Shoot indoors and it’s never a bad idea to use flash. Not that flash is perfect, mind you – it has certain limitations that it’s useful to be aware of. One of those limitations is your camera’s flash sync (or X-sync) speed, which affects the upper range of shutter speeds you can select when using flash. (At least with cameras that have focal plane shutters, such as DSLRs and mirrorless cameras. Cameras with a leaf shutter don’t have the same flash sync limitations.)
The flash sync speed varies between camera models, but is typically in the range of 1/180-1/250. Try to select a shutter speed faster than this and either the camera won’t let you or the shutter won’t fire, or the image is partially obscured by a dark horizontal band. We’ll come back to these two issues in a moment.
Before going further though, it’s worth noting that you don’t necessarily need to be shooting at or above the camera’s sync speed. In fact, if you’re shooting in low-light – which is often when flash is truly necessary – the camera will in all likelihood select a much slower shutter speed anyway. (Firing the flash either at the start of the exposure if you select 1st curtain sync, or at the end if you choose 2nd curtain sync.) Shooting flash in conjunction with a long shutter speed ensures that both your subject and background are correctly exposed; the former by the light from the flash, the latter by the ambient light of the scene.
The limitations of your camera’s flash sync speed is most keenly felt when you want to use flash as a fill light, particularly when shooting in full daylight with a large aperture to limit depth of field. (An aesthetic visual style that’s used frequently for portraiture.) To avoid overexposing the background you may need a shutter speed that’s far faster than the flash sync speed will allow.
Use a camera’s built-in flash there’s not much you can do once you hit the sync speed. You could stop your aperture down until the background is correctly exposed, but this will increase depth of field – which in the example above we were trying to avoid. The same is also true when you fit a proprietary flashgun – a flashgun specifically designed for a particular camera system – to your camera’s hotshoe.
It’s only when you attach a simple generic flashgun to the hotshoe, or via a sync cable, that you’ll be able to exceed the sync speed limit. And this is when you’ll see the dark horizontal band mentioned above. This is one of the shutter blades getting in the way. Flash requires the shutter to be fully open to work properly. It isn’t at shutter speeds faster than the sync speed, which is what causes that mysterious dark band across the image.
However, some flashguns have a nifty mode that gets around the problem of sync speed: High speed sync (HSS). As the name suggests HSS lets you use a shutter speeds faster than the sync speed, typically any shutter speed up to the camera’s maximum. With HSS selected you can shoot flash in bright conditions with impunity.
So. Problem solved and all is good.
Well, er. Yes. However, there’s a but. In solving the sync speed problem another issue raises its head. Switch to HSS and the effective range of the flashgun decreases. The solution to this is to either increase ISO and/or select a larger aperture, or move your subject closer to the camera. (Photography can be a lot like putting up wallpaper. You push one bubble down successfully and another pops up somewhere else. The only consolation is that photography is far more fun than decorating.)
The reason that HSS reduces the effective range of flash is entirely to the way that it works. Normally, a flash discharges light in one brief burst. In HSS mode the flash rapidly pulses light – so fast that it is essentially firing continuously – during the exposure.
Despite the reduction in range, it’s well worth getting to grips with HSS. Careful use of the option expands the type of images you’re able to shoot. One interesting technique made easier with HSS is deliberate underexposure of the background to a scene. It’s a look you often see in fashion photography, as it emphasises the (flash-lit) model and makes the background darker and more moody. Shutter speed has no effect on flash exposure so it’s just a case of selecting the right shutter speed to darken the background to suit. (This is generally easier to achieve in Manual exposure mode. In this mode aperture and shutter speed can easily be set separately, unlike semi-automatic modes such as aperture or shutter priority.)
If you own an external flashgun go and take a look at what modes it offers. If HSS is there then give a go. It’s fun to use and you’ll create images you couldn’t achieve any other way.