Understanding Shutter Speed
Shutter speed affects the amount of ambient light entering the camera. Shooting at "one over 30" or "1/30th" means that the shutter speed is open for one thirtieth of a second. Cutting the shutter speed in half will increase the ambient light exposure by one stop (i.e. let in twice as much light as 1/60th).
The following shutter speeds all have a difference of one stop:
1/15th, 1/30th, 1/60th, 1/125th, 1/250th, 1/500th, 1/1000th, 1/2000th
Changing the shutter speed - what does it do?
Affects ambient light, which is the background light in strobe-lit shots. The slower the shutter speed, the more ambient light is let in.
Does not affect strobe light. This is important to understand, and is counter-intuitive to many people new to underwater photography.
Is used to freeze or blur motion. See freezing motion.
Is used to reduce camera shake. See camera shake below.
Can't be faster than the camera sync speed when using a strobe.
The faster the shutter speed, the less light that comes in. Using fast shutter speeds is very difficult in low-light situations, without added strobe power.
The portion of the photograph lit by ambient light will be sharper with faster shutter speeds, to a point. Read the sharpness chapter for more details.
Understanding shutter speed
High shutters are used to freeze motion. 1/125th is the minimal speed usually needed. Fast moving dolphins or sea lions may need 1/200th or faster to get sharp photos. Fast speeds also help capture sun rays.
Wtih medium shutter speeds, 1/30th to 1/100th, moving objects may or may not be sharp. Enough light is coming in to use mid-range apertures.
Slow shutter speeds, 1 second to 1/15th, motion trails may appear, and creative effects can be done. Anything that moves in the photo will be blurred. Small apertures must be used unless light levels are very low. If the camera is moved, the entire photo will be blurred, except for any portion frozen by a strobe light.
Shutter speed and wide-angle photography
When shooting wide-angle underwater photography with strobes, a close subject is illuminated with a strobe, while an interesting reef, diver, sunball, fish, or silhouette is composed in the background. Adjusting the shutter speed effects the background exposure.
What shutter speed does not do
When shooting macro photography with strobes, changing the shutter speed doesn't affect the photo, and doesn't affect sharpness. (Most of the time). How can that be, you ask? Many beginners struggle with understanding this. You have to remember that in macro photography, your strobe supplies most of the light. The strobe fires faster than 1/500th of a second, and is not affected by shutter speed changes. (See the caveat below in the technical note).
Ok, I lied a little in the above paragraph. Changing the shutter speed can effect the photo when shooting macro, but only if the ambient light is bright enough, and if your aperture is low enough. For example, in dark waters, shooting at F16, ISO 100 you're not getting any ambient light into the photo, whether you shoot at 1/250th or 1/30th of a second. However, if you're shooting strobe-lit macro at F5.6, ISO 200, 1/100th in bright water near the surface, now you are getting a lot of ambient light into the photo. In this case, the less ambient light you let in (using a faster shutter speed), the better colors your photo will have, and the sharper they will be.
Using a slow shutter speed can also be used to get a blue background, see the creative underwater shooting section for more detail.
Technical note - some electronic shutters (e.g. - in compact cameras such as a Canon G10) can fire as fast as 1/4000th of a second. So if your strobe is doing a "full dump" that takes 1/500th of a second (full dump durations vary), and you set your shutter speed faster than that, it's possible you are cutting down your strobe power. However, a strobe doesn't always do a full dump.
Synchronizing with strobes - sync speed
Mechanical shutters have a maximum shutter speed that will work with an internal or external flash or strobe. This is called the strobe sync speed. On most recent dSLR's this shutter speed is 1/200th or 1/250th of a second. I can push my D300 up to 1/320th. If you try to use your strobes, and have the shutter speed set faster than this speed, you will see a black or partially black photo. This is due to the "curtain" of the shutter coming down or raising while the strobe was firing. Of course, the faster the sync speed, the better, especially when taking underwater photos with the sun in the photo.
