Turning day into night

It’s been a bit of a sore point with my sister-in-law, Ríoghnach, that she hasn’t appeared on the blog and so with her playing the role of god mother to her newest cousin at Faye’s christening on Saturday, I was on a mission to get some photographs of her that would make for suitable blog material.  As it turned out I got quite a few photos that I could blog, but the one above gives me the most scope to write about its creation, and indirectly leads into the concept of high-speed-sync flash photography.  That’s because this photograph was taken outdoors in the middle of the afternoon on a bright day, and in the middle of a yard.  Creating this image involved turning day into night, and there are basically two ways of doing that.

Both techniques require us to use flash, because what we are actually doing when we turn day into night like this is removing all ambient light from the scene in front of the camera, which requires us to add light back in to illuminate our subject. We use flash for that purpose. But first we must get rid of the ambient light.

The traditional way of doing this is to put the shutter speed to the max sync speed of the camera – in my case 1/250th second, use a low ISO (say 200) and close down your aperture until you have a totally black frame.  That’s what I did for the exposure in the photograph of Ríoghnach, and I found myself at f/22 which is an incredibly small aperture.  But that’s pretty much what it took to kill all the ambient light from the scene.

There are a couple of issues with shooting at f/22 for a portrait.  Firstly, creatively it might not be desirable because it’s going to keep pretty much the entire frame in pretty sharp focus, and normally for a portrait we don’t want this.  But given that here it’s also throwing the background black, that’s not too much of a concern.  The background may be in sharp focus (and it is), but you can’t tell because it’s also totally black.  In hindsight I should have shown how much was in focus by slowing down my shutter speed to allow that ambient light back in and you’d have seen that background quite clearly.

The second problem with shooting at f/22 is not only does that small aperture remove all that ambient light from the frame, but it also means only a little bit of light from the flash gets through, or to put it another way, you need to have your flash either very close, or at a very high power setting, to illuminate the subject sufficiently.  And if you are trying to diffuse the flash in any way (a softbox or an umbrella for instance) you’ll be making it work really hard – perhaps too hard.  It simply may not be able to output enough light to light the subject.

That’s what was of interest to me here when I was trying out the photo above, and I put the flash in TTL mode to see if it could find a power level that worked.  As it turns out I had first-time-right success, despite also having a small Stofen diffuser on the flash to give a little softness to the edge of the light.  What saves me is that Rioghnach is lighting herself, by holding the flash at arms length in front of her, and that means the light is close to the subject.  That’s not to say that Ríoghnach has short arms… but, well, she kind of does!

There is a second way, with some newer flashes at least, to turn day into night, and that is to use what’s known as high speed sync.  This is where the flash, instead of outputing a single very short burst of light, outputs a much longer duration, but lower power, spread of light acting almost more like a continuous light source than like a flash.  The duration is still pretty quick, but the flash starts to fire just before the shutter opens and fires continually throughout the exposure until it closes.  It allows you to use shutter speeds beyond the max sync speed, which is typically the fastest shutter speed that guarantees the entire frame sees the flash burst.  In this high-speed-sync mode, the entire frame sees light from the flash even though at no point is the entire frame exposed at once, because of the way in which the flash behaves.

Your ability to use this mode is totally dependent on your flash then, but what it allows you if you can use it is to shoot at shutter speeds faster than the max sync speed.

For the shot at the top of this post remember I was at f/22, 1/25os, ISO 200.  Let’s suppose I wanted to achieve shallow depth of field that is more traditional for a portrait.  If I wanted to shoot at f/2.8, what shutter speed/ISO combo would I need to ensure that the ambient light was still removed from the exposure.  Firstly lets dial the ISO down a stop to 100.  Working out the maths, f/22 @ ISO 200 is equivalent to f/16 @ ISO 100.  Going from f/16 to f/2.8 (my desired working aperture for this theoretical experiment) is a 5 stop difference, so I need to speed up by shutter speed by 5 stops.  That takes me from 1/250s to 1/8000s.

For the photo below of Ríoghnach’s sister (and my wife) Aoife, this is almost what I did, except that I didn’t go all the way to 1/8000s.  I stopped at 1/2000s because I was shooting facing the other direction, and the background was naturally darker as it was more in shade. Also, perhaps f/22 in the shot above was a stop or so more than I needed to go to nuke the background.  You can see a hint of background detail coming through in the photo below, but what’s more significant is that here I’m able to shoot wide open at f/2.8 and you can see how the focus falls off – Aoife’s left eye is sharp while her right eye, further from the camera, is out of focus.  In reality I’d be inclined to hover at f/5.6 for the required depth of field, but wanted to experiement with the two extremes of aperture.

f/2.8 @ 1/2000s, ISO 100, 24-70mm lens at 70mm, Nikon D700

The lighting in this shot is exactly the same – a diffused SB-800 hand held by Aoife in front of her and in TTL mode – but in this case the characteristics of how that flash is emitting light in high speed sync mode are very different.  But in a way that is transparent to the final photo.

At close working distances, high speed sync flash is a very useful thing to be able to use because it still allows you have creative freedom with aperture, but without having to also live with what might be undesirable ambient light or unsightly backgrounds.