When 30 Seconds is Too Short

Most cameras can keep the shutter open for 30 seconds when you select that shutter speed. But you might find yourself in a dim lighting situation where 30 seconds is not long enough for a proper exposure. That's when you can turn to Bulb. Depending on your camera, "bulb" might be a separate letter B on the exposure dial or a choice under your shutter speed settings. With the bulb setting, the shutter stays open as long as you hold down the shutter button, allowing you to make extended exposures of minutes or even hours.

2 minutes, f/11, ISO 100
Face Rock & Kittens at Sunset
Bandon, Oregon

In order to use bulb mode, your camera must be on a tripod to ensure a sharp image. And keeping your finger on the shutter button for any length of time is likely to jiggle the camera. So you need a remote shutter release (either wireless or wired) that you can use to lock the shutter open without touching the camera. Then you can time the exposure using a watch, a smartphone, or just by counting the seconds (one-thousand one, one-thousand two, one-thousand three, etc.) if the exposure won't be too long. If you want to get fancy, you can purchase an intervalometer which allows you to set timed exposures of many hours. (Some cameras now have an intervalometer built in.)

63 seconds (counted), f/22, ISO 400
Barn Interior
Huson, Montana

Being able to time exposures longer than 30 seconds is obviously advantageous for night photography, but it can also be useful indoors, such as in the barn photo above, or when using neutral density filters to intentionally lengthen exposure times for creative effects as in the water photo below.

4 minutes, f/16, ISO 100
Variable neutral density filter
Pilings along the Siuslaw River
Florence, Oregon

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