Electronic shutters can sync must faster, up to 1/1000th or 1/2000th of a second. The nikon D70 camera has an electronic shutter, this gives the camera a nice advantage when it comes to including sunbursts in a photo, it really helps to shoot sunbursts with a fast shutter speed.
Read more about sync speeds here:
If you've ever tried to take a photo in dim light without the flash of stationary subjects, and the photo came out blurry, you've experienced the effects of camera shake. When the shutter speed is slow, you must hold the camera still. There is rule of thumb, that says a photo lit by ambient light may loose sharpness due to camera shake if the shutter speed is less than one divided by 35mm equivalent focal length of the lens. For example, according to the rule, if you are shooting at 30mm focal length on a Nikon D200 camera, which has a 1.5 crop factor, you will want to use at least a 1/45th second shutter speed to avoid camera shake blur. Of course, someone's technique will effect the result, and one can shoot at slower speeds with IS or VR lenses
However, if a strobe is the main source of lighting, this only applies to the background (ambient) lighting. This is because the strobe fires very quickly. See the section on strobe sync speed.
In general, camera shake is not a major concern UW as it is topside. Motion blur and depth of field issues are more relevant.
When shooting subjects in motion, a fast shutter speed or a strobe must be used to freeze the motion. Generally, 1/125th to 1/200th of a second will freeze motion underwater, depending on how fast the subject is moving. If the subject is lit by a strobe, this does not apply due to the strobe sync speed. The strobe will fire fast enough to freeze motion.
Some photos will have "trails" before or after a moving object. This happens when the lighting is a mix between strobe light and ambient light, and a slower shutter speed is used. The strobe "freezes" part of the subject, while the ambient light coming in at a slow shutter speed leads a motion trail either before or after the subject.
Most cameras use front curtain sync by default. This means that the strobe fires as the shutter is opened. In this case, the trail will appear in front of the moving subject. DSLR's can also be set to rear curtain sync mode. This will cause the trail to appear behind the subject, looking more natural looking. Creative shooting and Rear curtain sync is discussed more here.
These fusiliers were moving quite fast. This was an ambient light shot, so there were no strobes to help freeze their motion. Shot at F5, 1/160th, ISO 320. Using 1/160th of a second just did the trick. Taken at bunaken, Indonesia, D80, Tokina 10-17mm lens at 17mm.
Minimum shutter speed for sharpness and freezing motion
As you read above, shutter speed is mostly irrelevant when lighting up a subject with your strobe, unless you are trying to get a blue blackground.
However, if a subject is completely or mostly lit by ambient light, then read on.
Now that you have read about camera shake and motion blur, you understand that you should have a minimum shutter speed for sharp photos. Of course, many other factors come into play, make sure you read the sharpness section.
Here are some general guidelines for minimum shutter speeds underwater, but please re-read the previous sections to help you understand when you can go a little lower or higher.
Subject is still - 1/20th - 1/125th, depending on focal length of the lens. to eliminate camera shake blur.
Slowly moving subjects - 1/50th - 1/125th, to freeze their motion.
Fast moving subects - 1/125th - 1/250th, depending on their speed. Very fast moving subjects may even need a faster shutter speed.
Below are some examples showing how the shutter speed controls the ambient light. I lit the sea fan, my foreground subject, with my strobe. as the shutter speed got slower and slower, the scene got brighter, and less of the stobe light lit up the sea fan. I had my strobes on TTL, so the power of the strobe was automatically reduced as the shutter speed got slower. If I had my strobes on manual power, the same strobe intensity would have lit up the sea fan in every shot. You can also see how the blue color of the water was affected. Depth of field is the same in all photos. Photos were taken at Catalina island with a tokina 10-17mm lens at 10mm.
1/25th. Still objects are still fairly sharp, although the fins are slightly blurred now. Good exposure for a natural light shot. A touch of strobe still hits the lower right corner.
1/10th. the photo is slightly blurry, and no strobe light is being used, and the scene is overexposed.
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