firephotogblog.com - Shutter Speed

Shutter Speed

In the last article we discussed the aperture setting and how we can use it to maximize depth of field in our fire scene images. While choosing the right aperture can be nuanced, our next setting, shutter speed, is more straightforward. As we discussed in the article on the Exposure Triangle, the camera’s shutter speed setting controls the amount of light in our exposure by changing the amount of time that the camera shutter stays open. Most shutter speeds are measured in fractions of a second (1/60, 1/125, 1/250, etc.). Long shutter speeds can be measured in seconds or even minutes. Doubling the shutter speed reduces exposure by one stop which halves the amount of light that falls on the camera’s sensor. Reducing the shutter speed by one stop (from 1/500 to 1/250 for example) doubles the amount of light the sensor receives. Just like aperture, shutter speed can be used for artistic effect. Long shutter speeds can add motion blur to an image, giving a sense of movement. Fast shutter speeds can freeze motion at the peak of action.

"The Obligatory Waterfall"

There’s an unwritten rule that any tutorial on shutter speed must include pictures of running water at different shutter speeds. Here it is...

1/2 second exposure

1/250 second exposure

1/800 second exposure

Of course, as forensic photographers, we aren’t concerned with artistic effects. For the most part, our photographic subjects don’t move (or flow over rocks). The lack of subject motion gives us quite a bit of latitude when it comes to choosing a shutter speed. With forensic photos, our biggest consideration in terms of shutter speed is camera shake. In order to prevent motion blur in our images, both our subject AND our camera must remain relatively still. Holding the camera steady, therefore, is an important part of capturing a sharp image. Our ability to keep the camera steady enough for a hand-held exposure depends on the shutter speed. Just as a fast shutter speed can freeze subject motion, a sufficiently fast shutter speed can freeze the motion of a hand-held camera. The good news is that we don’t need an extremely fast shutter to eliminate minor camera movement. A tried and true rule of thumb for determining the slowest shutter speed for hand-held shooting is to use the reciprocal of your lens's focal length. For example, if you are shooting with a 50mm lens, use a shutter speed of 1/50 or faster to avoid camera shake. If you are shooting zoomed in to 100mm, shutter speed should be 1/100 or faster. The reason for the faster shutter speed in the latter case is that camera shake is exaggerated the more we zoom into an image. If you have ever used a non-stabilized scope or binoculars, you’re familiar with this effect.

Image Stabilization (I.S.)

While the reciprocal rule will give you an approximation for the slowest usable shutter speed, the low shutter speed limit will ultimately depend on the photographer and their equipment. Some people have steadier hands than others. Also, most cameras utilize image stabilization (I.S.), in the form of in-camera stabilization, lens stabilization, or both. This technology reduces the effect of camera shake and can provide for much longer hand-held exposures. I.S. can provide for shutter speeds 2 to 5 stops slower than can be achieved by non-stabilized systems. As an example, a photographer shooting at a 60mm focal length can typically hand-hold an exposure at shutter speed of 1/60 without I.S. With image stabilization, the same photographer may be able to achieve shutter speeds down to ½ second!

The effectiveness of I.S. depends on the camera and lens used. In general, smaller sensors are easier to stabilize. This is because a small sensor requires less movement to stabilize an image compared to a large sensor. This is part of the reason the stabilization on smartphone cameras is so effective. With some phone cameras it’s possible to take exposures with shutter speeds several seconds long with surprisingly good results. This is much more difficult to achieve with a larger full frame sensor. The best way to figure out your slowest effective shutter speed is to take some test shots at various shutter speeds. Zoom into the images to see what shutter speeds result in acceptably sharp images. Or just stick to the reciprocal rule and you’ll be good.

Camera Shake Example

The image on the left demonstrates the effect of camera shake due to a long exposure. As we can see in the cropped images below, the 1/40 second exposure resulted in a soft image. For the next shot I set my shutter speed using the reciprocal rule. With a 70mm lens, I knew I needed a shutter speed of 1/70 or faster. Using a shutter speed of 1/80 second I was able to capture a much sharper image.

70mm lens @ 1/40 sec

70mm lens @ 1/80 sec

Sensor Crop Factor and the Reciprocal Rule

The beauty of the reciprocal rule is that it’s simple and easy to remember. A 50mm focal length means a 1/50 second shutter speed lower limit. But there’s a catch. The reciprocal rule applies to cameras with full frame (35mm) sensors. Cameras with smaller sensors will have a more narrow field of view and will appear to be more “zoomed in” at a given focal length. Remember that the more zoomed in the camera is, the more camera shake is apparent.

The degree to which the smaller sensor image is “zoomed in” is known as the crop factor. To find the low shutter speed limit for a crop sensor camera, some photographers will tell you that you should multiply your lens focal length by the crop factor for your sensor and then take the reciprocal of the product. So much for simple and easy to remember!

I don’t think you need to do all that. Remember that most cameras have image stabilization and that smaller sensor cameras tend to have really good I.S. I’ve found that in most cases, I.S. will compensate for the apparent increase in camera movement caused by crop factor. The reciprocal rule still works as long as you have image stabilization….and you keep it turned on!

Camera Holding

Whatever approach you take to determine your slowest exposure, as you approach your low shutter speed limit, you should be mindful of good camera holding technique. There is a tendency for photographers to hold the camera out in front of them at an arm's length to shoot a photo. This “point and shoot” technique is especially common when composing with an LCD screen. This approach works just fine at fast shutter speeds. At slower shutter speeds, better holding techniques

can reduce camera shake and give you sharper images.

Crop Factor Examples

The scene above was shot with a 35mm lens on a full frame camera.

Same shot, same camera position. 35mm lens on an APSC sensor camera. (1.5X crop factor)

Same shot, same camera position. 35mm lens on a micro-four-thirds camera. (2X crop factor)

If you’re old-school like me and you prefer using the camera’s viewfinder to compose your shots, good for you! This is the most stable way to hold a camera. For long hand-held exposures its best to have three points of contact with the camera. The right hand should hold the camera grip. With the left hand, hold the bottom of the camera lens between your thumb and forefinger. This way the left hand can help steady the camera while still being able to zoom and/or manual focus. Your head provides the third point of contact. With the viewfinder to your eye, your forehead should touch the camera body (or the flash) just above the viewfinder. Tuck your elbows into your sides to lock your camera and body into a stable position.

When composing with the LCD screen you lose a point of contact. Nevertheless, good holding technique will help here too. As before, keep elbows tucked into your sides to lock your camera and body together. This drastically reduces side to side camera shake. If you’re wearing a neck strap you can use it to add additional stability. Pull down gently on the strap to reduce up and down movement. Remember that a steady camera makes for sharp photos.

Additional Support

If you know that you are pushing the limits for a hand-held exposure, look for additional ways to hold steady. If there is a convenient post, tree, vehicle, or furniture item, use it to steady the camera. Tripods can be awkward and cumbersome at the fire scene. Nevertheless, when you need a long exposure, a tripod can be a helpful tool to have in your kit.

Conclusion

Many times photographers will look at a blurry photo and automatically assume it was a focus issue. Sometimes it is. Sometimes depth of field problem due to too large an aperture. Many times though, the problem is camera shake. If your images aren't coming out sharp, check your camera settings. Ask yourself, will this aperture give me good depth of field? Is this shutter speed fast enough to eliminate camera shake? Better yet, get in the habit of quickly checking your settings before you shoot and head off problems before they start!

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