Ensemble introduces young composers in Jamaica Plain

'I feel like you lose the magic a little bit when I tell you what it all is," the composer Robert Honstein confessed, laughing, on Saturday afternoon at St. John's Episcopal Church in Jamaica Plain. Seated beside a string quartet representing the Boston Young Composers Ensemble, Honstein had been asked to explain how sounds and gestures in his 2013 piece "Arctic" corresponded to his impressions of the photographer Chris McCaw's elaborate "Sunburn" series of Arctic Circle images.