Optical illusion

[Trinity]

Or my eyesight.
I've done a double mount 6mm showing and when put it against the wall arghh - it looked 6 at the top but less at the bottom. Turned it upside down and blow me down the thinner bit stayed at te bottom. Time for glasses.

[Dave]

Somebody will correct me I'm sure, but I believe it's parallax. I've spent minutes chasing mounts round 90 degrees at a time convinced something was wrong.

Dave

[Trinity]

Chopping device or an illness Dave?

[Dave]

Familiar feature to shooters, things appearing different relative to each other when viewed from different angles. Imagine looking at a row of telegraph poles whilst driving past them. They stay where they are but appear to move relative to each other as your viewing angle changes.

Dave

[prospero]

It's a well known illusion. All to do with how we perceive things as 'stable' and therefore safe. Ask someone to draw a triangle for you. 99.9% of the time they will do it apex-up. Apex-down is unstable and therefore gives you a subconscious feeling of instability.

Same reason for bottom-weighting mount borders. Do them equal and the top will always look wider. In theory we should also bottom-weight double mount reveals, but no one does.

[kevin]

Bottom should be approx; 10 per cent greater than top margin. 6mm at top, about7mm at bottom solves the illusion.

[233ART]

The trouble is with that idea, Kevin, is that not everyone sees things in the same way...

[prospero]

A lot depends on the nature of the mount and the lighting conditions. How extreme the difference in tone/colour between top mount and bottom etc.

[Trinity]

Spot on Prospero, it was a dark brown bottom layer and cream top, which did accentuate it.

[guzzijim]

Bottom should be approx; 10 per cent greater than top margin. 6mm at top, about7mm at bottom solves the illusion.

I've never noticed this phonominium with double mounts, having said that I never go above 4mm.

I have however encountered that 'slipped down' look with mounts/matts but have always allowed for it by making the bottom slightly wider.

Is 10% a calculated amount, perhaps devised by an ancient Greek scholar or is it a figure generally agreed by framers? or neither!

[guzzijim]

[quote="kevin"

Is 10% a calculated amount, perhaps devised by an ancient Greek scholar or is it a figure generally agreed by framers? or neither!

This was a serious question.