shutter speed of of a second (one that began exposures 18.75 ms after a bulb was fired with M sync triggering), and which was set to of a second, would close its shutter 59 ms after triggering a flashbulb (18.75 ms + 40 ms = 58.75 ms) and would achieve the maximum rated guide number from the . So long as one used flashbulbs with leaf shutter-type cameras, faster exposures and larger apertures could be used to minimize motion blur or reduce depth of field at the expense of guide number. In the case of the GE Synchro-Press with M sync for instance, shutter speeds as long as of a second still diminished its guide number, though it still managed an impressive 140 (ft) at a second exposure. This relationship between shutter speed and guide number was reflected in the guide number tables printed on flashbulb packaging after the industry-wide adoption of the guide number system, as exemplified by the below-left table for the . Cameras with focal-plane shutters—even if they had PC connectors with X, F, M, or S-sync delays ("xenon sync" with zero delay and flashbulbs with peak delays of 5, 20, and 30 ms)—could not be used at speeds that attenuated guide numbers with most types of flashbulbs because their light curves were characterized by rapid rise and fall rates; the second shutter curtain would begin wiping shut during a period of rapid change in scene illuminance, causing uneven exposure across the image area that varied in nature depending on exposure duration and the type of bulb. With the GE SynchroPress for instance, a modern camera with a focal-plane shutter and X sync would require a shutter speed of of a second (67 ms) to obtain an even exposure across the entire image area—and a not-insignificant boost in the guide number by capturing all the luminous energy to the left of the 20 ms peak. A notable exception to this limitation with focal-plane shutters was when using FP sync in combination with "flat peak" (FP) bulbs, which had rise times followed by broad, relatively level plateaus in their light output curves. The FP bulbs, like GE's , allowed extraordinary flexibility with shutter speeds, ranging from the slowest speeds on the dial to the fastest where only a narrow slit passed over the film—at the expense of course, of guide number. Fill flash: Guide numbers vs. distance When