There may be occasions where a judge may rule that evidence initially thought to be relevant to an issue must be ignored. The Supreme Court of Canada has touched on this in the context of provocation: R v Tran [2010] SCC 58 (26 November 2010):
"[41] ... the evidence must be reasonably capable of supporting the inferences necessary to make out the defence before there is an air of reality to the defence (Fontaine 2004 SCC 27 (CanLII), 2004 SCC 27, [2004] 1 S.C.R. 702 at para. 56; R. v. Reddick, 1991 CanLII 106 (S.C.C.), [1991] 1 S.C.R. 1086, at p. 1088, citing Pappajohn v. The Queen, 1980 CanLII 13 (S.C.C.), [1980] 2 S.C.R. 120, at p. 133). ... ."
Tran is an example of the evidence being insufficient to establish a necessary element (here, of a defence), but I discuss it because it obliquely invites consideration of the admissibility/insufficiency boundary.
The "air of reality" requirement is really a relevance requirement.
To be relevant, evidence must be reasonably capable of supporting the fact it seeks to prove: Bain v R [2009] NZSC 16 per Elias CJ and Blanchard J at [43] (adopting R v Thomas [1970] VR 674 at p 679), per Wilson J at [91], agreeing, and adding that the threshold is "very low").
Judges may - unless there has been a pre-trial hearing - have to rule on admissibility when a party seeks to adduce the evidence, and that may be when there is only a narrow context. But on the "air of reality" test, the judge can take a wider perspective and place the challenged evidence in the context of all the other evidence in the case.
The danger is that, in a jury trial, the judge will usurp the jury's function by removing an issue (often, but not necessarily, a defence) from their consideration. The law of provocation, surveyed in Tran, brings out this difficulty, which reflects the sometimes controversial difference between questions of law and questions of fact.
The trial was by judge alone, and the judge held that the prosecution had failed to exclude the partial defence of provocation. The Court of Appeal of Alberta held that the offender should have been convicted of murder, not manslaughter. The Supreme Court dismissed the offender's appeal. Broadly, the offender could not have been acting under provocation because his estranged wife's adultery was known to him before he entered her home and it could not have been an "insult" (s 232 Criminal Code) that caused him to lose his self-control.
So the trial judge must have thought it was reasonably possible that the offender had lost his self-control (a question of fact), while the appellate courts did not (because of a requirement of law: the facts did not establish an "insult" although if they had, that would have been a question of fact under s 232). There is an objective element to the loss of self control:
"[29] ... while one spouse undoubtedly has a legal right to leave his or her partner, in some circumstances the means by which that spouse communicates this decision may amount in fact to an "insult", within the ordinary meaning of the word. However, to be recognized at law, the insult must be of sufficient gravity to cause a loss of self-control, as objectively determined. The fact that the victim has the "legal right", in the broad sense of the term, to leave the relationship is an important consideration in the assessment of this objective standard."
Tran was not so much an "air of reality" case, as a case of loss of self control being outside the scope of the partial defence of provocation because it was unreasonable. But to avoid the hideous spectacle of an appellate court acting like a jury, it is more diplomatic to pretend that the evidence was irrelevant.
