Frames per second query

How is a cameras "frames per second" calculated? For instance,

manufacturers say a camera can shoot at 3 frames per second - but

this surely depends upon the shutter speed? If the shutter speed is

10 seconds then, obviously, the camera cannot shoot 3 frames in one

second. Is it the speed of the motordrive?

FPS specs usually apply to shutter speeds of maximum X sync or faster. The reason is that the X sync speed is the fastest speed at which the entire shutter is open during exposure. At slower speeds that that, the shutter remains open until the second curtain begins to close, causing a delay before the film can be wound on. At faster speeds the second curtain follows the first curtain exposing a moving window of film (of narrower width at faster speeds), but the time taken for the curtains to move across the film is about the same since the first curtain takes nearly 1/2 max X sync to travel across regardless of shutter speed.

Generically speaking (film cameras), the shutter speed and "frames per second" indicate two very different things. Here is why: Each frame shot comprises of a sort of "CYCLE" - the exposure has to be set (generally), the reflex mirror goes up, aperture stops down & the shutter opens (or exposes) for the specified duration. Once the exposure is complete, the shutter closes, the aperture opens back up, and the mirror is released back to its "released" position. As the mirror comes down into its resting place, the film motor winds the film (advances) to the next frame. The next exposure cannot start until the film winding is complete and the exposure is set for the next frame by the camera body.

 

Now, while the shutter can certainly go much faster, the "fps" speed is decided NOT by shutter speed, but by a combination of the other parameters - how fast can the mirror go up (and stabilize in its position) and down as also how fast the next frame of the film can be put into position. The faster this "cycle" time, the faster the "fps" speed. In some cameras, things like the AI Servo (or equivalent) may require additional time to continually compute the next focus point and proper exposure between continuous shots and hence that may make the speed faster in the single shot mode (where there is no predictive focus) with the focus stationary.

 

AFAIK.

 

- Harman

Harman has supplied many additional details. I should mention I made a small error - a shutter curtain takes nearly the entire max X sync speed to cross the film regardless of shutter speed, not 1/2 that. Of course with a digital body "winding the film" is dumping the sensor data to buffer memory - which can take longer if the buffer has to be processed and cleared and stored to the long term memory (CF card,etc.). With the complexity of options, you often see fps quoted for different specified operating conditions for the same camera body.