Without his glasses, David Bain had limited sight and he did not have them on the day he is accused of killing his family, a High Court jury has been told.
David Bain, 37, is being retried for the murders of five of his family members. The Crown says he shot his father Robin, his mother Margaret and siblings Stephen, Laniet and Arawa in their Dunedin home in 1994.
The defence contends Robin Bain killed his wife and children before using the accused's rifle to shoot himself.
On Thursday, evidence was read to the jury in the High Court in Christchurch from optometrists who treated various members of the Bain family.
One said David Bain was near-sighted and that anything more than 33 centimetres away would be blurred without his glasses.
She said she had seen Mr Bain two years before the killings when he bought his glasses in for repair. He was wearing his mother's glasses in the meantime.
At the time of the killings in 1994, Mr Bain was again without his glasses, as he had taken them in to be fixed three days earlier.
His mother's old glasses frame and right lens were found in his bedroom and taken as evidence, along with the left lens that was discovered in Stephen Bain's bedroom.
Suicide theory impossible - pathologist
Earlier, a pathologist has told the David Bain retrial he believes it would have been impossible for Robin Bain to have shot himself.
James Ferris analysed gunpowder scatter patterns, by comparing test fire cards with the photos of the wound in Robin Bain's temple.
Professor Ferris told the court the pattern of powder scattered on Robin Bain's skin showed the gun would have been fired from a distance of between 30cm and 42cm.
He said in his view it would have been impossible for the gunshot wound to be self-inflicted from that range.
Professor Ferris also gave evidence that Robin Bain's body may have been moved, as that would have been the only way bloodstains could have ended up on a curtain in the room where his body was found.
Jammed bullet evidence
The jury also heard evidence on Thursday of how a jammed bullet could be cleared from the rifle used in the killings.
The defence says a misfed bullet found near Robin Bain's body shows the gun must have jammed before the fatal shot was fired.
A misfed bullet is one that has failed to fire and has to be removed from the gun. It is noticeable by the markings on it.
The defence says its unlikely Robin Bain was shot by someone else, because he would have heard the noise and then had to wait while the killer cleared the jammed bullet.
But police armourer Robert Ngamoki, who tested the rifle, said it would be possible to clear a jammed bullet within a few seconds and without making much noise.