If you mix artificial Continuous light of some sort and daylight – you may have quantity issues (as we’ll discuss later) and you may well have colour temperature issues (as we’ll also discuss later) but essentially, you can eyeball the effect, and see what’s happening, as daylight and an artificial continuous light source effectively function the same way. Working with mixed light sources isn’t too tricky though, but you’ll need to pay attention.
Things get much more tricky when you mix flash with continuous light of some sort – either daylight, or artificial. Flash, of course, is a very brief pulse of light, and therefore very hard to judge how much of it there is in relation to your continuous light.
Exposure 1/30s @f5.6 @ ISO 100
What we have here is a mix of continuous light and flash light. I’ve used a tungsten 2Kw light here to give me quite a lot of ambient, continuous light to play with, and have shot indoors rather than outside so that I can be certain the numbers all stay consistent. Exactly the same principles that I’m about to describe occur in daylight, the difference is I might struggle to find consistent daylight, whereas this way I can be in control.
The flash light is a small flash head, firing through a 60cm square softbox. Since flash is roughly 5000K colour temperature, and tungsten is more like 2800-3000K there’s a full sheet of orange gel on the flash to bring its colour temperature down to the same as the tungsten light.
Watch what happens when we alter the shutter speed. As we slow the shutter speed down, and therefore let more light in, the “background” of the image gets brighter, and so does the subject to some extent.
Exposure – 1/8s @f5.6 ISO 100
As we speed the shutter speed up, the rest of the image gets much darker, but the subject lit by the flash stays pretty much the same.
Exposure 1/125s @f5.6 ISO 100
Think of shutter speed as a dimmer switch – you can choose how light or dark to make the areas not lit by your flash simply by slowing down or speeding up your shutter speed.
If we alter the aperture, then both the subject and the background will get lighter or darker – aperture affects flash as much as it affects continuous light. The same goes for ISO – only shutter speed allows us to separate out the effects of flash from those of continuous light.
Exposure 1/15s @f5.6 @ISO 400
Exposure 1/15s @f11 ISO 100
This works because the pulse of light from a flash is so brief that it makes no real difference how long the shutter is open for. As long as you use a shutter speed slower than the maximum flash sync speed, which will vary from camera to camera, but is usually in the region of 1/250s, changing your shutter speed will not alter the exposure of your flash.
Hopefully you’ll remember (either from the technical foundations course) or from your own experience, that if you shoot at a shutter speed faster than the max sync speed on your camera, you’ll start to get black bands on your image. This is caused by the inability of the camera to synchronise the brief pulse of the flash with the short period the shutter is open, so you start to see one of the shutter curtains covering part of the image. You can get round this by using some form of High Speed synchronisation. I won’t detail this here, as the various different forms vary from camera to camera and flash to flash. It’s worth knowing however that even if you can use some form of high speed sync, there’s always a trade off of some sort, usually in power output, so don’t think it’s the magic bullet.
High speed sync – labelled Hi-S
So, when mixing flash and continuous light we need to be aware of colour temperature, if we’re using something like tungsten, but more importantly we need to be aware of different quantities. We can alter the ratio between how much of the continuous light we see, and how much of the flash, by leaving the aperture and ISO constant, but altering the shutter speed. Remember – altering the shutter speed will only affect how much of the continuous light you see – you won’t notice a change in the portions of the image lit by flash until you go past the maximum sync speed. I suggest you go and play around with this – it’s a really useful trick to know, but takes a little bit of getting used to.