evidence. Even if he had been satisfied that the proposed evidence
met the threshold requirements for admission, the trial judge
would have excluded it in the exercise of his gatekeeper function.
 In connection with the threshold requirements, the trial
judge concluded that the opinion Dr. Harlow proposed to express
extended beyond his acknowledged expertise as a physicist. The
opinion involved an attempt by Dr. Harlow to transpose calculations based on his measurements of the appellant’s face to scenes
shown in the video in order to calculate various distances. Yet,
these calculations were made without any knowledge about
the positioning and operation of the cameras; the distance individuals were away from the cameras; potential distorting effects;
the two-dimensional nature of the images; and the lack of clarity
in the videos.
 In addition, the trial judge was satisfied that the proposed
evidence was neither relevant nor necessary. The opinion was
inconclusive: the white spot could have been, but was not likely
the chain and medallion. The jury was equally capable of watching the video and deciding whether the white spot was or was not
the medallion. This was a simple matter of watching the video.
Dr. Harlow’s calculations were not necessary for jurors to decide
this issue. In addition, the proposed evidence related to an issue
the jurors were not required to decide — the origin of the
white spot. The introduction of the expert evidence was more
apt to sow confusion and divert the fact-finding function from the
jury to the expert.
 At the gatekeeper stage of the expert evidence analysis,
the trial judge considered the proposed evidence was of limited
probative value on an issue that the jury could correctly decide
without Dr. Harlow’s opinion. Further, the prejudicial effect
of the evidence was significant in that it risked distracting the
jury from its core task — determining whether the appellant
was the shooter.
The arguments on appeal
 The appellant says the trial judge got it wrong. Not once,
but twice. At both the threshold and gatekeeper stages of the
 According to the appellant, the trial judge approached the
threshold admissibility decision too strictly. The criticisms he
expressed about Dr. Harlow’s testimony were relevant considerations for the jury to take into account in assessing the weight to
assign to the evidence, but they were not barriers to admissibility.
What is more, the opinion was necessary. The presence of the
medallion was critical to the identification of the shooter. The