Flash and shutter speed outdoors
My last article introduced the importance (or non-importance) of the shutter speed when used with flash indoors, this one deals with flash outdoors, which is very different. And it’s different because, outdoors in daylight, the ambient light produced by the sun is bright enough to affect the shot, and often is so bright that the flash makes little difference to the shot.
And the reason that the flash often makes little real difference is that the shutter speeds that normally work with flash let in so much of the daylight that it overpowers the flash.
When I started out in photography, many years ago, most of the cameras that were fitted with focal plane shutters could only be used at 1/30th second with flash, compared to something like 1/250th second with many of today’s cameras, and at just 1/30th second, the daylight came into the camera for 8x as long as with most of today’s cameras, making the flash only 1/8th of its present day effectiveness – a difference of 3 stops! We can now use a (relatively) short shutter speed to cut down the effect of the daylight, and this is what we need to do to actually create light from our flash (rather than just to add to the light that’s already there) because, at a shutter speed of say 1/250th second, the shutter will normally be fully open when the flash fires, so we get the full power of the flash, but the shutter is only open for a relatively short period of time, so the daylight doesn’t have to dominate.
But, often, a normal flash at 1/250th second is still dominated by the ambient light, so what are our options?
Well, in no particular order, we can do one of the following, or sometimes combine them.
1. Use a more powerful flash, such as the Lencarta Safari 1200Ws kit, which is way more powerful than almost all others, and many times more powerful than an ordinary flashgun.
2. Use an Atom flashgun, smaller, lighter and cheaper than the Safari but still much more powerful than an ordinary flashgun.
3. Use HSS, which allows a much faster shutter speed
4. Use a tail end sync radio trigger, which again allows a much faster shutter speed.
With both HSS and tail end sync, we lose flash power but we can lose even more ambient light, which means that the flash can dominate the part of the subject that it lights, and the background can go darker.
The principle of HSS (high speed sync) is an old idea, first introduced by Zeiss in 1953, but updated to work with modern flashguns, or at least with those flashguns that have HSS capability. It involves the flashgun firing a burst of flashes so close to each other that, in effect, the flash becomes a continuous light that is on for long enough for the shutter to complete its cycle, which means that literally any shutter speed can be used with it. HSS requires a suitable trigger, and also requires that the flashgun uses IGBT technology – IGBT is normal for flashguns, but not for studio flash, so you’re normally limited to using a flashgun. HSS also dramatically reduces the light output, so the method does have limitations that I personally find a bit much, and so, for this tutorial, I’m going to concentrate on tail end sync and forget about HSS.
Tail end synch (also known as Tail Hyper-sync) uses the long flash tail common to most (but not all) studio-type heads most of the time, and also works with most other flash units when used at full power output. The advantage, when used with a radio trigger designed for the purpose, is that literally any shutter speed can be used. However, there’s a big loss of brightness with tail end sync, and exposure also varies significantly down the frame. This may or may not be a problem. If you’re using it to photograph a white wall in the studio the uneven exposure will be a very real problem, but you wouldn’t need to use it for that… Use it to photograph a ‘busy’ subject outdoors, and although the uneven exposure is still there, you probably won’t even see it. And if you position your light so that it is closest to the part that would otherwise be the darkest, the fall off of light over distance caused by the Inverse Square Law helps even more.
I mentioned that flashguns use IGBT technology. What happens with IGBT is that the flash always fires at full power, regardless of the power setting used, and when a setting of less than full power is used, the flash is simply switched off abruptly (quenched). The by product of this system is that the flash durations halve with each halving of the power setting, which can be extremely useful for freezing action in the studio, or even outdoors when it’s pretty dark, but the downside is that we actually need fairly long flash durations when using tail end sync, because the flash duration needs to be long enough for the focal plane shutter to complete its cycle. Because of this, a flashgun used with tail end sync can normally only be used at full power, or possibly half power (depending on both the flashgun and the speed of the camera shutter).
Radio triggers capable of use with tail end sync are normally available to fit EITHER Canon or Nikon Cameras, and Yongnuo offer both a Canon-specific model, YN622C, and a Nikon one, YN622N, with offerings also available from Pocket Wizard and some other manufacturers. Adverts for tail end sync triggers typically claim that they can be used at 1/8000th second shutter speed and, although this is technically true, the moving slit in the camera shutter at 1/8000th is typically only about 1mm or so wide and such a fast shutter speed should only be used if there is a real need to freeze subject movement – if the objective is to reduce the amount of ambient light, it just doesn’t make sense to use a shutter speed faster than around 1/2000th sec, because faster speeds will not only increase the inevitable uneveness of the illumination, they will also cut down the effective power of the flash at about the same ratio as they cut down the effect of the ambient light, so are pointless.
If you haven’t got a tail end sync trigger, what are your options?
Well, there are always workarounds and bodges that can help. Remember the shot on the left, from my last tutorial? It was taken in the studio, with no real contribution from ambient light, so we have that big ugly black band caused by the second shutter curtain being half closed when the flash fired. And it doesn’t work because of it.
But, suppose that the shot was taken outside in normal daylight – the part of the subject that isn’t lit by the flash would still be lit by the daylight, so it might be OK in some circumstances.
What would happen if we turned the camera upside down for this shot? Well, the part that isn’t lit by the flash would be at the top of the frame instead of the bottom, and if all that’s at the top of the frame is sky then it would be OK, and probably better…
What would happen if the camera was turned on its side (portrait shape)? Well, the part that isn’t lit by the flash would now be on one side or the other, and if that part isn’t important, or is cropped out, then again it would be fine.
And doing this would reduce the effect of the ambient light by 1 stop, effectively making your flash twice as powerful.
Of course, other options that would increase the effective power of your flash, and which may be usable,
are to either move the flash closer to your subject or to remove any diffusers that spread the light over a larger area, or both. Coming back to the shot on the right, taken with the Safari, the backlight that highlights her back had to be placed a long way away, to get it out of shot, but I simply fitted a high intensity reflector to the flash head – this not only narrowed the beam of light so that it didn’t go where it wasn’t wanted, it also increased the effective power of the flash by at least 3 times!
Neutral density filters
Even if you have a powerful flash such as the Safari, you will often need a neutral density filter if you want to create light (rather than to just produce a fill light) when you use it outdoors in sunlight. The reason for this is that a typical bright sunlight exposure at 1/250th second and 100 ISO is around f/11 before you add the flash, which might increase the required aperture to around f/22 – which, even if you’re happy with f/22 from a depth of field viewpoint, might cause diffraction limitation. A 3 stop ND filter will reduce f/22 to f/8. It’s use will of course affect both the ambient and the flash equally, but by reducing the amount of both types of light passing through the lens, you will be able to use as much flash power as you wish. That’s what I did with the shot above left. I took that shot on a beautiful bright sunny day, I had hoped that it would be dull and raining but was out of luck – but using a neutral density filter turned my luck around 🙂