What is Nikon high speed sync?
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Can you explain in simple words what is Nikon high speed sync;
is there any speedlight that can be used on nikon d3100 that has manual functions and also ttl functions?
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High speed sync allows you to use flash at shutter speeds faster than 1/200th. Useful for fill-in but not avaialble on a D3100.
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The high speed flash sync is 1/320th second but it is only available on certain cameras.
Nikon flash sync is between 1/200 to 1/250th depending upon the model number. Look in your user manual to discover what shutter speed your specific cameras sync's with flash.