DSLR cameras have only a few moving parts. The two largest of these are the main reflex mirror (which enables one to look through the lens via the viewfinder and which swings up and out of the way when one takes the photo) and the shutter (which consists of two curtains opening and closing to allow light to fall onto the sensor. The hard-working mechanical shutter is delicate and can be prone to failure over the life of the camera.
Much like the odometer on a car, a shutter actuation count can be especially significant when considering buying a second hand camera. It indicates how many times the shutter on a particular camera has been fired.
Camera models are released with an actuation rating (how long the shutter should last). Take the shutter actuation rating on face-value though, as firstly, they are computed to be statistically accurate. While there are always exceptions in any set of statistics, and you may end up with a shutter that lasts half as long or twice as long as its rated lifetime, but generally speaking they should last for as long as they are rated. Secondly, statistical ratings tend to be pessimistic, rather than optimistic. For example on the Nikon D3 the shutter is rated for 300,000 shutter actuations. That doesn’t mean the shutter will fail within one or two shots past the rating. It’s just a guideline, but a great indication of how hard a camera has worked. Shutter actuation count ratings are usually available on the internet for most DSLR models.
There are several ways to check the shutter count of a camera and all of them rely on either having access to the camera, access to an image created by the camera, or both. Fortunately many manufacturers embed the number of shutter actuations in the EXIF data of the pictures produced with that camera so you can examine a recent photo taken with a given camera and see the actuation count.
The easiest way to check is with CameraShutterCount.com
Using the EXIF data this website works for many camera models. You can upload a picture (Jpeg) to the site, which will read the EXIF data, and will give you the shutter count and also the life cycle of the camera (based on the manufacturer’s estimated shutter life for the camera model). (Other sites: www.nikonshuttercount.com and www.myshuttercount.com)
But before you panic and start making funeral arrangements for your work-horse check out http://www.olegkikin.com/shutterlife/. This is a crowd-sourced database of camera shutter actuations and when the camera died (or if it is still alive). The database has been running since 2007 and will give you a good idea of your camera’s real life expectancy.
If you look up the stats for the Nikon D700, for example, the camera is rated for 150 000 actuations, but the real world data on this website indicates that 86% of the 200 cameras sampled are still going at 250 000 actuations.