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off-camera flash in low light – choosing your shutter speed

June 5, 2012

off-camera flash in low light – choosing your shutter speed

In the article on maximum flash sync speed, a question came up whether this is where we’d be at even in low light.

The answer is, that we’d most likely be at a shutter speed where the ambient light shows up. To remain at max flash sync speed in low light, isn’t the immediate best choice, for the background would usually go too dark. And we would like context. Our photographs usually look best with our subject in surroundings we can recognize, or complements our subject in some way.

Regarding our choice of max flash sync speed, this is the best go-to camera setting when you use flash in bright ambient light. High-speed flash sync kills too much of our flash output, to be our first choice, unless we are specifically chasing the higher shutter speeds or wider apertures. If you use a softbox or umbrella (or some light modifier) with high-speed flash, then you stand the risk of the flash simply not being able to pump out enough light to match the bright light. For this, generally our sweet spot is maximum flash sync speed.

Shutter speed choice when using flash, will vary depending on what we’re photographing, or trying to achieve. In low light, we’re most often dragging the shutter. But we have to be able to adapt what we do, against what we’re trying to achieve with our photos.

As recap example of using flash in low light, let’s have a closer look at the top image:

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I was busy photographing a model, Molly K, as part of a forthcoming review of the Canon 600EX-RT speedlite and Canon RT-E3 speedlite controller. For a final image, I wanted her against the awesome backdrop of the early evening Manhattan skyline.

The photograph above was my first test shot: 1/30 @ f/5.6 @ 800 ISO.  You can’t use your camera’s light-meter in a general way, since there are dark areas and bright areas. Selectively metering and guesstimating, along with a few test shots, will get us there. I eventually settled on 1/20 @ f/5.0 @ 800 ISO for the settings of the final image.

Working at shutter speeds as low as 1/20 hand-held, will strike fear with some photographers. Alternately, some might be okay with it, saying that “flash will freeze the movement.”  Well, yes … but not necessarily. It depends. Or if this was a Facebook relationship status, it would fall under “it’s complicated.”

We’ve discussed shutter speed choices with flash in that article, for various scenarios. But let’s look specifically at flash with low light.

Flash will only effectively freeze motion, if
- the ambient light on your subject is 4 stops or more under the (proper) flash exposure, and
- there isn’t a bright area behind your subject that will reveal there was movement.

In the example above – a straight-forward portrait of our model, there isn’t any movement to contend with, except for potential camera shake. This is where a stabilized lens works wonders. The Nikon 24-120mm f/4 VR (B&H) and Canon 24-105mm f/4L IS (B&H), are both superb lenses for general photography. With the image at the top, there was minimal available light on her, so the flash froze any potential camera shake. Along with a the stabilized lens, a careful stance and breathing, this all ensured that I get crisp images.

Since she wasn’t going to move or jump, the brighter sky behind her wouldn’t show the blur of movement.  Flash will freeze action, as long as the subject isn’t moving against a much brighter background – for then we will see some kind of ghosting effect, or parts of our subject might disappear into the brighter areas.

Doing another test shot at my final settings of: 1/20 @ f/5.0 @ 800 ISO, I could easily confirm that camera shake would not be a problem. In this case flash would freeze any movement, whether the model’s or my own. On top of that, I was shooting at a focal length of 24mm, and in comparison, especially with a stabilized lens, a shutter speed of 1/20th isn’t all that scary.

camera settings:  1/20 @ f/5.0 @ 800 ISO

I used a softbox to camera-right, with the Canon 600EX-RT (B&H). The speedlight was in manual exposure mode, and was controlled by the ST-E3-RT Speedlite Transmitter (B&H).

I didn’t specifically meter for the flash. From experience, I have a good idea of how powerful my light will be at specific distances with that softbox. With manual flash, we have that specific relationship between Aperture + ISO + Distance + Power, to give us the correct flash exposure. And this remains consistent, even if we shoot on another day in another place.

equipment used:
Canon 5D Mark II;  Canon 24-105mm f/4L IS (B&H)
Canon 600EX-RT (B&H);  ST-E3-RT Speedlite Transmitter (B&H)
Lastolite Hot Shoe EZYBOX Softbox Kit (24″x24″) (B&H)

related articles:
- max flash sync speed
- the advantage of a higher max sync speed
- tutorial: high-speed flash sync
- using high-speed flash sync / Auto FP   (model – Aleona)
- dragging the shutter
- shutter speed choice with flash
- shooting in low light – flash and incandescent light
- when aperture does NOT control flash exposure
- ‘urban legends’ of flash
- shutter speed does not necessarily control your background exposure!

 

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