8. The computer-implemented method of claim 7 , further comprising: outputting a first pair of image frames including a sixth image frame and a seventh image frame; receiving for the first pair of image frames, a second indication indicating which of the first pair of image frames is sharper; generating, using the first feature metric, a fifth feature value associated with the sixth image frame; generating, using the first feature metric, a sixth feature value associated with the seventh image frame; receiving a fourth score for the sixth image frame, the fourth score output by the image classifier and based on the fifth feature value; receiving a fifth score for the seventh image frame, the fifth score output by the image classifier and based on the sixth feature value; generating first results indicating which of the fourth score and the fifth score is higher; comparing the second indication with the first results; determining a first accuracy of the first feature metric based on a first consistency of the second indication with the first results; determining that the first accuracy of the first feature metric satisfies a second selection criteria; and selecting the first feature metric for use in image frame selection.
9. The computer-implemented method of claim 8 , further comprising: generating, using a second feature metric, a seventh feature value associated with the sixth image frame; generating, using the second feature metric, a eighth feature value associated with the seventh image frame; receiving a sixth score for the sixth image frame output by the image classifier based on the seventh feature value; receiving a seventh score for the seventh image frame output by the image classifier based on the eighth feature value; generating second results indicating which of the sixth score and the seventh score is higher; comparing the second indication with the second results; determining a second accuracy of the second feature metric based on a second consistency of the second indication with the second results; determining that the second accuracy of the second feature metric does not satisfy the second selection criteria; and retraining the image classifier using a plurality of feature metrics and the first indication, wherein the plurality of feature metrics comprises the first feature metric but omits the second feature metric.