Focal-plane Shutter - Disadvantages

Disadvantages

The main disadvantage of the focal-plane shutter is that a durable and reliable one is a complex (and often expensive) device. While the concept of a travelling slit shutter is simple, a modern FP shutter is a computerized microsecond accurate timer, governing sub-gram masses of exotic materials, subjected to hundreds of g's acceleration, moving with micron precision, choreographed with other camera systems for 100,000+ cycles. This is why FP shutters are seldom seen in cheap or low quality cameras.

In addition, the typical focal-plane shutter has flash synchronization speeds that are slower than the typical leaf shutter's 1/500 sec., because the first curtain has to open fully and the second curtain must not start to close until the flash has fired. In other words, the very narrow slits of fast speeds will not be properly flash exposed. The fastest X-sync speed on a 35 mm camera is traditionally 1/60 sec. for horizontal Leica-type FP shutters and 1/125 sec. for vertical Square-type FP shutters. Modern FP shutters have increased X-sync to 1/250 sec. with the use of exotic ultra-strong materials and computer control, and 1/8000 sec. through electronic sleight of hand. (See The quest for higher speed and Breaking the X-sync barrier, below.)

Focal-plane shutters may also produce image distortion of very fast moving objects or when panned rapidly. A large relative difference between a slow wipe speed and a narrow curtain slit results in cartoonish distortion, because one side of the frame is exposed at a noticeably later instant than the other and the object's interim movement is imaged.

For a horizontal Leica-type FP shutter, the image is stretched if the object moves in the same direction as the shutter curtains, and compressed if travelling in the opposite direction of them. For a downward firing vertical Square-type FP shutter, the top of the image leans forward. In fact, the use of leaning to give the impression of speed in illustration is a caricature of the distortion caused by the slow wiping vertical FP shutters of large format cameras from the first half of the twentieth century.