’m completely confused 😐.

Can I use a flash on a Sony a7c that, during bright daylight I would be able to make the background black/low key (like a black wall - the ambient exposure ) while the subject to be correctly exposed ?

How important is the flash sync speed and max shutter speed ? Should I skip a7c and go for a7iii instead ?

Answer 1

Can I use a flash on a Sony a7c that, during bright daylight I would be able to make the background black/low key (like a black wall - the ambient exposure ) while the subject to be correctly exposed ?

Assuming full sun falling on the subject and the wall? With a really big flash (like a 600-1200 Ws studio strobe) it might be possible, but it would be a lot easier with smaller lights in the shade or in a studio setting rather than outdoors in direct sunlight. To kill the ambient, you need to light at about +5EV above the ambient.

How important is the flash sync speed and max shutter speed?

It's important if you need faster shutter speeds while shooting flash, either to keep from overexposing in the ambient, or to achieve a thin DoF with daylight fill flash.

There is a flash feature called high-speed sync (HSS, aka focal plane or FP flash) where the flash can "fake" being a continuous light source for the duration of the exposure by pulsing continuously. But this feature also lowers the power output of a flash by about -2EV or more.

There's also a another form of syncing that I call "tail syncing" (commercially it goes by hypersync, hisync, HS, supersync, etc.), where a flash is typically fired at full power (longest pulse) with a slightly delayed shutter so that the flatter more even tail end of the pulse can be used as a continuous light source for a faster (1/1000s or so) exposure. So there are ways to use flash with a faster shutter speed. They're just going to rob you of flash light output as well and can be a game of diminishing returns.

If all you need is thin DoF with daylight fill, you can also consider putting a neutral density filter on your lens to allow you to shoot at sync speed or slower shutter speed without overexposing.

Should I skip a7c and go for a7iii instead?

I honestly don't think it makes that much of a difference for this specific feature. All focal plane shutter camera bodys have a flash sync speed limit. And while 1/250s is faster than 1/160s, it's only by 2/3 of a stop. And you'll still have to use HSS/FP flash or some other technique to get around that limit.

Answer 2

It is possible using a flash with high speed sync. High Speed Sync (HSS) allows faster shutter speeds in exchange for delivering less power for the increased period of time required to evenly expose a picture using a slit shutter configuration rather than firing when the shutter is fully open.

However, to achieve the look you want may require the use of a narrow aperture, management of the distance between the subject and background, and placing the flash close to the subject in order to mitigate the reduced output in HSS mode and the increased light requirements of a narrow aperture…basically you will need to live with the inverse square law.

For example 1/4000 at ISO 100 at f16 is approximately six stops under exposed in full daylight. If the effective guide number of your flash is 16 in HSS mode, the light to subject distance would need to be 1m or less for proper exposure of the subject, and the distance from subject to background 3m or more in the worst case.

Orienting the flash not to fall on the background, using a dark background, placing the subject out of the sun, post processing, etc. can all increase your possibilities.

There’s no substitute for experimentation when it comes to lighting because no amount of theory produces pictures and making pictures is the only way to gain experience.

Answer 3

Maximum flash sync speed is about the camera shutter, not about the flash. Your Sony model has a focal plane shutter, meaning any and every shutter speed takes about the 1/160 second for it to fully open. The flash has to wait until the shutter is fully open, or it will see the shutter partially open making some of the frame dark (but in full sun, still likely brighter than black). That means Maximum shutter speed of 1/160 second with flash before shutter is fully open. Some camera shutters are slightly faster, 1/200 or 1/250 second to open, but the problem is the same. The flash must wait for the shutter to fully open.

In bright sunlight at 1/160 maximum flash sync, exposure (at ISO 100) will be near f/16. Making the background dark will require an exposure and flash power and/or higher ISO to be able to make subject substantially brighter than the background (which also greatly depends on background distance being sufficiently away from subject of course).

HSS is another flash mode allowing faster shutter at wider aperture (Equivalent Exposure concept for HSS flash too), but at reduced flash power. Your Sony can do HSS, but you need a flash (with HSS) also compatible with the Sony HSS for both camera and flash to enable HSS.

Electronic shutters (instead of mechanical shutters) don't have a maximum flash sync speed issue (however not many allow fast shutters with flash because they don't offer HSS anyway).