Motion Blur Photography: Front Versus Rear Curtain

The more chaotic the environment, the more interesting your slow shutter images can become! Shallow, salmon-filled rivers are an excellent example. Each image looks completely different and unique as nothing in the environment is static (Nikon D500, Nikonos RS 13mm Fisheye lens, Nauticam housing, dual Retra Flash strobes, f/9, 1/10s, ISO 160)
 

The underwater world is a chaotic place in constant motion. So, how is one to convey this sense of movement and chaos in a still image? By using a slow shutter, of course! However, creating compelling and successful slow shutter images is not as simple as just lowering your shutter speed and hoping for the best. Slow shutter shooting also gives the chance to create interesting shadows, which give a three dimensional quality to images.
 

Panning with front curtain sync emphasizes the movement of these spadefish. The extra camera movement allows for exaggerated slow shutter effect (Sony a7R Mark V, Sony 28–60mm f/4–5.6, Nauticam housing, Nauticam WACP-C, dual Backscatter Hybrid Flash strobes, f/14, 1/3s, ISO 50)

Front Curtain Sync vs Rear Curtain Sync

Most cameras have several different flash settings that you are able to select to control external flashes, or strobes in underwater shooting. The two that are important for this discussion are front curtain (sometimes called “fill flash”) and rear curtain. Front curtain flash means the flash triggers at the beginning of your exposure whereas with rear curtain, the flash fires at the end of the exposure.
 

This lemon shark was photographed with rear curtain sync and no extra camera movement (Sony a7S Mark III, Nikonos RS 13mm Fisheye lens, Isotta housing, dual Kraken KR-S160 strobes, f/16, 1/20s, ISO 64)
 

Certain cameras like the Canon EOS R7 are not able to sync to the rear curtain when using non-Canon brand flashes. This mean front curtain sync is the only option. For subjects like sharks, whose movement is easy to follow with a camera pan, that isn't an issue. For unpredictable subjects, it could be (Canon EOS R7, Tokina 10–17mm Fisheye lens, Ikelite housing, dual Ikelite DS230 strobes, f/14, 1/15s, ISO 100)

How to Set Up the Image

When your flash fires during your exposure is important, as it can determine the aesthetic of your picture, or the way you approach the image. Flash will freeze your subject, which is why underwater photographers are able to get away with shooting such long exposures and still having sharp subjects in the frame. Whatever the flash lights up will be sharp, so it is what happens outside the strobe light and after the strobe fires that really determines how your image will look. 

Let’s say you have an image of a subject swimming right to left across your frame. If you shoot front curtain sync, with the flash firing at the start of the exposure, you will need to pan the camera in the same direction your subject is moving, or you will end up with a “ghost” of your subject in front of your frozen subject. Panning with your subject will keep the motion blur behind your subject, which looks far more natural. Thus, front curtain sync with slow shutter tends to be useful for subjects whose path of travel is easy to track, like a shark.
 

Even frogfish can be photographed with a slow shutter! This individual was quite active moving along the bottom. Front curtain sync could be used, as the fish was easy to track with a quick pan, allowing for an extra sense of motion that wouldn't have been achievable with rear curtain sync (Sony a7S Mark III, Sony 90mm Macro lens, Isotta housing, dual Retra Flash Pro Max strobes, f/16, 1/3s, ISO 640)
 

Using the same example as above, with a shark moving right to left across your frame, using rear curtain sync, with the flash firing at the end of the exposure, you would shoot your picture and keep the camera static or move it in the opposite direction of the path of travel of your subject. This results in the motion blur being behind your subject. Rear curtain sync can be used in situations where the movement of the subject after you press the shutter is unpredictable or when you can’t necessarily pan the camera.
 

This pair of Florida gar was photographed with rear curtain sync so the flash fired at the end of the exposure. The slow shutter effect turned the spring ecosystem into a painterly aesthetic (OM System OM-1, Olympus 8mm f/1.8 Pro Fisheye lens, AOI housing, dual Ikelite DS230 strobes, f/13, 1/3s, ISO 100)
 

A key aspect of slow shutter photography that is often overlooked or not thought about is that ambient light is required to create the motion blur effect. Without ambient light, your strobe flash freezes whatever it lights, so no matter how long your exposure is, without ambient light, there will be no sense of motion. During the day, this usually isn’t an issue, as you can often get enough ambient light from the sun. However, in dark scenarios or at night, an artificial light will need to be introduced to replicate the ambient light and allow for motion.
 

This blue claw crab was photographed at night. A constant light in the background substituted for ambient light, making it possible to create a feeling of movement (Sony a9, Nikonos RS 13mm Fisheye lens, Seafrogs housing, dual Kraken KR-S160 strobes, Kraken LTD 10,000 video light, f/9, 1/20s, ISO 320)

Final Thoughts

Slow shutter imagery can bring a unique and beautiful aesthetic to your portfolio, but understanding how to make compelling slow shutter images is key. Learning front curtain vs rear curtain sync, when to use which, and how to pull it off is key. The underwater realm is constantly in motion; why not do your best to bring a little bit of that motion into your still pictures?