1/250 of a second is the maximum flash sync speed on Nikon D7000. This is the fastest shutter speed where sensor is fully exposed at a time. On faster shutter speeds, second shutter curtain starts moving before the first curtain has fully opened and there would be no moment where the flash could illuminate whole scene captured by sensor.

This speed is often used for flash photography because the ratio of flash illumination to ambient light is most efficient at that speed. Efficiency is desirable for

• flash battery life,
• maximum flash reach,
• time it takes to recharge the flash before next shot,
• and using widest possible aperture for given ambient exposure.

While some flashes can also be used on higher speeds in High Speed Sync mode, it requires flash to output multiple light impulses per shot - that takes more power (the flash still lights up whole scene during each impulse) and the ratio to ambient light does not improve.

Slower shutter speeds than maximum sync speed can be used with any flash that is compatible with your camera or remote triggering system. At these slower speeds, you might want to look into using rear curtain sync.